{"id":926,"date":"2024-08-09T20:24:17","date_gmt":"2024-08-09T20:24:17","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=926"},"modified":"2025-02-10T20:41:55","modified_gmt":"2025-02-10T20:41:55","slug":"member-class-objects-initializer-lists","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/classes\/member-class-objects-initializer-lists\/","title":{"rendered":"Member Class Objects & Initializer Lists"},"content":{"rendered":"\n

<\/span>Member Class Objects & Initializer Lists<\/span><\/h2>\n\n\n\n

Diving into the next set of topics…<\/p>\n\n\n\n

<\/span>Member Class Objects<\/span><\/h3>\n\n\n\n

A class can contain members which are themselves instances of other classes — like a struct within a struct.  <\/p>\n\n\n\n

These are member class objects<\/em>.  For example:<\/p>\n\n\n\n

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<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Complex.h<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> Complex_h<\/span><\/span>\n#define<\/span> Complex_h<\/span><\/span>\n<\/span>\n\/\/ complex.h<\/span><\/span>\nclass<\/span> <\/span>Complex<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>Complex<\/span>(<\/span>double<\/span> <\/span>real<\/span> = <\/span>0.0<\/span>, <\/span>double<\/span> <\/span>imag<\/span> = <\/span>0.0<\/span>);<\/span><\/span>\n  <\/span>Complex<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>src<\/span>);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Print<\/span>();<\/span><\/span>\n  \/\/ ...<\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>double<\/span> m_real, m_imag;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>ComplexLine<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>ComplexLine<\/span>();<\/span><\/span>\n  <\/span>ComplexLine<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>start<\/span>,<\/span><\/span>\n              <\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>end<\/span>);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Print<\/span>();<\/span><\/span>\nprivate:<\/span><\/span>\n  Complex m_start, m_end;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n<\/span>\n#endif<\/span> \/* Complex_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Complex.cpp<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>double<\/span> real, <\/span>double<\/span> imag)<\/span><\/span>\n{<\/span><\/span>\n  m_real = real;<\/span><\/span>\n  m_imag = imag;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>const<\/span> Complex &src)<\/span><\/span>\n{<\/span><\/span>\n  m_real = <\/span>src<\/span>.<\/span>m_real<\/span>;<\/span><\/span>\n  m_imag = <\/span>src<\/span>.<\/span>m_imag<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>Complex<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex: ["<\/span> << m_real << <\/span>", "<\/span> << m_imag << <\/span>"]"<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\n\/\/ Class ComplexLine<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  m_start = <\/span>0.0<\/span>; <\/span><\/span>\n  m_end = <\/span>0.0<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>(<\/span><\/span>\n              <\/span>const<\/span> Complex &start,<\/span><\/span>\n              <\/span>const<\/span> Complex &end)<\/span><\/span>\n{<\/span><\/span>\n  m_start = start; <\/span><\/span>\n  m_end = end;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>ComplexLine<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine: ["<\/span>;<\/span><\/span>\n  <\/span>m_start<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>", "<\/span>;<\/span><\/span>\n  <\/span>m_end<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"]"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n<\/div>\n\n\n\n
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Here’s a test program for this:<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[]) <\/span><\/span>\n{<\/span><\/span>\n  ComplexLine line1;<\/span><\/span>\n  <\/span>line1<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span><\/span>\n  ComplexLine <\/span>line2<\/span>(<\/span>Complex<\/span>(<\/span>2.0<\/span>, <\/span>1.5<\/span>),<\/span><\/span>\n                    <\/span>Complex<\/span>(<\/span>3.9<\/span>, <\/span>2.5<\/span>));<\/span><\/span>\n  <\/span>line2<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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Which produces the following output:<\/p>\n\n\n\n
ComplexLine: [Complex: [0, 0], Complex: [0, 0]]\nComplexLine: [Complex: [2, 1.5], Complex: [3.9, 2.5]]\nProgram ended with exit code: 0<\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
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But suppose we remove the default constructor:<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Complex.h<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> Complex_h<\/span><\/span>\n#define<\/span> Complex_h<\/span><\/span>\n<\/span>\n\/\/ complex.h<\/span><\/span>\nclass<\/span> <\/span>Complex<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  \/\/ Complex(double real = 0.0, double imag = 0.0);<\/span><\/span>\n  <\/span>Complex<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>src<\/span>);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Print<\/span>();<\/span><\/span>\n  \/\/ ...<\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>double<\/span> m_real, m_imag;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>ComplexLine<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>ComplexLine<\/span>();<\/span><\/span>\n  <\/span>ComplexLine<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>start<\/span>,<\/span><\/span>\n              <\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>end<\/span>);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Print<\/span>();<\/span><\/span>\nprivate:<\/span><\/span>\n  Complex m_start, m_end;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n#endif<\/span> \/* Complex_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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Now, we get compile-time errors in Complex.cpp<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Complex.cpp<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\n\/*<\/span><\/span>\n Complex::Complex(double real, double imag)<\/span><\/span>\n{<\/span><\/span>\n  m_real = real;<\/span><\/span>\n  m_imag = imag;<\/span><\/span>\n}<\/span><\/span>\n *\/<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>const<\/span> Complex &src)<\/span><\/span>\n{<\/span><\/span>\n  m_real = <\/span>src<\/span>.<\/span>m_real<\/span>;<\/span><\/span>\n  m_imag = <\/span>src<\/span>.<\/span>m_imag<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>Complex<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex: ["<\/span> << m_real << <\/span>", "<\/span> << m_imag << <\/span>"]"<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\n\/\/ Class ComplexLine<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  m_start = <\/span>0.0<\/span>; <\/span><\/span>\n  m_end = <\/span>0.0<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>(<\/span><\/span>\n              <\/span>const<\/span> Complex &start,<\/span><\/span>\n              <\/span>const<\/span> Complex &end)<\/span><\/span>\n{<\/span><\/span>\n  m_start = start; <\/span><\/span>\n  m_end = end;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>ComplexLine<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine: ["<\/span>;<\/span><\/span>\n  <\/span>m_start<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>", "<\/span>;<\/span><\/span>\n  <\/span>m_end<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"]"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n<\/div>\n\n\n\n
Complex.cpp:32:14 Constructor for 'ComplexLine' must explicitly initialize the member 'm_start' which does not have a default constructor\n\nComplex.cpp:32:14 Constructor for 'ComplexLine' must explicitly initialize the member 'm_end' which does not have a default constructor\n\nComplex.cpp:34:11 No viable overloaded '='\n\nComplex.cpp:35:9 No viable overloaded '='\n\nComplex.cpp:38:14 Constructor for 'ComplexLine' must explicitly initialize the member 'm_start' which does not have a default constructor\n\nComplex.cpp:38:14 Constructor for 'ComplexLine' must explicitly initialize the member 'm_end' which does not have a default constructor\n<\/code><\/pre>\n\n\n\n
The problem is that we wrote code to assign<\/em>, not<\/strong> initialize<\/em> the member class object.<\/p>\n\n\n\n
The compiler automatically generated a call to the default constructor<\/em> for the member class object to perform the initialization.<\/p>\n\n\n\n
When we don’t have a default constructor for the member class object, the compiler doesn’t know how to initialize the object<\/em>.<\/p>\n\n\n\n
<\/span>Constructor Initializer Lists<\/span><\/h3>\n\n\n\n
In order to initialize the member class object, we have to use a constructor initializer list<\/em>:<\/p>\n\n\n\n
class X\n{\npublic:\n\tX(int x, int y) : <initializer-list><\/strong>\n\t{\n\t\t\/\/ Body\n\t}\n\t\/\/ ...\n};<\/code><\/pre>\n\n\n\n
Constructor initializer lists are only used with constructors<\/em>.<\/p>\n\n\n\n
An initializer list (preceded by a colon) follows the constructor’s argument list, and precedes the constructor’s body.<\/p>\n\n\n\n
An initializer list contains one or more items, separated by commas.<\/p>\n\n\n\n
An initializer list item looks somewhat like a function call.<\/p>\n\n\n\n
Here’s the previous example, converted to using initializer lists:<\/p>\n\n\n\n
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The Complex.h file is unchanged:<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Complex.h<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> Complex_h<\/span><\/span>\n#define<\/span> Complex_h<\/span><\/span>\n<\/span>\n\/\/ complex.h<\/span><\/span>\nclass<\/span> <\/span>Complex<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>Complex<\/span>(<\/span>double<\/span> <\/span>real<\/span> = <\/span>0.0<\/span>, <\/span>double<\/span> <\/span>imag<\/span> = <\/span>0.0<\/span>);<\/span><\/span>\n  <\/span>Complex<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>src<\/span>);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Print<\/span>();<\/span><\/span>\n  \/\/ ...<\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>double<\/span> m_real, m_imag;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>ComplexLine<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>ComplexLine<\/span>();<\/span><\/span>\n  <\/span>ComplexLine<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>start<\/span>,<\/span><\/span>\n              <\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>end<\/span>);<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Print<\/span>();<\/span><\/span>\nprivate:<\/span><\/span>\n  Complex m_start, m_end;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n#endif<\/span> \/* Complex_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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The initializer lists are added to Complex.cpp:<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Complex.cpp<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>double<\/span> real, <\/span>double<\/span> imag)<\/span><\/span>\n  : <\/span>m_real<\/span>(real), <\/span>m_imag<\/span>(imag)<\/span><\/span>\n{ }<\/span><\/span>\n<\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>const<\/span> Complex &src)<\/span><\/span>\n  : <\/span>m_real<\/span>(<\/span>src<\/span>.<\/span>m_real<\/span>), <\/span>m_imag<\/span>(<\/span>src<\/span>.<\/span>m_imag<\/span>)<\/span><\/span>\n{ }<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>Complex<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex: ["<\/span> << m_real << <\/span>", "<\/span> << m_imag << <\/span>"]"<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\n\/\/ Class ComplexLine<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>()<\/span><\/span>\n  : <\/span>m_start<\/span>(<\/span>0.0<\/span>), <\/span>m_end<\/span>(<\/span>0.0<\/span>)<\/span><\/span>\n{ }<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>(<\/span><\/span>\n              <\/span>const<\/span> Complex &start,<\/span><\/span>\n              <\/span>const<\/span> Complex &end)<\/span><\/span>\n  : <\/span>m_start<\/span>(start), <\/span>m_end<\/span>(end)<\/span><\/span>\n{ }<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>ComplexLine<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine: ["<\/span>;<\/span><\/span>\n  <\/span>m_start<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>", "<\/span>;<\/span><\/span>\n  <\/span>m_end<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"]"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span><\/code><\/pre><\/div>\n<\/div>\n<\/div>\n\n\n\n
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The test program is unchanged:<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[]) <\/span><\/span>\n{<\/span><\/span>\n  ComplexLine line1;<\/span><\/span>\n  <\/span>line1<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span><\/span>\n  ComplexLine <\/span>line2<\/span>(<\/span>Complex<\/span>(<\/span>2.0<\/span>, <\/span>1.5<\/span>),<\/span><\/span>\n                    <\/span>Complex<\/span>(<\/span>3.9<\/span>, <\/span>2.5<\/span>));<\/span><\/span>\n  <\/span>line2<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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Which produces the same output as before:<\/p>\n\n\n\n
ComplexLine: [Complex: [0, 0], Complex: [0, 0]]\nComplexLine: [Complex: [2, 1.5], Complex: [3.9, 2.5]]\nProgram ended with exit code: 0<\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
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Note that, in this case, this leaves the constructor bodies empty — which is not unusual!<\/em><\/p>\n<\/blockquote>\n\n\n\n
<\/span>Recommendation<\/span><\/h4>\n\n\n\n
While you can get away with using assignment in constructors, it is something to be discouraged!<\/p>\n\n\n\n
Why?<\/em><\/p>\n\n\n\n
    \n
  • Because it’s important to understand that you’re initializing<\/em>, not assigning, and…<\/li>\n\n\n\n
  • Because it’s more efficient<\/em><\/strong> to use initialization.  
    Remember that the compiler will try to initialize things for you, even if you don’t do it.<\/li>\n<\/ul>\n\n\n\n
    <\/span>Efficiency: Assignment vs Initializer Lists<\/span><\/h3>\n\n\n\n
    Let’s examine what happens when we use assignment instead of initialization.  <\/p>\n\n\n\n
    First, in the previous Complex\/ComplexLine example, we’ll return to using assignments within the constructors, add some operator=<\/strong> functions to track when assignment happens, and we’ll instrument the classes with printouts. <\/p>\n\n\n\n
    \n
    \n
    Here are the Complex.h and Complex.cpp files:<\/p>\n\n\n\n
    <\/path><\/path><\/svg><\/span>
    \/\/<\/span><\/span>\n\/\/  Complex.h<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> Complex_h<\/span><\/span>\n#define<\/span> Complex_h<\/span><\/span>\n<\/span>\n\/\/ complex.h<\/span><\/span>\nclass<\/span> <\/span>Complex<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>Complex<\/span>(<\/span>double<\/span> <\/span>real<\/span> = <\/span>0.0<\/span>, <\/span>double<\/span> <\/span>imag<\/span> = <\/span>0.0<\/span>);<\/span><\/span>\n  <\/span>Complex<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>src<\/span>);<\/span><\/span>\n  <\/span>~Complex<\/span>();<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Print<\/span>();<\/span><\/span>\n  <\/span><\/span>\n  <\/span>Complex<\/span> <\/span>&<\/span>operator=<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>rhs<\/span>);<\/span><\/span>\n  <\/span>Complex<\/span> <\/span>&<\/span>operator=<\/span>(<\/span>double<\/span> <\/span>rhs<\/span>);<\/span><\/span>\n<\/span>\n  \/\/ ...<\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>double<\/span> m_real, m_imag;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>ComplexLine<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>ComplexLine<\/span>();<\/span><\/span>\n  <\/span>ComplexLine<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>start<\/span>,<\/span><\/span>\n              <\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>end<\/span>);<\/span><\/span>\n  <\/span>~ComplexLine<\/span>();<\/span><\/span>\n  <\/span><\/span>\n  <\/span>void<\/span> <\/span>Print<\/span>();<\/span><\/span>\n  <\/span><\/span>\n  <\/span>ComplexLine<\/span> <\/span>&<\/span>operator=<\/span>(<\/span>const<\/span> <\/span>ComplexLine<\/span> <\/span>&<\/span>rhs<\/span>);<\/span><\/span>\n<\/span>\nprivate:<\/span><\/span>\n  Complex m_start, m_end;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\n#endif<\/span> \/* Complex_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
    \n
    <\/path><\/path><\/svg><\/span>
    \/\/<\/span><\/span>\n\/\/  Complex.cpp<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>double<\/span> real, <\/span>double<\/span> imag)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex(double real, double imag)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n  m_real = real;<\/span><\/span>\n  m_imag = imag;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>const<\/span> Complex &src)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex(const Complex &src)"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n  m_real = <\/span>src<\/span>.<\/span>m_real<\/span>;<\/span><\/span>\n  m_imag = <\/span>src<\/span>.<\/span>m_imag<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span>::~<\/span>Complex<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"~Complex()"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>Complex<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex: ["<\/span> << m_real << <\/span>", "<\/span> << m_imag << <\/span>"]"<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span> <\/span>&<\/span>Complex<\/span>::<\/span>operator=<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>rhs<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex::operator=(const Complex &rhs)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span>if<\/span> (<\/span>this<\/span> != &rhs)<\/span><\/span>\n  {<\/span><\/span>\n    m_real = <\/span>rhs<\/span>.<\/span>m_real<\/span>;<\/span><\/span>\n    m_imag = <\/span>rhs<\/span>.<\/span>m_imag<\/span>;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>return<\/span> *<\/span>this<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span> <\/span>&<\/span>Complex<\/span>::<\/span>operator=<\/span>(<\/span>double<\/span> <\/span>rhs<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex::operator=(double rhs)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n  m_real = rhs;<\/span><\/span>\n  m_imag = <\/span>0.0<\/span>;<\/span><\/span>\n  <\/span>return<\/span> *<\/span>this<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\n<\/span>\n\/\/ Class ComplexLine<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine()"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n  m_start = <\/span>0.0<\/span>;<\/span><\/span>\n  m_end = <\/span>0.0<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>(<\/span><\/span>\n              <\/span>const<\/span> Complex &start,<\/span><\/span>\n              <\/span>const<\/span> Complex &end)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine(const Complex & start, "<\/span><\/span>\n                            <\/span>"const Complex &end)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n  m_start = start;<\/span><\/span>\n  m_end = end;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplexLine<\/span>::~<\/span>ComplexLine<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"~ComplexLine()"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>ComplexLine<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine: ["<\/span>;<\/span><\/span>\n  <\/span>m_start<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>", "<\/span>;<\/span><\/span>\n  <\/span>m_end<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"]"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplexLine<\/span> <\/span>&<\/span>ComplexLine<\/span>::<\/span>operator=<\/span>(<\/span>const<\/span> <\/span>ComplexLine<\/span> <\/span>&<\/span>rhs<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine::operator=(const Complex &rhs)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span>if<\/span> (<\/span>this<\/span> != &rhs)<\/span><\/span>\n  {<\/span><\/span>\n    m_start = <\/span>rhs<\/span>.<\/span>m_start<\/span>;<\/span><\/span>\n    m_end = <\/span>rhs<\/span>.<\/span>m_end<\/span>;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>return<\/span> *<\/span>this<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n
    The test program is again unchanged:<\/p>\n\n\n\n
    <\/path><\/path><\/svg><\/span>
    \/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[]) <\/span><\/span>\n{<\/span><\/span>\n  ComplexLine line1;<\/span><\/span>\n  <\/span>line1<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span><\/span>\n  ComplexLine <\/span>line2<\/span>(<\/span>Complex<\/span>(<\/span>2.0<\/span>, <\/span>1.5<\/span>),<\/span><\/span>\n                    <\/span>Complex<\/span>(<\/span>3.9<\/span>, <\/span>2.5<\/span>));<\/span><\/span>\n  <\/span>line2<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
    \n
    And it now produces the following output:<\/p>\n\n\n\n
    Complex(double real, double imag)\nComplex(double real, double imag)\nComplexLine()\nComplex::operator=(double rhs)\nComplex::operator=(double rhs)\n\nComplexLine: [Complex: [0, 0], Complex: [0, 0]]\n\nComplex(double real, double imag)\n\nComplex(double real, double imag)\n\nComplex(double real, double imag)\nComplex(double real, double imag)\nComplexLine(const Complex & start, const Complex &end)\nComplex::operator=(const Complex &rhs)\nComplex::operator=(const Complex &rhs)\n~Complex()\n~Complex()\n\nComplexLine: [Complex: [2, 1.5], Complex: [3.9, 2.5]]\n\n~ComplexLine()\n~Complex()\n~Complex()\n~ComplexLine()\n~Complex()\n~Complex()\nProgram ended with exit code: 0<\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
    Go through that output and see if you can understand where each line comes from in the code.<\/p>\n\n\n\n
    Now, let’s change to initializer lists:<\/p>\n\n\n\n
    \n
    \n
    The only file to change is Complex.cpp:<\/p>\n\n\n\n
    <\/path><\/path><\/svg><\/span>
    \/\/<\/span><\/span>\n\/\/  Complex.cpp<\/span><\/span>\n\/\/  Member Class Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/22\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>double<\/span> real, <\/span>double<\/span> imag)<\/span><\/span>\n  : <\/span>m_real<\/span>(real), <\/span>m_imag<\/span>(imag)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex(double real, double imag)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>const<\/span> Complex &src)<\/span><\/span>\n  : <\/span>m_real<\/span>(<\/span>src<\/span>.<\/span>m_real<\/span>), <\/span>m_imag<\/span>(<\/span>src<\/span>.<\/span>m_imag<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex(const Complex &src)"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span>::~<\/span>Complex<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"~Complex()"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>Complex<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex: ["<\/span> << m_real << <\/span>", "<\/span> << m_imag << <\/span>"]"<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span> <\/span>&<\/span>Complex<\/span>::<\/span>operator=<\/span>(<\/span>const<\/span> <\/span>Complex<\/span> <\/span>&<\/span>rhs<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex::operator=(const Complex &rhs)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span>if<\/span> (<\/span>this<\/span> != &rhs)<\/span><\/span>\n  {<\/span><\/span>\n    m_real = <\/span>rhs<\/span>.<\/span>m_real<\/span>;<\/span><\/span>\n    m_imag = <\/span>rhs<\/span>.<\/span>m_imag<\/span>;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>return<\/span> *<\/span>this<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplex<\/span> <\/span>&<\/span>Complex<\/span>::<\/span>operator=<\/span>(<\/span>double<\/span> <\/span>rhs<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Complex::operator=(double rhs)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n  m_real = rhs;<\/span><\/span>\n  m_imag = <\/span>0.0<\/span>;<\/span><\/span>\n  <\/span>return<\/span> *<\/span>this<\/span>;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\n<\/span>\n\/\/ Class ComplexLine<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>()<\/span><\/span>\n  : <\/span>m_start<\/span>(<\/span>0.0<\/span>), <\/span>m_end<\/span>(<\/span>0.0<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine()"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplexLine<\/span>::<\/span>ComplexLine<\/span>(<\/span><\/span>\n              <\/span>const<\/span> Complex &start,<\/span><\/span>\n              <\/span>const<\/span> Complex &end)<\/span><\/span>\n  : <\/span>m_start<\/span>(start), <\/span>m_end<\/span>(end)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine(const Complex & start, "<\/span><\/span>\n                            <\/span>"const Complex &end)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplexLine<\/span>::~<\/span>ComplexLine<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"~ComplexLine()"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>ComplexLine<\/span>::<\/span>Print<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine: ["<\/span>;<\/span><\/span>\n  <\/span>m_start<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>", "<\/span>;<\/span><\/span>\n  <\/span>m_end<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"]"<\/span> << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nComplexLine<\/span> <\/span>&<\/span>ComplexLine<\/span>::<\/span>operator=<\/span>(<\/span>const<\/span> <\/span>ComplexLine<\/span> <\/span>&<\/span>rhs<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"ComplexLine::operator=(const Complex &rhs)"<\/span><\/span>\n            << <\/span>std<\/span>::endl;<\/span><\/span>\n  <\/span>if<\/span> (<\/span>this<\/span> != &rhs)<\/span><\/span>\n  {<\/span><\/span>\n    m_start = <\/span>rhs<\/span>.<\/span>m_start<\/span>;<\/span><\/span>\n    m_end = <\/span>rhs<\/span>.<\/span>m_end<\/span>;<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>return<\/span> *<\/span>this<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
    \n
    … and here’s the new output:<\/p>\n\n\n\n
    Complex(double real, double imag)\nComplex(double real, double imag)\nComplexLine()\nComplexLine: [Complex: [0, 0], Complex: [0, 0]]\nComplex(double real, double imag)\nComplex(double real, double imag)\nComplex(const Complex &src)\nComplex(const Complex &src)\nComplexLine(const Complex & start, const Complex &end)\n~Complex()\n~Complex()\nComplexLine: [Complex: [2, 1.5], Complex: [3.9, 2.5]]\n~ComplexLine()\n~Complex()\n~Complex()\n~ComplexLine()\n~Complex()\n~Complex()\nProgram ended with exit code: 0<\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
    Compare the outputs side-by-side:<\/p>\n\n\n\n
    \n
    \n
    Complex(double real, double imag)\nComplex(double real, double imag)\nComplexLine()\nComplex::operator=(double rhs)\nComplex::operator=(double rhs)\n\nComplexLine: [Complex: [0, 0], Complex: [0, 0]]\n\nComplex(double real, double imag)\n\nComplex(double real, double imag)\n\n\nComplex(double real, double imag)\nComplex(double real, double imag)\nComplexLine(const Complex & start, const Complex &end)\nComplex::operator=(const Complex &rhs)\nComplex::operator=(const Complex &rhs)\n~Complex()\n~Complex()\n\nComplexLine: [Complex: [2, 1.5], Complex: [3.9, 2.5]]\n\n~ComplexLine()\n~Complex()\n~Complex()\n~ComplexLine()\n~Complex()\n~Complex()\nProgram ended with exit code: 0<\/code><\/pre>\n<\/div>\n\n\n\n
    \n
    Complex(double real, double imag)\nComplex(double real, double imag)\nComplexLine()\n\n\n\nComplexLine: [Complex: [0, 0], Complex: [0, 0]]\n\nComplex(double real, double imag)\n\nComplex(double real, double imag)\nComplex(const Complex &src)\nComplex(const Complex &src)\n\n\nComplexLine(const Complex & start, const Complex &end)\n\n\n~Complex()\n~Complex()\n\nComplexLine: [Complex: [2, 1.5], Complex: [3.9, 2.5]]\n\n~ComplexLine()\n~Complex()\n~Complex()\n~ComplexLine()\n~Complex()\n~Complex()\nProgram ended with exit code: 0<\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
    See if you can explain the differences.<\/p>\n\n\n\n
    Note that using initializer lists has removed the need for a number of operations!<\/p>\n\n\n\n
    …and this was a trivially simple class.  In non-trivial classes, you can produce significant savings!<\/p>\n\n\n\n
    <\/span>When Constructor Intializer Lists are Required<\/span><\/h3>\n\n\n\n
    There are cases where you don’t have a choice — where you must use initializer lists<\/em>:<\/p>\n\n\n\n
      \n
    • const<\/strong> data members<\/li>\n\n\n\n
    • reference data members<\/li>\n\n\n\n
    • member class objects with no default constructor<\/li>\n<\/ul>\n\n\n\n
      For example:<\/p>\n\n\n\n
      \n
      \n
      <\/path><\/path><\/svg><\/span>
      \/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Required Constructor Initializer Lists<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/23\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n<\/span>\nclass<\/span> <\/span>NoDefaultConstructor<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>NoDefaultConstructor<\/span>(<\/span>double<\/span> <\/span>value<\/span>)<\/span><\/span>\n    : <\/span>m_value<\/span>(value)<\/span><\/span>\n  { }<\/span><\/span>\n  \/\/ ...<\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>NoDefaultConstructor<\/span>();<\/span> \/\/ Disable<\/span><\/span>\n    <\/span><\/span>\n  <\/span>double<\/span> m_value;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>RequiresInitializerList<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>RequiresInitializerList<\/span>(<\/span>const<\/span> <\/span>int<\/span> <\/span>constValue<\/span>,<\/span><\/span>\n                          <\/span>int<\/span> <\/span>&<\/span>refValue<\/span>,<\/span><\/span>\n                          <\/span>NoDefaultConstructor<\/span> <\/span>&<\/span> <\/span>ndc<\/span>)<\/span><\/span>\n  {<\/span><\/span>\n    m_constValue = constValue;<\/span><\/span>\n    m_refValue = refValue;<\/span><\/span>\n    m_ndc = ndc;<\/span><\/span>\n  }<\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>const<\/span> <\/span>int<\/span>            m_constValue;<\/span><\/span>\n  <\/span>int<\/span>                 &m_refValue;<\/span><\/span>\n  NoDefaultConstructor m_ndc;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[]) <\/span><\/span>\n{<\/span><\/span>\n  NoDefaultConstructor <\/span>ndc<\/span>(<\/span>4.2<\/span>);<\/span><\/span>\n  <\/span>int<\/span> value1 = <\/span>13<\/span>;<\/span><\/span>\n  <\/span>int<\/span> value2 = <\/span>73<\/span>;<\/span><\/span>\n    <\/span><\/span>\n  <\/span>RequiresInitializerList<\/span>(value1, value2, ndc);<\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
      \n
      This gives compile time errors:<\/p>\n\n\n\n
      main.cpp:26:3 Constructor for 'RequiresInitializerList' must explicitly initialize the const member 'm_constValue'\n\nmain.cpp:26:3 Constructor for 'RequiresInitializerList' must explicitly initialize the reference member 'm_refValue'\n\nmain.cpp:26:3 Field of type 'NoDefaultConstructor' has private default constructor\n\nmain.cpp:30:18 Cannot assign to non-static data member 'm_constValue' with const-qualified type 'const int'\n<\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
      \n
      \n
      If we change to using initializers,<\/p>\n\n\n\n
      <\/path><\/path><\/svg><\/span>
      \/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Required Constructor Initializer Lists<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/23\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n<\/span>\nclass<\/span> <\/span>NoDefaultConstructor<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>NoDefaultConstructor<\/span>(<\/span>double<\/span> <\/span>value<\/span>)<\/span><\/span>\n    : <\/span>m_value<\/span>(value)<\/span><\/span>\n  { }<\/span><\/span>\n  \/\/ ...<\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>NoDefaultConstructor<\/span>();<\/span> \/\/ Disable<\/span><\/span>\n    <\/span><\/span>\n  <\/span>double<\/span> m_value;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>RequiresInitializerList<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>RequiresInitializerList<\/span>(<\/span>const<\/span> <\/span>int<\/span> <\/span>constValue<\/span>,<\/span><\/span>\n                          <\/span>int<\/span> <\/span>&<\/span>refValue<\/span>,<\/span><\/span>\n                          <\/span>NoDefaultConstructor<\/span> <\/span>&<\/span> <\/span>ndc<\/span>)<\/span><\/span>\n    : <\/span>m_constValue<\/span>(constValue), <\/span>m_refValue<\/span>(refValue), <\/span>m_ndc<\/span>(ndc)<\/span><\/span>\n  { }<\/span><\/span>\nprivate:<\/span><\/span>\n  <\/span>const<\/span> <\/span>int<\/span>            m_constValue;<\/span><\/span>\n  <\/span>int<\/span>                 &m_refValue;<\/span><\/span>\n  NoDefaultConstructor m_ndc;<\/span><\/span>\n};<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>(<\/span>int<\/span> <\/span>argc<\/span>, <\/span>const<\/span> <\/span>char<\/span> <\/span>*<\/span> <\/span>argv<\/span>[]) <\/span><\/span>\n{<\/span><\/span>\n  NoDefaultConstructor <\/span>ndc<\/span>(<\/span>4.2<\/span>);<\/span><\/span>\n  <\/span>int<\/span> value1 = <\/span>13<\/span>;<\/span><\/span>\n  <\/span>int<\/span> value2 = <\/span>73<\/span>;<\/span><\/span>\n    <\/span><\/span>\n  <\/span>RequiresInitializerList<\/span>(value1, value2, ndc);<\/span><\/span>\n  <\/span>return<\/span> <\/span>0<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
      \n
      the program runs correctly.
      <\/p>\n<\/div>\n<\/div>\n\n\n\n
      \n
      It is always better to use initializer lists to perform initialization of class data members.<\/em><\/p>\n<\/blockquote>\n\n\n\n
      \n
      If you don’t use initializer lists, you’re probably doing assignment in the constructor body.  This is very inefficient, because the compiler is doing implicit initialization (perhaps incorrectly) for you anyway.<\/em><\/p>\n<\/blockquote>\n\n\n\n
      <\/span>Summary<\/span><\/h2>\n\n\n\n
      Well, we’ve learned still more about classes:<\/p>\n\n\n\n
        \n
      • Member Class Objects<\/li>\n\n\n\n
      • Constructor Initializer Lists<\/li>\n<\/ul>\n\n\n\n
        As you can see, classes are the central feature of C++!<\/p>\n","protected":false},"excerpt":{"rendered":"
        Member Class Objects & Initializer Lists Diving into the next set of topics… Member Class Objects A class can contain members which are themselves instances of other classes — like a struct within a struct.  These are member class objects.  For example: Here’s a test program for this: Which produces the following output: But suppose […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1901,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-926","page","type-page","status-publish","hentry"],"blocksy_meta":[],"_links":{"self":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/926","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/comments?post=926"}],"version-history":[{"count":24,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/926\/revisions"}],"predecessor-version":[{"id":2255,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/926\/revisions\/2255"}],"up":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/pages\/1901"}],"wp:attachment":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/wp-json\/wp\/v2\/media?parent=926"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}