{"id":924,"date":"2024-08-09T20:23:01","date_gmt":"2024-08-09T20:23:01","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=924"},"modified":"2025-01-26T18:31:12","modified_gmt":"2025-01-26T18:31:12","slug":"this-temporary-objects","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/classes\/this-temporary-objects\/","title":{"rendered":"this & Temporary Objects"},"content":{"rendered":"\n

<\/span>this & Temporary Objects<\/span><\/h2>\n\n\n\n

Here is the next stage of our saga about C++ classes…<\/p>\n\n\n\n

<\/span>The this<\/strong> Pointer<\/span><\/h3>\n\n\n\n

When you invoke a (non-static) class member function:<\/p>\n\n\n\n

Complex c(1.0, 2.0);\n \/\/ ...\nc.Print();<\/strong><\/code><\/pre>\n\n\n\n
within the member function, there is available a pointer called<\/em> this<\/strong>.<\/p>\n\n\n\n
The this<\/strong> pointer for a class T<\/strong> is implicitly defined<\/em> as:<\/p>\n\n\n\n
T * const this;  \/\/ means what? Can you say it in words?<\/strong><\/code><\/pre>\n\n\n\n
(later, we\u2019ll see cases where this changes)<\/p>\n\n\n\n
this<\/strong> is implicitly initialized to point to the instance of the class that was used to invoke the member function<\/em>.<\/p>\n\n\n\n
For example, assuming the existence of a String<\/strong> class, the call:<\/p>\n\n\n\n
String s1(\"Hello!\");\nString s2(\"\");\ns2 = s1.Left(5);<\/strong><\/code><\/pre>\n\n\n\n
is equivalent to something like:<\/p>\n\n\n\n
String s1(\"Hello!\");\nString s2(\"\");\ns2 = Left(&s1<\/strong>, 5);<\/code><\/pre>\n\n\n\n
In other words, there is a hidden ‘parameter’<\/em> supplied to the member function behind the scenes.<\/p>\n\n\n\n
Access via this<\/strong> is only within class scope<\/em>, since it’s only available from within member functions of the class.<\/p>\n\n\n\n
In most cases, references via this<\/strong> are implied.  <\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
Complex::Complex(double r)\n{\n  real = r;\n  imag = 0.0;\n}<\/strong>\n<\/code><\/pre>\n\n\n\n
is equivalent to:<\/p>\n\n\n\n
Complex::Complex(double r)\n{\n  this-><\/strong>real = r;\n  this-><\/strong>imag = 0.0;\n}<\/code><\/pre>\n\n\n\n
However, there are cases where you need to use this<\/strong> explicitly.  We’ll see some of these in a short while.<\/p>\n\n\n\n
<\/span>Temporary Objects<\/span><\/h3>\n\n\n\n
There are cases where the compiler needs to construct temporary objects<\/em>.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
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Here’s a very simple Complex class, together with its class method functions, annotated to produce output at suitable times.<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Complex.h<\/span><\/span>\n\/\/  Temporary Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/19\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> Complex_h<\/span><\/span>\n#define<\/span> Complex_h<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>Complex<\/span><\/span>\n{<\/span><\/span>\npublic:<\/span><\/span>\n  <\/span>Complex<\/span>(<\/span>double<\/span> <\/span>r<\/span>, <\/span>double<\/span> <\/span>i<\/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>void<\/span> <\/span>Print<\/span>() <\/span>const<\/span>;<\/span><\/span>\n  <\/span><\/span>\n  <\/span>double<\/span> m_r, m_i;<\/span> \/\/ Real, imaginary<\/span><\/span>\n  <\/span><\/span>\n   \/\/ ...<\/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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<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  Complex.cpp<\/span><\/span>\n\/\/  Temporary Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/19\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n<\/span>\n#include<\/span> <\/span>"Complex.h"<\/span><\/span>\n<\/span>\nComplex<\/span>::<\/span>Complex<\/span>(<\/span>double<\/span> r, <\/span>double<\/span> i)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Constructing instance of Complex from doubles ["<\/span><\/span>\n            << r << <\/span>","<\/span> << i << <\/span>"]"<\/span> << <\/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><\/span>\n  <\/span>std<\/span>::cout << <\/span>"Constructing instance of Complex using copy constructor from "<\/span>;<\/span><\/span>\n  <\/span>src<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/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>"Destroying instance of Complex "<\/span>;<\/span><\/span>\n  <\/span>this<\/span>-><\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>std<\/span>::endl;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>Complex<\/span>::<\/span>Print<\/span>() <\/span>const<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"["<\/span> << m_r << <\/span>","<\/span> << m_i << <\/span>"] "<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n<\/div>\n\n\n\n
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And here’s a test program to try things out, with its own annotated output:<\/p>\n\n\n\n
<\/path><\/path><\/svg><\/span>
\/\/<\/span><\/span>\n\/\/  TestComplex.cpp<\/span><\/span>\n\/\/  Temporary Objects<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 8\/19\/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>f<\/span>(<\/span>Complex<\/span> <\/span>c<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>"f: Complex argument c: "<\/span>;<\/span><\/span>\n  <\/span>c<\/span>.<\/span>Print<\/span>();<\/span><\/span>\n  <\/span>std<\/span>::cout << <\/span>std<\/span>::endl;<\/span><\/span>\n  \/\/ Do something...<\/span><\/span>\n  <\/span>return<\/span> c;<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nint<\/span> <\/span>main<\/span>()<\/span><\/span>\n{<\/span><\/span>\n  Complex <\/span>a<\/span>(<\/span>1.0<\/span>);<\/span><\/span>\n  Complex b = <\/span>f<\/span>(<\/span>Complex<\/span>(<\/span>2.0<\/span>));<\/span><\/span>\n  a = <\/span>f<\/span>(a);<\/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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Here’s the output the program produces:<\/p>\n\n\n\n
Constructing instance of Complex from doubles [1,0]\nConstructing instance of Complex from doubles [2,0]\nf: Complex argument c: [0,6.95161e-310] \nConstructing instance of Complex using copy constructor from [0,6.95161e-310] \nDestroying instance of Complex [0,6.95161e-310] \nConstructing instance of Complex using copy constructor from [6.95167e-310,6.95161e-310] \nf: Complex argument c: [0,0] \nConstructing instance of Complex using copy constructor from [0,0] \nDestroying instance of Complex [0,6.95161e-310] \nDestroying instance of Complex [0,0] \nDestroying instance of Complex [6.95161e-310,6.95161e-310] \nDestroying instance of Complex [0,6.95161e-310] \nProgram ended with exit code: 0<\/code><\/pre>\n\n\n\n
Can you explain this output? Can you see any strangenesses?<\/p>\n\n\n\n
What’s going on?<\/p>\n\n\n\n
How many temporary objects (i.e. without a name) can you identify above?<\/p>\n<\/div>\n<\/div>\n\n\n\n
<\/span>Creation and Destruction of Temporary Objects<\/span><\/h3>\n\n\n\n
There are two major questions about temporary objects:<\/p>\n\n\n\n