{"id":1980,"date":"2025-01-26T21:09:23","date_gmt":"2025-01-26T21:09:23","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/?page_id=1980"},"modified":"2025-01-27T18:47:20","modified_gmt":"2025-01-27T18:47:20","slug":"exception-specifications","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalCPlusPlusProgramming\/index.php\/topics\/c-details\/exceptions-2\/exception-specifications\/","title":{"rendered":"Exception Specifications"},"content":{"rendered":"\n

<\/span>Exception Specifications<\/span><\/h2>\n\n\n\n

Whether a function throws an exception or not should really be part of its interface.  <\/p>\n\n\n\n

So, a function declaration may specify a set of exceptions that might be thrown<\/em> by this function:<\/p>\n\n\n\n

void myFunction(int myParam)\n  throw (exc1, exc2, exc3)<\/strong>;<\/code><\/pre>\n\n\n\n
This specifies that myFunction<\/strong> may throw exceptions exc1<\/strong>, exc2<\/strong>, and exc3<\/strong> (and exceptions derived from these), but not others.<\/p>\n\n\n\n
When a function specifies something about its exceptions, it is effectively making a guarantee to its caller.   If during execution, something happens to violate that guarantee, then the attempt will be transformed into a call to unexpected()<\/strong>.<\/p>\n\n\n\n
By default, unexpected()<\/strong> calls terminate()<\/strong> which, by default, calls abort()<\/strong>.<\/p>\n\n\n\n
A function declared with no exception specification<\/em> is assumed to potentially throw any exception<\/em>:<\/p>\n\n\n\n
int myFunction(); \/\/ can throw any exception<\/strong>\n<\/code><\/pre>\n\n\n\n
This is consistent with C (and earlier C++) usage.<\/p>\n\n\n\n
A function may be declared as throwing no exceptions<\/em>:<\/p>\n\n\n\n
int myOtherFunction() throw (); \/\/ throws no exceptions<\/strong><\/code><\/pre>\n\n\n\n
<\/span>Compile-time Checking of Exception Specifications<\/span><\/h3>\n\n\n\n
Some compile-time checking is done with exception specifications:<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  ExcSpecs.h<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> ExcSpecs_h<\/span><\/span>\n#define<\/span> ExcSpecs_h<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>SomethingBadHappened<\/span> {};<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomething<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>);<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomethingElse<\/span>() <\/span>throw<\/span>();<\/span><\/span>\n<\/span>\n#endif<\/span> \/* ExcSpecs_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  ExcSpecs.cpp<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span>"ExcSpecs.h"<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomething<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  \/\/ Does not throw the specified exception<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomethingElse<\/span>()<\/span> \/\/ doesn't match declaration<\/span><\/span>\n{<\/span><\/span>\n  \/\/ Throws an exception not specified<\/span><\/span>\n  <\/span>throw<\/span> <\/span>SomethingBadHappened<\/span>();<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
This produces a compile-time error:<\/p>\n\n\n\n
ExcSpecs.cpp:15:13 'doSomethingElse' is missing exception specification 'throw()'\n<\/strong><\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
So, we fix it:<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  ExcSpecs.h<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> ExcSpecs_h<\/span><\/span>\n#define<\/span> ExcSpecs_h<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>SomethingBadHappened<\/span> {};<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomething<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>);<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomethingElse<\/span>() <\/span>throw<\/span>();<\/span><\/span>\n<\/span>\n#endif<\/span> \/* ExcSpecs_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  ExcSpecs.cpp<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span>"ExcSpecs.h"<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomething<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  \/\/ Does not throw the specified exception<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomethingElse<\/span>() <\/span>throw<\/span>()<\/span> \/\/ Now matches declaration<\/span><\/span>\n{<\/span><\/span>\n  \/\/ Throws an exception not specified<\/span><\/span>\n  <\/span>throw<\/span> <\/span>SomethingBadHappened<\/span>();<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
But it now produces a compile-time warning:<\/p>\n\n\n\n
ExcSpecs.cpp:18:3 'doSomethingElse' has a non-throwing exception specification but can still throw\n<\/strong><\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
So, we fix that, by adding an exception specification to the doSomethingElse<\/strong><\/code> declaration and definition:<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  ExcSpecs.h<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> ExcSpecs_h<\/span><\/span>\n#define<\/span> ExcSpecs_h<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>SomethingBadHappened<\/span> {};<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomething<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>);<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomethingElse<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>);<\/span><\/span>\n<\/span>\n#endif<\/span> \/* ExcSpecs_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  ExcSpecs.cpp<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span>"ExcSpecs.h"<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomething<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  \/\/ Does not throw the specified exception<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomethingElse<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  \/\/ Throws a specified exception<\/span><\/span>\n  <\/span>throw<\/span> <\/span>SomethingBadHappened<\/span>();<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n<\/div>\n\n\n\n
Which, when combined with this main program…<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  main.cpp<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span><iostream><\/span><\/span>\n#include<\/span> <\/span>"ExcSpecs.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  <\/span>try<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>doSomething<\/span>();<\/span><\/span>\n    <\/span>doSomethingElse<\/span>();<\/span><\/span>\n  }<\/span><\/span>\n  <\/span>catch<\/span> (SomethingBadHappened)<\/span><\/span>\n  {<\/span><\/span>\n    <\/span>std<\/span>::cout << <\/span>"Caught SomethingBadHappened exception<\/span>\\n<\/span>"<\/span>;<\/span><\/span>\n  }<\/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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… produces the following:<\/p>\n\n\n\n
Caught SomethingBadHappened exception<\/strong>\nProgram ended with exit code: 0<\/strong><\/code><\/pre>\n\n\n\n
In other words, it works as expected.<\/p>\n<\/div>\n<\/div>\n\n\n\n
But that was with the compiler set to use  C++11. But once we switch to C++17, we get:<\/p>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  ExcSpecs.h<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#ifndef<\/span> ExcSpecs_h<\/span><\/span>\n#define<\/span> ExcSpecs_h<\/span><\/span>\n<\/span>\nclass<\/span> <\/span>SomethingBadHappened<\/span> {};<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomething<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>);<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomethingElse<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>);<\/span><\/span>\n<\/span>\n#endif<\/span> \/* ExcSpecs_h *\/<\/span><\/span><\/code><\/pre><\/div>\n<\/div>\n\n\n\n
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\/\/<\/span><\/span>\n\/\/  ExcSpecs.cpp<\/span><\/span>\n\/\/  Exception Specifications<\/span><\/span>\n\/\/<\/span><\/span>\n\/\/  Created by Bryan Higgs on 10\/5\/24.<\/span><\/span>\n\/\/<\/span><\/span>\n<\/span>\n#include<\/span> <\/span>"ExcSpecs.h"<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomething<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  \/\/ Does not throw the specified exception<\/span><\/span>\n}<\/span><\/span>\n<\/span>\nvoid<\/span> <\/span>doSomethingElse<\/span>() <\/span>throw<\/span> (<\/span>SomethingBadHappened<\/span>)<\/span><\/span>\n{<\/span><\/span>\n  \/\/ Throws a specified exception<\/span><\/span>\n  <\/span>throw<\/span> <\/span>SomethingBadHappened<\/span>();<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
Now it produces errors:<\/p>\n\n\n\n
ExcSpecs.h:13:20 ISO C++17 does not allow dynamic exception specifications\nExcSpecs.h:15:24 ISO C++17 does not allow dynamic exception specifications\n\nExcSpecs.cpp:10:20 ISO C++17 does not allow dynamic exception specifications\nExcSpecs.cpp:15:24 ISO C++17 does not allow dynamic exception specifications<\/strong>\n<\/code><\/pre>\n<\/div>\n<\/div>\n\n\n\n
Why? Because Exception Specifications have been deprecated.<\/strong><\/em><\/p>\n\n\n\n
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Warning:<\/strong><\/p>\n\n\n\n
C++ exception specifications, specifically the throw(...)<\/strong><\/code> syntax, were deprecated in C++11 and completely removed in C++17. Here’s why:<\/p>\n\n\n\n
Problems with Exception Specifications:<\/strong><\/p>\n\n\n\n