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Diagnostic directives

Shows the given error message and renders the program ill-formed, or shows the given warning message without affecting the validity of the program (since C++23).

Syntax

`#error` diagnostic-message	(1)
`#warning` diagnostic-message	(2)	(since C++23)

Explanation

1) After encountering the #error directive, an implementation displays the message diagnostic-message and renders the program ill-formed (the compilation stops).

2) Same as (1), except the validity of the program is not affected and the compilation continues.

diagnostic-message can consist of several words not necessarily in quotes.

Notes

Before its standardization in C++23, #warning has been provided by many compilers in all modes as a conforming extension.

Example

#if __STDC_HOSTED__ != 1
#   error "Not a hosted implementation"
#endif
 
#if __cplusplus >= 202302L
#   warning "Using #warning as a standard feature"
#endif
 
#include <iostream>
 
int main()
{
    std::cout << "The implementation used is hosted\n";
}

Possible output:

The implementation used is hosted

References

C++23 standard (ISO/IEC 14882:2023):

15.8 Error directive [cpp.error]

C++20 standard (ISO/IEC 14882:2020):

15.8 Error directive [cpp.error]

C++17 standard (ISO/IEC 14882:2017):

19.5 Error directive [cpp.error]

C++14 standard (ISO/IEC 14882:2014):

16.5 Error directive [cpp.error]

C++11 standard (ISO/IEC 14882:2011):

16.5 Error directive [cpp.error]

C++03 standard (ISO/IEC 14882:2003):

16.5 Error directive [cpp.error]

C++98 standard (ISO/IEC 14882:1998):