Diagnostic directives

 

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Preprocessor

#if#ifdef#ifndef#else#elif#elifdef#elifndef#endif (C++23)(C++23)

#define#undef#,## operators

#include__has_include (C++17)

#error#warning (C++23)

#pragma_Pragma (C++11)

#line

 

Shows the given error message and renders the program ill-formed, or given warning message without affect 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 that 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

Run this code

#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";
}

Possible output:

The implementation used is hosted

References

• C++23 standard (ISO/IEC 14882:2024):

• 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):

• 16.5 Error directive [cpp.error]

See also

C documentation for Diagnostic directives