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Diagnostic directives

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Preprocessor

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) .

Contents

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

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\n";
}

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