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      Diagnostic directives

      From cppreference.com
      <cpp‎ |preprocessor
       
       
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      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

      [edit]Syntax

      #errordiagnostic-message (1)
      #warningdiagnostic-message (2)(since C++23)

      [edit]Explanation

      1) After encountering the#error directive, an implementation displays the messagediagnostic-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.

      [edit]Notes

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

      [edit]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

      [edit]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]

      [edit]See also

      C documentation forDiagnostic directives
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