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.  (od C++23)

Syntax

1	#error		diagnostic-message
2	#warning		diagnostic-message	(od 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";
}

Result

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):
  • 16.5 Error directive [cpp.error]