# Diagnostic directives

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< cpp‎ | 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).

### 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

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