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Compiler Warning (level 4) C4843

  • Grein

'type1': An exception handler of reference to array or function type is unreachable, use 'type2' instead

Remarks

Handlers of reference to array or function type are never a match for any exception object. Starting in Visual Studio 2017 version 15.5, the compiler honors this rule and raises a level 4 warning. It also no longer matches a handler of char* or wchar_t* to a string literal when /Zc:strictStrings is used.

This warning is new in Visual Studio 2017 version 15.5. For information on how to disable warnings by compiler version, see Compiler warnings by compiler version.

Example

This sample shows several catch statements that cause C4843:

// C4843_warning.cpp
// compile by using: cl /EHsc /W4 C4843_warning.cpp
int main()
{
    try {
        throw "";
    }
    catch (int (&)[1]) {} // C4843 (This should always be dead code.)
    catch (void (&)()) {} // C4843 (This should always be dead code.)
    catch (char*) {} // This should not be a match under /Zc:strictStrings
}

The compiler generates these warnings:

warning C4843: 'int (&)[1]': An exception handler of reference to array or function type is unreachable, use 'int*' instead
warning C4843: 'void (__cdecl &)(void)': An exception handler of reference to array or function type is unreachable, use 'void (__cdecl*)(void)' instead

The following code avoids the error:

// C4843_fixed.cpp
// compile by using: cl /EHsc /W4 C4843_fixed.cpp
int main()
{
    try {
        throw "";
    }
    catch (int (*)[1]) {}
}