C++ PROGRAMMING

# Warning And Error Message Help

 Date Published: January 21, 2014 Last Modified: January 21, 2014

## warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:

• Compilers: Clang
• Default Severity: Warning

This is a form of overload ambiguity. It occurs because a candidate function is only better than another candidate function if NONE of it’s parameters are a worst match than the parameters of the other, and at least one conversion is better.

## error: undefined reference to typeinfo for [classname]

• Compilers: Clang
• Default Severity: Error

This can come about if you have run-time info disabled via the -fno-rtti GCC compiler option.

This can also come about from forgetting to declare non-pure virtual functions, as the following snippet from GCC suggests.

If the class declares any non-inline, non-pure virtual functions, the first one is chosen as the “key method” for the class, and the vtable is only emitted in the translation unit where the key method is defined.

## no out-of-line virtual method definitions

MyClass has no out-of-line virtual method definitions; its vtable will be emitted in every translation unit

• Compilers: Clang
• Default Severity: Warning

This warning is emitted by Clang whenever it can’t find any virtual methods for a class outside of the class declaration. This condition typically arises when writing header-only classes. If you are writing a header-only class, you can safely ignore this warning, understanding that more code space will be consumed due to vtable duplication (which should be a big issue in most situations).

To silence this warning, you can add:

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wweak-vtables"

// Your code here

#pragma clang diagnostic pop


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