How can I force a compile error in C++?

Question

I would like to create a compile-time error in my C++ code with a custom error message. I want to do this for a couple of reasons:

• to force compilation to fail while I'm working on new features which haven't been implemented yet. (compile time ! TODO reminder)
• to create a more readable error when attempting to implement an unsupported template specialization.

I'm sure there is a trick to doing this but I cant find a resource explaining the method. I would wrap the code in a #define of the form COMPILE_FAIL("error message");

Thanks D

Answer 1

Use #error:

#error "YOUR MESSAGE"

This produces an error from the preprocessor. If you want to detect an error at a later stage (e.g. during template processing), use static_assert (a C++11 feature).

Answer 2

Look into static_assert.

Example:

#include <iostream>
#include <type_traits>

template<typename T>
class matrix {
    static_assert(std::is_integral<T>::value, "Can only be integral type");
};

int main() {
    matrix<int*> v; //error: static assertion failed: Can only be integral type
}