How can I print the type of an object as part of a static_assert statement?

Question

I have written some type conversion operators which only make sense in the context of a subset of types.

An example is below

explicit virtual operator DataId<float>() const
{
    static_assert(std::is_same_v<T, DataId<float>>, "std::is_same_v<T, DataId<float>>");
    return data; // T data
}

This class contains an object of type T=DataId<U> , where U=float, int, double, std::string .

static_assert seems to demand that the argument passed to it to create the error message is a const char* .

Is there a way to print the type of T in the message?

I tried, but failed, with this attempt:

constexpr auto message(
    (std::string("std::is_same_v<T=") + typeid(T).name() + ", DataId<float>>").c_str()
);
static_assert<..., message>;

Answer 1

the message need to be a string literal, so you cannot do it.

---

Newer gcc/clang already output the types for std::is_same as well. (gcc 11/clang 8)

---

For compilers that doesn't do it and make it harder to get the real types.

A possible workaround is wrap it inside a template and make it a hard error, then compiler would probably tell you the instantiated types.

template<typename T, typename U>
consteval void assert_same(){
    static_assert(
        std::is_same_v<T, U>
    );
}

// usage:
template<typename U>
struct X{
    using T = std::vector<U>;
    void foo(){
        assert_same<T,float>();
    }
};

would result in a report of

In instantiation of 'consteval void assert_same() [with T = XXX; U = XXX]' 'consteval void assert_same() [with T = XXX; U = XXX]' :

...

error: static assertion failed

https://godbolt.org/z/Y7TEhM51f

---

Answer 2

According to https://en.cppreference.com/w/cpp/language/static_assert :

Since message has to be a string literal, it cannot contain dynamic information or even a constant expression that is not a string literal itself. In particular, it cannot contain the name of the template type argument.

So if it's explicitly said that it can't contain the template name, I don't think it can be done.