c++ - Can a compilation error be forced if a string argument is not a string literal?

Question

Let's say I have these two overloads:

void Log(const wchar_t* message)
{
    // Do something
}

void Log(const std::wstring& message)
{
    // Do something
}

Can I then in the first function add some compile-time verifiction that the passed argument is a string literal?

EDIT: A clarification on why this would be good in my case; my current high-frequency logging uses only string literals and can hence be optimized a lot when there are non-heap allocation guarantees. The second overload doesn't exist today, but I might want to add it, but then I want to keep the first one for extreme scenarios. :)

Answer 1

So this grew out of Keith Thompson's answer... As far as I know, you can't restrict string literals to only normal functions, but you can do it to macro functions (through a trick).

#include <iostream>
#define LOG(arg) Log(L"" arg)

void Log(const wchar_t *message) {
    std::wcout << "Log: " << message << "
";
}

int main() {
    const wchar_t *s = L"Not this message";
    LOG(L"hello world");  // works
    LOG(s);               // terrible looking compiler error
}

Basically, a compiler will convert "abc" "def" to look exactly like "abcdef". And likewise, it will convert "" "abc" to "abc". You can use this to your benefit in this case.

---

I also saw this comment on the C++ Lounge, and that gave me another idea of how to do this, which gives a cleaner error message:

#define LOG(arg) do { static_assert(true, arg); Log(arg); } while (false)

Here, we use the fact that static_assert requires a string literal as it's second argument. The error that we get if we pass a variable instead is quite nice as well:

foo.cc:12:9: error: expected string literal
    LOG(s);
        ^
foo.cc:3:43: note: expanded from macro 'LOG'
#define LOG(arg) do { static_assert(true, arg); Log(arg); } while (false)