gcc和clang预处理器不了解L前缀

Question

Assume this code:

enum class Foo
{
    One,
    Two,
    Three,
    Four,
    Five
};

#define HANDLE_FOO(f) case Foo::f: std::wcout << L#f << std::endl;
Foo x = Foo::Five;
switch (x)
{
    HANDLE_FOO(One);
    HANDLE_FOO(Two);
    HANDLE_FOO(Three);
    HANDLE_FOO(Four);
    HANDLE_FOO(Five);
}    

msvc compiles it just right, but gcc gives this error: "'L' was not declared in this scope"

So, the question is: how can I workaround this thing? Not only 'L' prefix doesn't work, but 'u' and so on doesn't work neither.

This code compiles just fine, though:

std::wstring str = L"This is a string";

Thank you in advance.

Answer 1

I believe that construct is invalid, and MSVC should diagnose it.

The # operator "stringizes" its argument, so One becomes "One" -- but by preceding it with L you have two separate tokens, L and "One" .

What you want is a single token, the string literal L"One" . You need to use the ## token-pasting operator as well as the # stringizing operator:

#define HANDLE_FOO(f) case Foo::f: std::wcout << L ## #f << std::endl;

Experiment shows that this works with both g++ 5.3.0 and clang++ 3.6.

Answer 2

Checked with -E option of GCC, a space was produced between L prefix and the string in your code and I guessed the space is harmful.

This code which uses ## operator to concatenate the tokens worked:

#include <iostream>
enum class Foo
{
    One,
    Two,
    Three,
    Four,
    Five
};

#define HANDLE_FOO(f) case Foo::f: std::wcout << L## #f << std::endl;
int main(){
    Foo x = Foo::Five;
    switch (x)
    {
        HANDLE_FOO(One);
        HANDLE_FOO(Two);
        HANDLE_FOO(Three);
        HANDLE_FOO(Four);
        HANDLE_FOO(Five);
    }
}