C ++ macro only works with file preprocessing

Question

I am trying to create a matching mechanism for the new std::optional<T>in C ++. I wrote the following macro:

#define EXPAND(x) x
#define CAT_(x, y) x##y
#define CAT(x, y) CAT_(EXPAND(x), EXPAND(y))
#define if_opt__(xalt, bval, x, y)  \
auto xalt = y;                      \
bool bval = true;                   \
if (xalt.has_value())               \
for (auto x = xalt.value(); bval; bval = false)
#define if_opt_(xalt, x, y) if_opt__(xalt, CAT(xalt, _b), x, y)
#define if_opt(x, y) if_opt_(CAT(x, __LINE__), x, y)

And I created the following sample program for it:

std::optional<int> get(int a) {
    if (a < 0) {
        return {};
    }
    return a;
}

int main(void) {
    if_opt(a, get(0)) {
        std::cout << "optional matched!" << std::endl;
    }
    return 0;
}

However, when I try to compile a program, I get errors such as' redefinition; different base types. "Then I pre-processed it into a file, copied the result and compiled it and worked fine. Macro rated for:

auto a24 = get(0);
bool a24_b = true;
if (a24.has_value()) 
    for (auto a = a24.value(); a24_b; a24_b = false) {
        std::cout << "optional matched!" << std::endl;
    }

Why doesn't it compile using the macro itself? I am using MSVC.

Answer 1

How your macro CATperforms an additional extension should be fixed:

//#define EXPAND(x) x // not needed
#define CAT_(x, y) x##y
#define CAT(x, y) CAT_(x, y)

- , EXPAND(x)##EXPAND(y), , ## EXPAND.

? , VS ( ) . Gcc .