C++ preprocessor token pasting for namespace qualification

问题

I am having trouble with the preprocessor token pasting operator in gcc 4.7.1 (std=c++11). Namely, consider the following code:

// Create a name for a global map (this works)
#define GLOBAL_MAP(name) g_map_ ## name // This works fine

// Now, namespace qualify this map (this fails to compile when used)
#define NS_QUAL_GLOBAL_MAP(name) SomeNamespace:: ## GLOBAL_MAP(name)

Usage scenarios - first the map definitions:

std::map<std::string,std::string> GLOBAL_MAP(my_map);

namespace SomeNamespace
{

std::map<std::string,std::string> GLOBAL_MAP(my_map);

}

Now the usage:

void foo()
{
    bar(GLOBAL_MAP(my_map)); // This compiles fine
    baz(NS_QUAL_GLOBAL_MAP(my_map)); // This fails to compile with:
                                     // error: pasting "::" and "NAME_MAP" does not give a
                                     // valid preprocessing token
}

What I believe might be happening is that it is interpreting GLOBAL_MAP after ## as a token for pasting rather than a macro to be further expanded. How do I get around this?

回答1:

Token pasting generates a single token for the compiler to read. This isn’t what you want here — :: is a valid C++ token on its own, but ::g_map_my_map isn’t a token that the compiler knows.

Hence, remove the token pasting operator:

#define NS_QUAL_GLOBAL_MAP(type) SomeNamespace::GLOBAL_MAP(type)

回答2:

You don't need the ## operator after ::. The ## operator is used to form a single token, but SomeNamespace::g_map_mymap are multiple tokens anyway. Just do

#define NS_QUAL_GLOBAL_MAP(type) SomeNamespace::GLOBAL_MAP(type)

回答3:

You just want SomeNamespace:: GLOBAL_MAP(name).

You can't join a name like g_map_my_map to the :: token, because ::g_map_my_map is not a valid token, it's two tokens. So just put them next to each other, don't try to join them.

来源：https://stackoverflow.com/questions/32252643/c-preprocessor-token-pasting-for-namespace-qualification

标签

c++

c++11

c-preprocessor