我如何为std :: string专门化这个类模板？

Question

I am writing a TMP to count the number of elements passed to a struct as template parameters using variadic templates. This is my code:

template<class T, T... t>
struct count;

template<class T, T h, T... t> 
struct count<T, h, t...>{
 static const int value = 1 + count<T, t...>::value;
};

template<class T>
struct count<T>{
 static const int value = 0;
};

template<>
struct count<std::string, std::string h, std::string... l>{
 static const int value = 1 + count<std::string, l...>::value;
};

template<>
struct count<std::string>{
 static const int value = 0;
};

int main(){
 std::cout << count<int, 10,22,33,44,56>::value << '\n';
 std::cout << count<bool, true, false>::value << '\n';
 std::cout << count<std::string, "some">::value << '\n';
 return 0;

}

I get an error on the third instantiation of count with std::string because g++ 4.7 tells me error: 'class std::basic_string<char>' is not a valid type for a template non-type parameter . Any workaround for this?

Answer 1

The problem is not the type std::string but the literal "some" in your call

std::cout << count<std::string, "some">::value << '\n';

Unfortunately, it is not possible to pass a string or floating point literal to a template as is also written in this answer or in that one .

Answer 2

Sorry to disappoint you, but there's no way around this. Non-type template parameters can only be primitive types such as:

• integral or enumeration
• pointer to object or pointer to function
• reference to object or reference to function
• pointer to member

std::string or other types simply don't work there.