为什么需要在此C ++模板中指定类型？

Question

I have this minimal, contrived example of C++ code, with a template struct with a default type parameter:

#include <iostream>
using namespace std;

template <class T=int>
struct AddsFourtyTwo {
    template <class U>
    static U do_it(U u) {
        return u + static_cast<T>(42);
    }
};

int main() {
    double d = 1.24;
    std::cout << AddsFourtyTwo::do_it(d) << std::endl;
}

When I try to compile this code, I get the following error with g++ 4.9.1:

$ g++ test.cpp
test.cpp: In function 'int main()':
test.cpp:14:18: error: 'template<class T> struct AddsFourtyTwo' used without template parameters
     std::cout << AddsFourtyTwo::do_it(d) << std::endl;
                  ^

If I specify int for T, then it compiles and produces the expected output (43.24). My question is, why is this necessary to do? What does the default type parameter do in the definition of AddsFourtyTwo, if you need to specify the type anyway?

Answer 1

您不需要指定类型，但语言不允许使用模板作为实际类型而不指定某个参数列表：

std::cout << AddsFourtyTwo<>::do_it(d) << std::endl;

Answer 2

Using the template class requires the following syntax:

std::cout << AddsFourtyTwo<>::do_it(d) << std::endl;

will compile and use the default parameter. That's confusing since the same is not required for templated methods.