Why can't I have template and default arguments?

问题

I changed a paremeter in a function to accept any kind of object using a template but I can't use it in conjunction with other default parameters, is there something I am missing?

#include <string>
#include <iostream>

class MyClass {
    public:
    std::wstring msg = L"hey";
    MyClass(){};
};

class MyClass2{
    public:
    template<class T> MyClass2(T* t, int i);
};
template<class T>
MyClass2::MyClass2(T* t,int i=0){ std::wcout << t->msg << std::endl; }

int main(int argc, char **argv)
{
    MyClass mc;
    MyClass2 mc2(&mc);
    return 0;
}

Output:

practice.cpp:16:32: error: redeclaration of 'MyClass2::MyClass2(T*, int)' may not have default arguments [-fpermissive]

I thought it was reasonable to not use default values in the template but is there a reason for the other parameters?

回答1:

You certainly can; put the default argument on the declaration, not the definition.

Putting the default in the definition's argument list instead of the declaration's is an added extra that is not available for function templates:

[C++14: 8.3.6/4]: For non-template functions, default arguments can be added in later declarations of a function in the same scope. [..]

I don't really know why this restriction is in place.

Similar rule:

[C++14: 8.3.6/6]: Except for member functions of class templates, the default arguments in a member function definition that appears outside of the class definition are added to the set of default arguments provided by the member function declaration in the class definition [..]

来源：https://stackoverflow.com/questions/32105975/why-cant-i-have-template-and-default-arguments