类模板专门化时的前向声明

Question

Is the following forward declaration, at the class template specialisation stage, legal C++ code?

template<typename>
struct Basic
{};

template<>
struct Basic<struct Foo> //<-- Fwd declaration?
{};

struct Foo
{
    Basic<Foo> m_a;
};

int main()
{
    Foo test;
}

It does compile , but I'm not sure if it's legal

Answer 1

template<>
struct Basic<struct Foo>
{};

is legal.

From C++11 Standard :

A class declaration introduces the class name into the scope where it is declared and hides any class, variable, function, or other declaration of that name in an enclosing scope.

Answer 2

Yes, it is legal C++ and it does forward declare struct Foo .

You can also do this in function declarations:

void fun(struct foo);

struct foo {};

void fun(struct foo) {}

or with pointers.

struct foo* pointer;
struct foo {};

Basically anywhere where a complete type is not required. (Works with class as well.)