如何将模板 class 的内部 class 分离到其他文件中

Question

I want to do:

typedef MyTemplateClass<Type01, Type02> TClass;
TClass t;
TClass::InnerClass i;
i.test();

i think the solution could be:

template <typename A, typename B>
class MyTemplateClass
{
public:
    class InnerClass
    {
        //here I can do stuff with A and B
        A test() { return 0; }
    };

    friend class InnerClass;
};

but I want to have ma templates in separate *.inl file included at the bottom of header file

how to define such behaviour in other file?

when i do just

//file.inl
class InnerClass
{
    //here I can do stuff with A and B
    A test() { return 0; }
};

A and B are undefined.

but

template <typename A, typename B>
class InnerClass
{
...
};

makes my method template independent of MyTemplateClass types...

Once again in one sentence: How to make InnerClass of MyTemplateClass in other file with providing

TClass::InnerClass i;
i.test();

behaviour?

Answer 1

It works the same as when using a non-template: you have to fully qualify the name of the member:

struct outer {
    struct nested;
};

struct outer::nested {
    int
    test();
};

int
outer::nested::test()
{ /* whatever */ }

In your case that'd be:

template <typename A, typename B>
class MyTemplateClass {
public:
    class InnerClass;

    friend class InnerClass;
};

// note the use of the <> brackets
template<typename A, typename B>
class MyTemplateClass<A, B>::InnerClass {
public:
    A
    test();
};

// ditto
template<typename A, typename B>
A
MyTemplateClass<A, B>::InnerClass::test()
{ return 0; }

You do have to be wary of the definition order though: if eg MyTemplateClass has function members that use InnerClass , then the definition of InnerClass has to be visible at the point of use.

Simple rule of thumb: inline everything or nothing. Either you define everything inline (the InnerClass class definition and the function members of both classes) inside MyTemplateClass , or you put all the definitions of all function members (of both MyTemplateClass and InnerClass ) at the very end after InnerClass has been defined.

Don't fret though, if you mess up your compiler will only be too happy to help you with error messages.

Answer 2

You can't. In a nutshell. Templates must be wholly defined before instantiation- that means that whatever you do, you'll have to define the inner class in the template class anyway.