c++ Template class member struct initialization syntactic sugar

Question

Given a class:

template <typename T>
class foo
{
public:
    struct bar
    {
        bar(T value) { m_foo = value; }
        T m_foo;
        ...
    }
    ...
}

and an instance:

foo<int> myfoo;

is it possible to create an instance of bar without having to specify myfoo 's types again like this:

myfoo::bar mybar;
// foo<int>::bar mybar;

when I try this I get an error-pair (VS 2015) :

'myfoo': is not a class or namespace name

'bar': identifier not found

UPDATE here is a more detailed example of what I want to achieve:

template <typename T, typename T2, typename T3>
class MyClass
{
public:
    struct MyClassCreateParameters
    {
        MyClassCreateParameters( T t, T2 t2, T3 t3 )
            :
            T1Param( t ), T2Param( t2 ), T3Param( t3 ) { }

        T T1Param;
        T2 T2Param;
        T3 T3Param;
    };

    MyClass( const MyClassCreateParameters& params )
        :
        m_t1( params.T1Param ),
        m_t2( params.T2Param ),
        m_t3( params.T3Param ) { }

private:
    const T m_t1;
    const T2 m_t2;
    const T3 m_t3;
};

int main()
{
    MyClass< int, char, char* >* myclass;
    myclass = new MyClass< int, char, char* >(
        myclass::MyClassCreateParameters( 1, 'A', "abc" ) );

    return 0;
}

Answer 1

使用C ++ 11，您应该能够：

decltype(myfoo)::bar mybar;

Answer 2

On the face of it, I would use a typedef:

template <typename T1, typename T2, typename T3>
class MyClassTemplate
{
public:
    struct Params { /* ... */ };
    MyClassTemplate(Params);

    // ...
};

using MyClass = MyClassTemplate<int, char, char*>;

MyClass x(MyClass::Params(a, b, c));

However, you could also endow your class template with a forwarding constructor template:

template <typename T1, typename T2, typename T3>
class MyClassTemplate
{
private:
    struct Params { /* ... */ };
    MyClassTemplate(Params);

public:
    template <typename ...Args>
    MyClassTemplate(Args ...args) : MyClassTemplate(Params(args...)) {}
    // ...
};

MyClassTemplate<int, char, char*> x(a, b, c);