Syntax: typedef typename F:: template

[RyanInt]

template <class F, class X>
struct twice
{
    typedef typename F::template apply<X>::type once;    // f(x) 
    typedef typename F::template apply<once>::type type; // f(f(x)) 
};

What is F:: template?

[MindSpace]

Its when "apply" is a meta-function of the class defined by F.

We will have a tendency to write F::apply, but the syntax is F::template apply

There is a brief explanation here: http://www.boost.org/doc/libs/1_40_0...her-order.html

[RyanInt]

Metaproduction function? Can you give me an example of code that apply

[MindSpace]

A meta-function is a function that applies to types. For example, you can define the meta-function AddPointer which skips the type int at "int*" like this:

struct AddPointer {
    template<typename T>
    struct apply {
        typedef T* type;
    };
};
int main() {
    typedef AddPointer::apply<int>::type TPInt;
    // And here TPInt is a pointer to int... 
    return 0;
}

Once in your sample before you took your arguments in a meta-function and a type to which you apply your meta-function. So you use it like this:

int main() {
    AddPointer::apply<int>::type TPInt;
    // and here TPInt is a pointer to int... 
    twice<AddPointer, int>::type TDPInt;
    return 0;
}

So the syntax is that when you want to call a structure in a generic type F, you do F:: template apply (apply with your structure is in F). To access IT F:: apply