c – 在模板类A中访问X和Y,如模板类> A类;

Foo< Param > tt;

在这里,您可以看到没有为Param提供任何值.您将获取模板模板参数,以便Foo本身可以使用它喜欢的任何参数来实例化Params.

例：

template < template < int,int > class X1>
struct Foo {

    X1<1,2> member;

    X1<42,100> foo();
};

template <int N,int P> struct A {};

template <int X,int Y> struct B {};

Foo<A> a_foo;  //has a member of type A<1,2>,foo returns A<42,100>
Foo<B> b_foo; //has a member of type B<1,foo returns B<42,100>

但是如果你想让你的Foo输出那些整数,它必须采用真实的类型,而不是模板.其次,模板参数(X和Y)的名称仅在它们在范围内时才有意义.否则它们完全是任意标识符.您可以使用简单的元编程来检索值：

#include <cstdio>

template <class T>
struct GetArguments;

//partial specialization to retrieve the int parameters of a T<int,int>
template <template <int,int> class T,int A,int B>
struct GetArguments<T<A,B> >
{
   enum {a = A,b = B};
};
//this specialization also illustrates another use of template template parameters:
//it is used to pick out types that are templates with two int arguments

template <class X1>
struct Foo {
  int foo() {
    printf("ok%d %dn",GetArguments<X1>::a,GetArguments<X1>::b);
    return 0;
  }
};

template < int X,int Y >
class Param {
public:
   void print();
};

//this is to illustrate X and Y are not essential part of the Param template
//in this method definition I have chosen to call them something else
template <int First,int Second> 
void Param<First,Second>::print()
{
   printf("Param<%d,%d>n",First,Second);
}

int main() {

    Foo< Param<10,20> > tt;
    tt.foo();
    Param<10,20> p;
    p.print();
    return 0;
}