CODEWITHSUNDEEP
c++templatesfunction-templates
Ralf
Ralf

Reputation: 1295

Calling member function template of class template

I have the following program (sorry, it is rather complex, but already a boiled-down version from a larger program):

#include <stdio.h>

// two different vector types

template <typename T, int N>
struct Vec1
{
  Vec1() { puts("Vec1()"); }
};

template <typename T, int N>
struct Vec2
{
  Vec2() { puts("Vec2()"); }
};

// a function wrapper

template <typename T, int N>
struct MyFct
{
  template <template <typename, int> class VEC>
  static inline VEC<T,N>
  apply()
  {
    puts("MyFct::apply()");
    return VEC<T,N>();
  }
};

// tester

#if 0
template <typename T, int N, template <typename, int> class FCT>
struct Tester
{
  static inline void test()
  {
    puts("Tester::test");
    Vec1<T,N> v1;
    v1 = FCT<T,N>::apply<Vec1>();
    Vec2<T,N> v2; 
    v2 = FCT<T,N>::apply<Vec2>();
  }
};
#endif

int
main()
{
  MyFct<float,16>::apply<Vec1>();
  MyFct<int,32>::apply<Vec2>();
  // Tester<float,16,MyFct>::test();
  return 0;
}

The program compiles with #if 0 (without Tester), but with #if 1 (with Tester) I get the error message

g++ -Wall -o templatetemplate2a templatetemplate2a.C
templatetemplate2a.C: In static member function 'static void Tester<T, N, FCT>::test()':
templatetemplate2a.C:41:30: error: missing template arguments before '>' token
     v1 = FCT<T,N>::apply<Vec1>();
                              ^
templatetemplate2a.C:41:32: error: expected primary-expression before ')' token
     v1 = FCT<T,N>::apply<Vec1>();
                                ^
templatetemplate2a.C:43:30: error: missing template arguments before '>' token
     v2 = FCT<T,N>::apply<Vec2>();
                              ^
templatetemplate2a.C:43:32: error: expected primary-expression before ')' token
     v2 = FCT<T,N>::apply<Vec2>();

This is surprising since the direct application (see main()) of apply<Vec1>() and apply<Vec2>() works fine. However, if I do the same inside of Tester::test(), I get the error messages. Is this some name-scope issue? Are the template classes Vec1 and Vec2 unknown inside of Tester? How can I make them known? Or is there some other problem?

Upvotes: 1

Views: 310

Answers (1)

songyuanyao
songyuanyao

Reputation: 172914

apply() is a member function template, you need to use the keyword template to tell the compiler that it's a template, when calling it with dependent name. Note the difference between FCT<T,N> and MyFct<float,16>, the former depends on template parameters FCT, T and N, while the later doesn't.

Inside a template definition, template can be used to declare that a dependent name is a template.

e.g.

v1 = FCT<T,N>::template apply<Vec1>();
v2 = FCT<T,N>::template apply<Vec2>();

Upvotes: 3

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