在带有类型后卫的模板文件中使用前向声明

Question

I'm trying to combine forward declarations in a template library, with type guards to intercept cases of the wrong type of template being used with the wrong type of class. Here is the simplest snippet of code I can get down to, in order to illustrate the problem...

Main.cpp

#include <iostream>
#include "templates.h"

int main()
{
    B<int_array> hello;
    return 0;
}

templates.h

#include <type_traits>

#define USE_GUARD
#define FORWARD_DECLARE

template<typename T>
using is_pod = typename std::enable_if<std::is_pod<T>::value>::type;

typedef struct {
    int *p; 
    int n;   
} int_array;

template <typename E, typename T >
class A
{

};

#ifdef USE_GUARD
template <typename T, typename = is_pod<typename std::remove_pointer<decltype(T::p)>::type> >
#else
template <typename T>
#endif
class B : public T, public A< B<T>, T >
{                                     // This is the 'curiously recurring template
                                      // pattern' (CRTP)
public:
    //! Default constructor
    B() noexcept
    {
        T::p = nullptr;
        T::n = 0;
    }

#ifdef FORWARD_DECLARE
    void reorder(const int_array & newOrder);
#endif
};

template <typename T> class C : public B<T>
{
public:
    C() noexcept : B<T>()
    {
        T::p = nullptr;
        T::n = 0;
    }
};

#ifdef FORWARD_DECLARE
#ifdef USE_GUARD
template <typename T, typename = is_pod<typename std::remove_pointer<decltype(T::p)>::type> >
#else
template <typename T>
#endif
void B<T>::reorder(const int_array & newOrder)
{
    C<T> myC;
}
#endif

I get error messages as follows:

Visual Studio 2015:

Neither USE_GUARD nor FORWARD_DECLARE defined - works fine

Only FORWARD_DECLARE defined - works fine

Only USE_GUARD defined - works fine

Both defined - C3860: template argument lost following class template name must list parameters in the order used in template parameter list

g++:

Neither USE_GUARD nor FORWARD_DECLARE defined - works fine

Only FORWARD_DECLARE defined - works fine

Only USE_GUARD defined - works fine

Both defined - Invalid use of incomplete type 'class B<T> at the point where the function 'reorder' is defined

So there's obviously something that I'm doing wrong in the way that I combine the type guard with the forward declaration, but neither compiler is giving me a very intelligible error message. Am I making an elementary error in my syntax, or am I just trying to do something fundamentally silly?

Answer 1

Default argument should not be repeated in member definition, and all arguments should be provided:

#ifdef FORWARD_DECLARE
# ifdef USE_GUARD
template <typename T, typename U>
void B<T, U>::reorder(const int_array & newOrder)
# else
template <typename T>
void B<T>::reorder(const int_array & newOrder)
# endif
{
    C<T> myC;
}
#endif

Demo