Question

I got a strange linker error on the following code:

The code uses type traits to provide a partial template specialization for all types A<T> where T is not a subtype of X.

class X{};

#include <type_traits>

//Enabler for all types that are not a subtype of X
#define enabler(T) typename std::enable_if<!std::is_base_of<X, T>::value>::type


//A template (second param is only for enabling partial specializations)
template <typename T, typename = void>
struct A{};

//Partial template specialization for instances
//that do not use a T which is a subclass of X
template <typename T>
struct A<T, enabler(T)>{
    static int foo(); //Declaration only!
};

//Definition of foo() for the partial specialization
template <typename T,enabler(T)>
static int foo(){
    return 4;
}

int bar = A<int>::foo();

int main(){}

Even though this is only one file, linking fails. The problem seems to be the non-inline definition of foo(). Once I inline it, everything works fine. In the real code, I cannot inline it due to circular dependencies.

The error is the following:

/tmp/ccS7UIez.o: In function `__static_initialization_and_destruction_0(int, int)':
X.cpp:(.text+0x29): undefined reference to `A<int, void>::foo()'
collect2: ld returned 1 exit status

So where is the problem?

Answer 1

The definition of static member function A<T, enabler(T)>::foo has syntax errors, should be :

template <typename T>
int A<T, enabler(T)>::foo(){
    return 4;
}