I'm struggling to get the below code to compile. I want to enable foo function for class A only when N=3.
#include <iostream>
template <size_t N>
class A
{
public:
template <size_t n = N, std::enable_if_t<(n == 3)>* = nullptr> int foo(int a);
};
template<size_t N>
template <size_t n = N, std::enable_if_t<(n == 3)>* = nullptr>
int A<N>::foo(int a)
{
return a * 4;
}
int main()
{
A<3> a1;
std::cout << a1.foo(10) << std::endl;
// the below should fail to compile
// A<4> a2;
// std::cout << a2.foo(7) << std::endl;
}
Output
<source>:12:20: error: default argument for template parameter for class enclosing 'int A<N>::foo(int)'
12 | int A<N>::foo(int a)
| ^
CodePudding user response：
Whenever you separate a function's declaration and definition, whether it is a template function or not, default argument values can only be in the declaration, not in the definition. So, simply remove the default values from foo's definition, eg:
#include <iostream>
template <size_t N>
class A
{
public:
template <size_t n = N, std::enable_if_t<(n == 3)>* = nullptr> int foo(int a);
};
template<size_t N>
template <size_t n, std::enable_if_t<(n == 3)>*>
int A<N>::foo(int a)
{
return a * 4;
}