add function member to a template class conditionally [duplicate] - c++

Question

This question already has answers here:
How to conditionally add a function to a class template?
(5 answers)
Closed 2 years ago.
I have a class template defined as follow
template<typename T>
class A
{
T t_;
// void f();
};
My question is how to add the f() method only if the type T is integer without compilation error.
int main()
{
A<int> a; // OK
A<string> b; // OK
}
Example :
#include <type_traits>
#include <new>
#include <iostream>
#include <string>
template <typename T>
struct Foo
{
T t;
template <typename..., typename U = T>
std::enable_if_t<std::is_same_v<T, int>> say_hello() { std::cout << "Hello"; }
};
int main()
{
Foo<int>();
Foo<double>();
}
Error C2938 'std::enable_if_t<false,void>' : Failed to specialize alias template
Thank you.

Answer 1

You can enable specific functions using type_traits and SFINAE:
#include <iostream>
#include <string>
#include <type_traits>
template<typename T>
class A
{
public:
template<typename U = T, std::enable_if_t<std::is_integral_v<U>, int> = 0>
void f() {
std::cout << "int " << t_ << '\n';
}
private:
T t_;
};
int main() {
A<int> a;
a.f();
A<std::string> s;
// s.f(); // error: no member named 'f' in 'A<std::__cxx11::basic_string<char> >'
}
If you have many integer specific functions you can put them in a class of its own and inherit that class only if T is integral.
#include <iostream>
#include <string>
#include <type_traits>
template<typename Atype>
class int_functions {
public:
Atype* This() { return static_cast<Atype*>(this); }
void f() {
std::cout << "int " << This()->t_ << '\n';
}
};
template<typename Atype>
class non_int_functions {
};
template<typename T>
class A : public std::conditional_t<std::is_integral_v<T>, int_functions<A<T>>,
non_int_functions<A<T>>>
{
friend std::conditional_t<std::is_integral_v<T>, int_functions<A<T>>,
non_int_functions<A<T>>>;
public:
private:
T t_;
};