Check template type in struct - c++

Question

I want to define additional variables in a struct based on the type in template, something like this:
template<typename CONFIG>
struct Test
{
int a;
int b;
if (std::is_same<CONFIG, MyClass>::value)
{
// additional variables if the CONFIG type is MyClass
int c;
int d;
}
// functions
void func()
{
a = 0;
b = 0;
if (std::is_same<CONFIG, MyClass>::value)
{
c = 0;
d = 0;
}
}
}
How should I do that? Thanks!

Answer 1

This is a use case for template specialization. You have a primary template like
template<typename CONFIG>
struct Test
{
int a;
int b;
// functions
void func()
{
a = 0;
b = 0;
}
};
and then a specialization for MyClass like
template <>
struct Test<MyClass>
{
int a;
int b;
int c;
int d;
// functions
void func()
{
a = 0;
b = 0;
c = 0;
d = 0;
}
};

Answer 2

A variant that follows your pseudo-code a bit more (not saying that it is a better approach than the other answer):
template<typename CONFIG>
struct TestAdditionalMembers {};
template<>
struct TestAdditionalMembers<MyClass>
{
int c;
int d;
};
template<typename CONFIG>
struct Test : TestAdditionalMembers<CONFIG>
{
int a;
int b;
// functions
void func()
{
a = 0;
b = 0;
if constexpr(std::is_same<CONFIG, MyClass>::value)
{
this->c = 0;
this->d = 0;
}
}
};
The additional members are provided through inheritance and explicit specialization of the class template. Since c and d are not dependent names, but only exist for certain template parameters, you will need to refer to them with this->c, etc. (this is always a dependent name).
Inside a function the condition needs to be checked by a if constexpr, since at-runtime check would be too late. (The code inside the if must compile even if the condition is always false.)

Answer 3

Based on walnuts answer, you could collect all your conditional actions and member variables in a specialization that you inherit if you want to avoid type traits and constexpr-if's.
template<typename T>
struct completions {
void func_completion() {} // does nothing
};
template<>
struct completions<MyClass> {
int c;
int d;
void func_completion() {
c = 0;
d = 0;
}
};
template<typename CONFIG>
struct Test : completions<CONFIG> {
int a;
int b;
// functions
void func() {
a = 0;
b = 0;
this->func_completion();
}
};