I am trying to generalize a function for a game engine I am writing to simplify the shader loading process.
Anyway, the difficulty arises in my attempts to templating the function. I attempted to adapt the solution found in this stack thread, and resulted in various versions of the following code without any ideal solutions:
template<typename T, typename U, U T::* x>
U testFunc(T& t)
{
// Simplification of the extremely generalized function;
// I want to return/utilize member variable x of input t
return x;
}
struct testS1 { int a = 100; };
struct testS2 { bool b = true; };
testS1 tS1;
testS2 tS2;
// would ideally return 100
int c = testFunc<testS1, int, &testS1::a>(tS1);
// would ideally return true
bool d = testFunc<testS2, bool, &testS2::b>(tS2);
Running the program gives the following error:
Severity: Error
Line: 46
Code: C2440
Description: 'return': cannot convert from 'int testS1::* ' to 'U'
I understand that the returned value x is not the same type as U because int testS1::* is not the same type as int.
However, I do not understand what syntax I would use to return the member variable x of type U from the struct T.
The real structs behind the placeholders testS1 and testS2 are very different from each other, so I would like to avoid using a base class/struct if at all possible.
The other answer has already provided the syntax for accessing the pointer to the member data. In c++20 using abbreviated function template, you could do it, less verbosely the same
template<auto x>
constexpr auto testFunc(auto const& t)
{
return t.*x;
}
Now you call with less explicate template arguments
int c = testFunc<&testS1::a>(tS1);
bool d = testFunc<&testS2::b>(tS2);
// ...