Question

Is it possible to write a template function for which a particular type of classes have some functions or overloaded operators? For instance

template <typename T>
void dosomething(const T& x){
    std::cout << x[0] << std::endl;
}

In this context I’m assuming that x is a class that behaves like an array, that is, I have overloaded the [] operator as well as the << so that it can work with std::cout.

The actual code that I have is slightly different but gcc is giving me

error: subscripted value is neither array nor pointer

This must be because it doesn’t know that I’m expecting T to be of some class that overloads the [] operator. Does anyone know if it is possible to overcome this? I’d like to let c++ know that the particular type T will have the [] overloaded.

Answer 1

You might need to provide a little more detail, as this short example works for me:

#include <iostream>
#include <vector>

template <typename T>
void dosomething(const T& x){
    std::cout << x << std::endl;
}

template <typename T>
void dosomething_else(const T& x){
    std::cout << x[0] << std::endl;
}

int main() {
    dosomething< int >(1) ;
    std::vector< int > vec( 3, 1 );
    //dosomething< std::vector< int > >(vec);
    dosomething_else< std::vector< int > >(vec);
}

However, if you were to uncomment this line you would get a compiler error as std::vector doesn’t implement the << operator:

//dosomething< std::vector< int > >(vec);

When you say this your thinking is on the right track:

I’d like to let c++ know that the particular type T will have the []
overloaded.

However, the C++ compiler will actually search for [] operator at compile-time for any functions that request it. If there is no [] operator defined, you will get a compiler error. For example, this will cause a compiler error if inserted into the main() function:

dosomething_else< int >(1);

You get this error message, similar to what you suggest in the question:

test.cpp: In function 'void dosomething_else(const T&) [with T = int]':
test.cpp:19:   instantiated from here
test.cpp:11: error: subscripted value is neither array nor pointer

You can actually check if the [] exists at compile-time using the method outlined in this question:
How to check whether operator== exists?