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c++templatestype-parameter
Ferenc Dajka
Ferenc Dajka

Reputation: 1051

C++ template member function already defined with same type

I have this simple code below, a template with 2 type parameters. If I declare my class with the same type (like BidirectionalMap<int,int>), I receive an error: 

int BidirectionalMap<T,S>::operator [](T) const' : member function already defined or declared

Here's my template code:

template <class T, class S>

class BidirectionalMap{
    int count(T t){
        return 1;
    }
    int count(S s){
        return 1;
    }
};

Upvotes: 2

Views: 1327

Answers (1)

Jarod42
Jarod42

Reputation: 217438

The error you got is normal, because after substitution you have

template <>
class BidirectionalMap<int, int>
{
    int count(int t){ return 1; }
    int count(int s){ return 1; } // Duplicated method
};

To solve that, you may provide partial specialization:

template <class T>
class BidirectionalMap<T, T>
{
    int count(T t) { return 1; }
};

In C++20, you might use requires to "discard" methods:

template <class T, class S>
class BidirectionalMap
{
    int count(T t) requires(!std::is_same<T, S>::value) { /*..*/ }
    int count(S s) requires(!std::is_same<T, S>::value) { /*..*/ }
    int count(T t) requires( std::is_same<T, S>::value) { /*..*/ }
};

Upvotes: 6

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