CODEWITHSUNDEEP
c++templatesoperator-overloading
George Skelton
George Skelton

Reputation: 1125

Unresolved external symbol error overloading operator+ template

I am trying to overload the '+' operator for a class template but getting unresolved external symbol error calling it using the infix notation:

// In main.cpp

template<class T>
struct B
{
    B(T t) : t_(t) {}
    friend B operator+(B const &lhs, B const &rhs);
    T t_;   
};

template<class T>
B<T> operator+(B<T> const &lhs, B<T> const &rhs)
{
    return B<T>(lhs.t_ + rhs.t_);
}

int main()
{
    B<int> b = 1;
    b = operator+<int>(b, 2); // works but clunky syntax
    // b = b + 2; // LNK2019: unresolved external symbol
}

It works fine for regular non-templated classes so wondering if it is possible to achieve the same thing here.

I'm using Visual C++ 2015.

Upvotes: 3

Views: 931

Answers (2)

Jarod42
Jarod42

Reputation: 217235

friend B operator+(B const &lhs, B const &rhs);

is a non template function.

The simpler would be to define it inline:

template<class T>
struct B
{
    B(T t) : t_(t) {}
    friend B operator+(B const &lhs, B const &rhs)
    {
       return B(lhs.t_ + rhs.t_);
    }
    T t_;   
};

Demo

Else you have to declare all template friend

template <typename T> struct B;

template <typename T> B<T> operator+(B<T> const &lhs, B<T> const &rhs);

template<class T>
struct B
{
    B(T t) : t_(t) {}

    template <typename U>
    friend B<U> operator+(B<U> const &lhs, B<U> const &rhs);
    T t_;   
};

Demo

or just the specific one

template <typename T> struct B;

template <typename T> B<T> operator+(B<T> const &lhs, B<T> const &rhs);

template<class T>
struct B
{
    B(T t) : t_(t) {}
    friend B operator+<>(B const &lhs, B const &rhs);
    // Note the <>

    T t_;   
};

template<class T>
B<T> operator+(B<T> const &lhs, B<T> const &rhs)
{
    return B<T>(lhs.t_ + rhs.t_);
}

Demo

Upvotes: 4

Mattia F.
Mattia F.

Reputation: 1740

You have not defined it as a template inside the class. The simplest solution is defining the function inside the class (inline):

template<class T>
struct B
{
    B (T t) : t_ (t) {}
    friend B operator+ (const B &lhs, const B &rhs)
    {
       return B (lhs.t_ + rhs.t_);
    }
    T t_;   
};

Upvotes: 0

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