C++ template functions

Question

This Vec template supports several functions such as multiplying a vector by scalar and adding vector to another vector.

The thing that is confusing me is that why the overloading of the second operator* is outside of the class template? The operator* which is declared in the class overloads vectorXscalar The one declared outside supports scalarXvector

template <class T>
class Vec {
public:
    typename list<T>::const_iterator begin() const {
        return vals_.begin();
    }
    typename list<T>::const_iterator end() const {
        return vals_.end();
    }
    Vec() {};

    Vec(const T& el);
    void push_back(T el);
    unsigned int size() const;
    Vec operator+(const Vec& rhs) const;
    Vec operator*(const T& rhs) const; //here
    T& operator[](unsigned int ind);
    const T& operator[](unsigned int ind) const;
    Vec operator,(const Vec& rhs) const;
    Vec operator[](const Vec<unsigned int>& ind) const;

    template <class Compare>
    void sort(Compare comp) {
        vals_.sort(comp);
    }

protected:
    list<T> vals_; 
};

template <class T>
Vec<T> operator*(const T& lhs, const Vec<T>& rhs); //and here!

template <class T>
ostream& operator<<(ostream& ro, const Vec<T>& v);

Possible duplicate of [Function declaration inside or outside the class](http://stackoverflow.com/questions/9075931/function-declaration-inside-or-outside-the-class) — Cristina_eGold, Jan 14 '16 at 22:21

pticawr · Accepted Answer · 2016-01-14T20:49:55.033

The operator* declared inside the template class could equally be written outside the class as

template <class T>
Vec<T> operator*(const Vec<T>& lhs, const T& rhs);

It can be written inside the class with a single argument (representing the rhs) because there is the implied *this argument used as the lhs of the operator.

The difference with the operator* defined outside the class is that the lhs of the operator is a template type. This allows the arguments to be supplied either way around when using the operator.

You are allowed to define the operator outside of a class with any arbitrary lhs and rhs types, but within the class are restricted to only varying the rhs. The compiler would select the best match of any defined operator* given the argument types.

Your first sentence with the code you provided made things that my Lecturer couldn't explain to me well! You sir know how to explain things dont you? Thanks!! :) — sijaan hallak, Jan 14 '16 at 20:57

C++ template functions

1 Answers1