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c++referencemember-functionsconst-reference

Const reference qualifier on a member function

I have seen in an anwser there: Is returning by rvalue reference more efficient?

The member function definition:

Beta_ab const& getAB() const& { return ab; }

I am familiar with the cv-qualifier (const) on member functions, but not const&.

What does the last const& mean?

Solution

  • The & is a ref-qualifier. Ref-qualifiers are new in C++11 and not yet supported in all compilers, so you don't see them that often currently. It specifies that this function can only be called on lvalues (and not on rvalues):

    #include <iostream>

class kitten
{
private:
    int mood = 0;

public:
    void pet() &
    {
        mood += 1;
    }
};

int main()
{
    kitten cat{};
    cat.pet(); // ok

    kitten{}.pet(); // not ok: cannot pet a temporary kitten
}

    Combined with the cv-qualifier const, it means that you can only call this member function on lvalues, and those may be const.