Whats the significance of return by reference?

Question

In C++,

function() = 10;

works if the function returns a variable by reference.

What are the use cases of it?

Answer 1

The named parameter idiom is a another use case. Consider

class Foo
{
public:
    Foo(
        int lions,
        float tigers,
        double bears,
        std::string zookeeper
    );
};

users of this class need to remember the position of each parameter

Foo foo( 1, 2.0, 5, "Fred" );

which can be non-obvious without looking at the header. Compared to a creator class like so

class CreateFoo
{
friend class Foo;
public:
    CreateFoo();

    CreateFoo& lions(int lions) {
        _lions = lions;
         return *this;
    }

    CreateFoo& tigers(float tigers) {
        _tigers = tigers;
        return *this;
    }

    CreateFoo& bears(double bears) {
        _bears = bears;
        return *this;
    }

    CreateFoo& zookeeper(const std::string& zookeeper) {
        _zookeeper = zookeeper;
        return *this;
    }

private:
    int _lions;
    float _tigers;
    double _bears;
    std::string _zookeeper;
};

which can then be used by clients like so

Foo foo = CreateFoo().
    lions(1).
    tigers(2.0).
    zookeeper("Fred").
    bears(5)
    ;

assuming Foo has a constructor taking a const CreateFoo&.