C++：未调用赋值运算符

Question

I have a following code which is below.

When I write o1 = o2 , void operator=(TestClass& _rhs) is called. It's ok.

But when I do o1 = test_function(); , first operator float() is called, then void operator=(float _value) . It's logically correct, but why is void operator=(TestClass& _rhs) not invoked?

class TestClass
{
public:
    TestClass(float _value)
    {
        value = _value;
    }

    operator float()
    {
        return value;
    }

    void operator=(float _value)
    {
    }

    void operator=(TestClass& _rhs)
    {
    }

private:
    float value;
};

TestClass test_function()
{
    TestClass result = 0;
    return result;
}

int main()
{
    std::cout << "Hello World!\n"; 

    TestClass o1(1), o2(1);

    o1 = o2;

    o1 = test_function();
}

Answer 1

why void operator=(TestClass& _rhs) is not involved?

Because that assignment operator is malformed. It should be:

void operator=(TestClass const& _rhs)
//                CONST! ^^^^^

Your form, void operator=(TestClass& _rhs) will not accept prvalues (temporaries), like the temporary returned from test_function() .

So, the only valid assignment in your code is void operator=(float _value) , which is possible because your class can implicitly convert to a float .

Answer 2

You should declare operator= differently for TestClass. Either use

void operator=(const TestClass& _rhs)

or

void operator=(TestClass _rhs)

This is because function test_functions returns a temporary value, and you cannot bind temporarily to a non-const lvalue reference. This is why the other operator=(float) is selected.

You may find overload resolution rules here: https://riptutorial.com/cplusplus/example/8117/steps-of-overload-resolution