overloading operator = for casting

Question

As title, can be to overloading operator = for casting? I have a simple class.

    class A{
protected:
    int m_int;
public:
    A& operator=( int& obj)
    {
        m_int = obj;
        return *this;
    }
};

I want:

A t_a = 1;

and

int t_int = t_a;

Is there a way to do this?

Possible duplicate of [What is an "operator int" function?](https://stackoverflow.com/questions/3814865/what-is-an-operator-int-function) — Dean Seo, Nov 03 '17 at 08:08
This is not necessarily what I need. Int is just an example. I want to be able to overload in the opposite direction, operator=(int& lhs, A& rhs) — Hoàn Trần, Nov 03 '17 at 08:14
`operator=` cannot be a cast. It’s an assignment operator. A cast is something you write in your source code to tell the compiler to do a conversion. — Pete Becker, Nov 03 '17 at 13:47

Vlad from Moscow · Accepted Answer · 2017-11-03T08:39:26.753

2

Just define conversion operator

operator int() const
{
    return m_int;
}

or

explicit operator int() const
{
    return m_int;
}

In the last case you have to use an explicit casting in the statement

int t_int = int( t_a );

Take into account that the assignment operator should be declared like

A& operator=( const int& obj)
{
    m_int = obj;
    return *this;
}

or like

A& operator=( int obj)
{
    m_int = obj;
    return *this;
}

Otherwise it will be impossible to bind the non-constant reference with integer literals or temporary values.

As for the assignment operator then you may define only a compound assignment operator for the type int and the type A.

For example you could define the operator += or something other operator.

edited Nov 03 '17 at 08:39

answered Nov 03 '17 at 08:14

Vlad from Moscow

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int() is cast-operator () ? – Hoàn Trần Nov 03 '17 at 08:25
@HoànTrần It is a conversion operator that can be used in casting constructions. You can make it either implicit or explicit as it is shown in the answer. – Vlad from Moscow Nov 03 '17 at 08:28
can not overloading operator = for casting? – Hoàn Trần Nov 03 '17 at 08:29
@HoànTrần You may not overload the assignment operator for fundamental types. – Vlad from Moscow Nov 03 '17 at 08:30
@HoànTrần You may overload only a compound assignment operator outside a class definition for fundamental types when one operand is a user defined type. – Vlad from Moscow Nov 03 '17 at 08:36
So if not int, which is a class B? And I can not fix the source code of B? – Hoàn Trần Nov 03 '17 at 08:43
@HoànTrần In this case you may define a conversion operator to the type B in the class A as it is written in my answer. – Vlad from Moscow Nov 03 '17 at 08:44

Martin Bonner supports Monica · Answer 2 · 2017-11-03T08:43:35.510

0

A t_a = 1;

This doesn't use assignment. You need a constructor which takes an int argument.

int t_int = t_a;

You will need operator int() for this.

Note that it is a really bad idea to have a class which has both an implicit constructor from a type, and an implicit cast to the type. You will get all sorts of confusing errors when you try to do overload resolution.

Instead, I would make the constructor explicit, and write an explicit conversion function. That means you have to write:

int t_int = t_a.to_int();

But at least it's explicit.

Edit: Note that you can overload operator = for casting (either inside or outside the class), but neither of the code samples you gave will use it. = is used both for assignment and initialization, and both your samples are initialization (so won't use operator =)

edited Nov 03 '17 at 08:43

answered Nov 03 '17 at 08:16

Martin Bonner supports Monica

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I want 2 classes can use equivalent, almost, call through the function is clear but annoying. – Hoàn Trần Nov 03 '17 at 08:36
@HoànTrần : You need a lot more motivation explaining what you are trying to do in order to explain why your case is an exception to the rule. I'm not sure SO is the right place to do that. – Martin Bonner supports Monica Nov 03 '17 at 08:40
I want overloading assignment operator, not cast operator. But it seems impossible to do that. – Hoàn Trần Nov 03 '17 at 08:55

score 0 · Answer 3 · answered Nov 03 '17 at 08:38

Yes, that’s possible. You need a custom ctor and assignment operator. But writing those disables some of the compiler generated ctors/assignment ops. If you still need/want them, you need to reintroduce them explicitly.

class A
{
protected:
    // Note the initializer. Without it m_int is uninitialized
    // when you default construct an A. That’s a common source
    // of bugs.
    int m_int = 0;

public:
    // Following two are the important ones
    A(int i) : m_int(i) {}

    A& operator=(int i)
    {
        m_int = i;
        return *this;
    }

    // if A needs to be default constructible as well
    A() = default;

    // The int ctor/assignment disable the compiler generated
    // normal copy ctor/assignment (the ones that take another A).
    // Reintroduce them like this:
    A(const A&) = default;
    A& operator=(const A&) = default;

    // Writing any copy ctor/assignment disables the compiler generated
    // move ctor/assignment. If you still want them, reintroduce them.
    A(A&&) = default;
    A& operator=(A&&) = default;
};

overloading operator = for casting

3 Answers3