Overload class function operator twice as setter and getter

Question

I have a class and i want to overload the fuction-call operator. But since the C++ standard forbids declaring two similar methods which only differ in return type, i get the compile error C2556. I want to use the functions as getter and setter methods. I know i can achieve this by creating a get and a set function. So the question is : Is there a way to achieve this somehow?

class Foo
{
    private:
        std::vector<int> m_vec;

    public:
        Foo()
        {
            m_vec.push_back(1);
            m_vec.push_back(2);
            m_vec.push_back(3);
        }

        //Getter
        int operator()(int i)
        {
            return m_vec.at(i);
        }

        //Setter (C2556)
        int& operator()(int i)
        {
            return m_vec.at(i);
        }
};

int main()
{
    Foo foo;
    foo(1) = 10; //use the setter 
    int i = foo(1); //use the getter
    return 0;
}

Mats Petersson · Accepted Answer · 2014-12-03T09:37:54.133

8

The traditional way of solving this problem is to use const, such as:

#include <vector>

class Foo
{
    private:
        std::vector<int> m_vec;

    public:
        Foo()
        {
            m_vec.push_back(1);
            m_vec.push_back(2);
            m_vec.push_back(3);
        }

        //Setter
        int& operator()(int i)
        {
            return m_vec.at(i);
        }
        //getter
        int operator()(int i) const
        {
            return m_vec.at(i);
        }
};

int main()
{
    Foo foo;
    foo(1) = 10; //use the setter 
    int i = foo(1); //use the getter
    const Foo& b = foo;
    int j = b(1);
    return 0;
}

Now the compiler will use the "appropriate" method when you want to modify vs. not modify the object. (You only need the const operator if you ever use Foo in a const setting)

edited Dec 03 '14 at 09:37

answered Dec 03 '14 at 09:00

Mats Petersson

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Then this code foo(1) = 10; results in error C2106: '=' : left operand must be l-value – Dmitry Ilukhin Dec 03 '14 at 09:04
@DmitryIlukhin this is the correct answer, I do not see your l-value error message - can you provide an example? – Wojtek Surowka Dec 03 '14 at 09:47
Setter has `&`, getter does not? – Martin F Apr 11 '16 at 16:29
1

This doesn't seem to work. Both `foo(1) = 10;` and `int i = foo(1);` call the setter. [Example](http://coliru.stacked-crooked.com/a/65b496c7fd5b0190). – TrebledJ Mar 03 '19 at 09:56

score 2 · Answer 2 · answered Dec 03 '14 at 09:02

2

To my understanding, the first overload is not necessary. It is sufficient to overload

int& operator()(int i)
{
    return m_vec.at(i);
}

which then serves as both getter and setter. The subject is also discussed here.

answered Dec 03 '14 at 09:02

Codor

17,447
9
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But it does not work for const objects, and getter should work for them – Wojtek Surowka Dec 03 '14 at 09:47
Ah sorry, i didn't say that i need both methods, since my class is a bit more complex and i need to do several other things in case of the setter function. – AquilaRapax Dec 03 '14 at 10:32
@AquilaRapax Then I do not think it is possible. Your operator cannot know if it is used on the left or on the right side of assignment. – Wojtek Surowka Dec 03 '14 at 10:52
@WojtekSurowka Unfortunately not, except i declare `Foo` as `const` (as mentioned in the solution). – AquilaRapax Dec 03 '14 at 12:24
@AquilaRapax Declaring a variable as `const` just to use a getter is not very practical. – Wojtek Surowka Dec 03 '14 at 12:58
@WojtekSurowka No, especially if i need both functionalities ;) i kicked the idea and wrote `get` and `set` methods – AquilaRapax Dec 03 '14 at 13:05

Overload class function operator twice as setter and getter

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