Lambda expression for operator overloading

Question

Is it possible to write lambda expression for overloading operators?

For example, I have the following structure:

struct X{
    int value;
    //(I can't modify this structure)
};

X needs == operator

int main()
{
    X a = { 123 };
    X b = { 123 };
    //[define equality operator for X inside main function]
    //if(a == b) {}
    return 0;
}

== operator can be defined as bool operator==(const X& lhs, const X& rhs){...}, but this requires adding a separate function, and my comparison is valid only within a specific function.

auto compare = [](const X& lhs, const X& rhs){...} will solve the problem. I was wondering if I can write this lambda as an operator.

I don't understand the problem in using a non-member operator function. Can you elaborate? — 4386427
– 4386427, Commented Apr 24, 2018 at 5:57
A lambda is an object. An object cannot have a name like operator==. Only a function can. — n. m. could be an AI
– n. m. could be an AI, Commented Apr 24, 2018 at 6:02

R Sahu · Accepted Answer · 2018-04-24 06:01:49Z

5

Is it possible to write lambda expression for overloading operators?

No.

Operator overload functions must be functions or function templates. They can be member functions, member function templates, non-member functions, or non-member function templates. However, they cannot be lambda expressions.

From the C++11 Standard/13.5 Overloaded operators, para 6:

An operator function shall either be a non-static member function or be a non-member function and have at least one parameter whose type is a class, a reference to a class, an enumeration, or a reference to an enumeration.

edited Apr 24, 2018 at 6:01

answered Apr 24, 2018 at 5:51

R Sahu

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2 Comments

Henri Menke Over a year ago

Not all operators can be non-member functions, e.g. operator=.

R Sahu Over a year ago

@HenriMenke, True. That is an additional constraint.

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