Nested lambda syntax and invocation?

Question

I didn't know how to name the title, hopefully its correct...

I stumbled upon below lambda definition, and don't understand the syntax, wht is the meaning of var = [=] and return [=]?

also second question in ConstexprLambda() function below, why can't we must call add(1, 2) instead of add(1, 2)() why need for additional () while in the call to identity(123) the code makes not use of additional () ?

question(s) is put into comments of the code.

auto identity = [](int n) constexpr
{
    return n;
};

constexpr auto add = [](int x, int y)
{
    auto L = [=] // what is = [=]?
    {
        return x;
    };
    auto R = [=]
    {
        return y;
    };
    return [=] // what return [=] means here?
    {
        return L() + R();
    };
};

void ConstexprLambda()
{
    static_assert(identity(123) == 123);
    static_assert(add(1, 2)() == 3); // why can't we just add(1,2) like above?
}

The example is taken from here

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

types

Let's start from the type

auto identity = [](int n) constexpr
{
    return n;
};

Here identity is a wrapper that stores a callable of type lambda(int). You can use std::function too:

std::function<int(int)> identity;

So identity take an int and return an int when you invoke it.

add on the other hand when invoked, takes two ints and returns a callable.

extra () operator

You need to invoke add(1, 2) because the return type of add is a callable, otherwise you cannot do the add(1, 2)() == 3 comparison because of type mismatch.

see the types live on godbolt

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Oct 6, 2019 at 12:45

Oblivion

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2 Comments

metablaster Over a year ago

does that mean (in above add function) that return [=] and auto T = [=] { } are actually nested lambdas (lambda within lambda)?

Oblivion Over a year ago

@metablaster add is a lambda that returns another lambda. [=] means capture by value.

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