Define and return a function from a function?

Question

How does one define and return a function inside a function?

For instance, we have a function like:

float foo(float val) {return val * val;}

Answer 1

Use std::function

std::function bar(float coeff)
{
    auto f = [coeff](float x)
    {
        return coeff * foo(x);
    };

    return f;
}

You would then use it like this:

auto f = bar(coeff);
auto result = f(x);