Lambda closure type constructors

Question

The cppreference shows that there are different rules for lambda closure type constructors.

Default Construction - Until C++14

Cl

Answer 1

There was a shortcoming. We couldn't use lambdas quite as "on the fly" as one might have wanted. C++20 (with the addition of allowing lambdas in unevaluated contexts) makes this code valid:

struct foo {
    int x, y;
};

std::map<foo, decltype([](foo const& a, foo const& b) { return a.x < a.y; })> m;

Note how we defined the compare function inline? No need to create a named functor (could be a good idea otherwise, but we aren't forced too). And there's no need to break the declaration in two:

// C++17
auto cmp = [](foo const& a, foo const& b) { return a.x < a.y; };
std::map<foo, decltype(cmp)> m(cmp); // And also need to pass and hold it!

Usages like this (and many more) were the motivating factor in making this change. In the example above, the anonymous functors type will bring all the benefits a named functor type can bring. Default initialization and EBO among them.