Why does for_each + lambda trigger -Waggregate-return warning?

Question

When trying the following example with gcc and the -Waggregate-return flag the warning: function call has aggregate value triggers:

struct Element {
// ... stuff ...
}

Container<Element> elements(10);
for_each(begin(elements),end(elements),[](Element& e){

// ... modify elements ...

});

As far as I could find out, the -Waggregate-return flag "Warns if any functions that return structures or unions are defined or called", because, if I understood correctly, you could potentially overflow the stack by returning a large enough object.

However, for_each returns the type of the lambda, whose type is void. Why does it trigger the warning? What have I missed? How can I improve my code?

Answer 1

Why does it trigger the warning?

Presumably, the warning is triggered by returning anything of class or union type. Lambdas have class type, and for_each returns its function argument, so that will trigger the warning.

It's also possible that the iterator type returned by begin(elements) and end(elements) might trigger the warning, depending on how the Container type implements iterators.

How can I improve my code?

I'd disable that warning; it's not really compatible with idiomatic C++, since it's very common to return small class objects from a function. It would also be triggered by, for example, std::map::insert(), which returns a pair, and many other standard library functions.