Not allowed to return a function from a function. How could I?

Question

8.3.5/8 Functions [dcl.fct] says

[...] Functions shall not have a return type of type array or function, although they may have a return type of type pointer or reference to such things. [...]

Why so explicit of a rule? Is there some syntax that would even allow returning a function as opposed to a function pointer?

Am I miss-interpreting the quote?

typedef void (*fp)();

void foo(){}
fp goo()
{
    return foo; //automatically converted to function pointer
}

Solution 1

This is quite a contrived example of a function trying to return a function:

void foo() { }

template<typename T>
T f() { return foo; }

int main(){
    f<decltype(foo)>();
}

This is the error I get from Clang 3.2:

Compilation finished with errors:
source.cpp:7:5: error: no matching function for call to 'f'
    f<decltype(foo)>();
    ^~~~~~~~~~~~~~~~
source.cpp:4:3: note: candidate template ignored: substitution failure 
[with T = void ()]: function cannot return function type 'void ()'
T f() { return foo; }
~ ^
1 error generated.

Solution 2

Is there some syntax that would even allow returning a function as opposed to a function pointer?

A syntax? Sure there is:

using fun = int (int);

fun function_that_returns_a_function();

That doesn’t compile because the rule in §8.3.5/8 forbids it. I don’t know why the rule specifically exists – but consider that the type “function” doesn’t have any size so you cannot create objects of function type in C++.

Solution 3

I know this probably does not answer your question completely but it does so partially

You can return a function from another function (that's what lambdas are)

std::function<int (int)> retLambda() {
    return [](int x) { return x; };
}

Comments

[Andy Prowl]

Technically, lambdas are not functions though, they are instances of functors

[Lightness Races in Orbit]

+1: Exactly. Also, this is just one of those rare instances where C++ makes things clear and explicit for us rather than leaving us to derive it from the grammar and other various rules (such as non-copyability). Praise him!

[Andy Prowl]

@LightnessRacesinOrbit: Indeed it is. I often feel like deducing a rule from the Standard is like proving a theorem of Number Theory. Not in this case.

[Lightness Races in Orbit]

Yeah; it's interesting that it's rare enough to actually generate an SO question from an alarmed OP when it does happen.