Why C++ 'Standard' won't support function definition inside another function?

Question

I have search through google/stack overflow etc. but I could not find the proper/exact solution for this. I know latest C++ revision supports Lambda function by which we can achieve, and also I know we should avoid defining a function inside another function. But I'm curious why can not we have function definition inside function according to C++ standard?.

like following:

int print() {
   void test(){
     // statements
   }
}

I know people will mark this as duplicate but in those questions no one has given satisfactory answer, to make this to be highlight I have asked here.

Because it didn't see the use in this without some sort of closure mechanic. Which we now have, they are called lambdas. — StoryTeller - Unslander Monica, Oct 20 '16 at 10:09
http://stackoverflow.com/questions/29967202/why-cant-i-define-a-function-inside-another-function — deviantfan, Oct 20 '16 at 10:09
Because it requires special hardware to implement: either a display or a load-address instruction. I haven't seen a language that allowed this since 1982. — user207421, Oct 20 '16 at 10:10
yeah that's correct @StoryTeller. But Im just curious why we cannot? — learn2code, Oct 20 '16 at 10:10
I don't know what kind of answer you expect for this kind of question. It's always *"Because that's what the committee decided."* Write a mail to Stroustrup if you want to ask him why he did not decide differently. vOv — Baum mit Augen, Oct 20 '16 at 10:10
@Suchendra I would not go as far as calling it "insane", but in the end you are asking us why some group of people made a particular choice. Hard to tell for people who are not part of said group. ("insane" changed to "useless", and that I would agree with.) — Baum mit Augen, Oct 20 '16 at 10:14
Since you can write `auto test = []{ statements; };` and call it as a function, pretty much yeah — krzaq, Oct 20 '16 at 10:14
@EJP why would it require special hardware? I can see a nested function easily implemented like a normal function but with a special access, so that it can be called only from the enclosing function (well... I imagine no closure for them) — bolov, Oct 20 '16 at 10:14
@bolov It requires special hardware if the nested function accesses a variable declared in the enclosing function, bearing in mind that the nested function could call itself recursively. — user207421, Oct 20 '16 at 10:17
@BaummitAugen: At least some of those people frequent this tag, though. — Lightness Races in Orbit, Oct 20 '16 at 10:36
@LightnessRacesinOrbit Fair enough. And there are also mailing lists, talks and what not, so admittedly, those questions can be answered. I did not vote this to be off-topic. — Baum mit Augen, Oct 20 '16 at 10:48
@LightnessRacesinOrbit What is this world we are living in!! If we ask curious questions , we get negative remarks. If you know answer please do. Im waiting for the answer, not for the arguments. — learn2code, Oct 20 '16 at 11:12
@Suchendra: The detailed, freely-written answer is on the duplicate question that we kindly marked for you in our own time. What is this world we are living in, where people are so lazy and entitled? I'm waiting for a little effort and gratitude on your part! — Lightness Races in Orbit, Oct 20 '16 at 12:08

StoryTeller - Unslander Monica · Answer 1 · 2016-10-20T10:21:00.937

0

I remembered now that you could emulate it in the pre c++11 days like this:

void foo() {
  struct detail {
    static void bar() {
      //statements
    }
  };

  detail::bar();
}

Can even pass that as a callback from within foo. You should note however that there is no closure here. You can't access variables of foo in bar.

edited Oct 20 '16 at 10:21

answered Oct 20 '16 at 10:18

StoryTeller - Unslander Monica

165,132
21
377
458

We can achieve it by your solution which you've provided. But my question why we cannot define a function inside another? Is there any particular reason Does it effects the efficiency or performance? – learn2code Oct 20 '16 at 10:20
1

@Suchendra, without any closure mechanic it doesn't affect any metric of the program, other than code clarity. – StoryTeller - Unslander Monica Oct 20 '16 at 10:24

score -2 · Answer 2 · answered Oct 20 '16 at 10:15

-2

Pretty sure you can:

int main() {
    auto curry = [] (auto f) {
        return [=] (auto x) {
            return [=] (auto y) {
                return f(x, y);
            };
        };
    };
}

answered Oct 20 '16 at 10:15

Michael Foukarakis

39,737
6
87
123

3

OP asks explicitly not about lambdas. – deviantfan Oct 20 '16 at 10:30

Why C++ 'Standard' won't support function definition inside another function?

2 Answers2