Infinite recursion in copy constructor

Question

I'm having trouble in a part of my program where I pass an object that acts as a lambda function to another function (I need to capture a const this pointer so I can't use an actual lambda). This causes the copy constructor of my lambda to be called, which again calls the copy constructor, and eventually the stack overflows. I understand what's happening but I'm not sure why the copy constructor is calling itself or how to fix this. I've reproduced the problem below.

Compiler: MSVC 2010

#include <functional>

void synchronizedExecution(std::function<void()> function) {
    function();
}

int main(int argc, char *argv[])
{

    int b = 0;

    class Function : public std::function<void()> {
    public:
        int& b;
        Function(int& b) :
            b(b) {}
        void operator()() {}
    } function(b);

    synchronizedExecution(function);

    return 0;
}

Class declaraion/definition inside `main()` looks wrong (at least missing semicolon after declaration!). — πάντα ῥεῖ, Feb 06 '14 at 19:54
Declaration looks just fine to me. The semicolon is right there after the instance. — Fred Larson, Feb 06 '14 at 19:56
It is fine (no recursion) under g++...are you using Visual Studio 2010 or 2012? — IdeaHat, Feb 06 '14 at 19:57
Class declaration/definition is fine - it's just using an alternative syntax to also declare a variable of the class and initialize it with a constructor, passing `b`. — Sion Sheevok, Feb 06 '14 at 19:57
Inheiriting from `std::function` is a horrible idea. Why are you doing that? — Yakk - Adam Nevraumont, Feb 06 '14 at 20:27
I had a compilation problem that seemed to go away when I inherited std::function. It didn't line up with my understanding of templates but my understanding of templates isn't that deep. I've since removed it and found it in fact didn't do anything. — HahaHortness, Feb 06 '14 at 20:39
@HahaHortness: You still have the recursion without the inheritance? — Fred Larson, Feb 06 '14 at 20:47

score 3 · Answer 1 · answered Feb 06 '14 at 19:53

3

I can help you with the "how to fix this" part - change your function to

void synchronizedExecution(const std::function<void()>& function)

answered Feb 06 '14 at 19:53

PaF

3,297
1
14
15

Depends, I can see where the user would like the function to actually be copied (since synchronizedExecution is a function, I'm guessing async is also an option) – IdeaHat Feb 06 '14 at 20:00
@MadScienceDreams: Passing by value would be [slicing](http://stackoverflow.com/q/274626/10077). Bad news. – Fred Larson Feb 06 '14 at 20:03
@FredLarson I was thinking that but it doesn't expload in g++ and I'm still trying to figure out why... – IdeaHat Feb 06 '14 at 20:05
@MadScienceDreams: I bet it doesn't call `Function::operator()()`, either. – Fred Larson Feb 06 '14 at 20:07
My code was growing in a way that made me think that even if I passed by reference I would have to copy the function at some point. But now I don't think that's the case, so I think this will work. – HahaHortness Feb 06 '14 at 20:07
@FredLarson I think (i could be wrong) that his code is actually interpreted as the implicite functor-type constructor for std::function rather than as a literal type, so it creates a copy of the Function class as a member rather than it being sliced. I have no proof of this. – IdeaHat Feb 06 '14 at 20:12
@MadScienceDreams: Actually, I don't think it will work either way, as `std::function::operator()()` is not virtual. It's not meant to be used as a base class. – Fred Larson Feb 06 '14 at 20:14
@FredLarson added manual copy constructor, `std::function fun = function` definitely calls Function::Function(const Function& fun), synchornizedExecution(function) calls the copy constructor twice (once for creating the implicitly casted std::function, once for the copy into the function). So no slicing, atleast with real C++11. – IdeaHat Feb 06 '14 at 20:18
@MadScienceDreams: I get it now, and you're right. There's no slicing here. – Fred Larson Feb 06 '14 at 20:45

score 0 · Answer 2 · answered Feb 06 '14 at 20:03

First, to solve your problem you can probably pass object by reference, not by value.

Second, your coding is very prone to errors. When you introduce a new name and it is the same as the existing one, you have a collision which compiler solves but you not always get what you expected. For example

   class Function
        int& b;
        Function(int& b) :
            b(b) {}

I am honestly not sure, which b will be substituted into (), the class member or the function parameter. AFAIR, the class member has precedence and this is not what you intended. I suspect your actual problem has the same cause.

Infinite recursion in copy constructor

2 Answers2