[cfe-dev] problem with std::function and virtual destructor

[Steve Ramsey]

On Jul 15, 2012, at 12:59 AM, Richard Smith wrote:
> On Sat, Jul 14, 2012 at 11:40 PM, Rich E <reakinator at gmail.com> wrote:
>> Can anyone explain why the following doesn't compile?
>> 
>> #include <functional>
>> 
>> struct X{
>> 	typedef std::function<void(X&)> callback_type;
>> 	virtual ~X() {}
>> private:
>> 	callback_type _cb;
>> };
>> 
>> int main(){}
>> 
>> 
>> If you comment out the virtual destructor, the code compiles fine.  Full error report:
> 
> I'm not sure why instantiating std::function<void(X&)>::operator= would require X to be complete -- libstdc++ does not behave that way. It's possible this is a bug in libc++, but it's also possible that the standard allows this behavior and libstdc++ is just more permissive. Hopefully Howard can help with that part.

This is correct behavior as per FDIS 12.8.18. Specifically, since there's a user-declared copy constructor, there needs to be a user-declared copy-assignment operator, if you want one; otherwise, the implicitly declared one is defined as deleted. Consider:

//////////
#include <functional>

struct X{
    typedef std::function<void(X&)> callback_type;
    virtual ~X() {}
//    X & operator = (const X &) = default;
private:
    callback_type _cb;
};

int main(){}
/////////

Uncommenting the copy-assignment operator function declaration makes the error go away. Note that in this case, the copy constructor is deleted, and the default constructor, move constructor, and move-assignment operator are suppressed.

			Steve