dimanche 22 février 2015

Abstract class as std::initializer_list object

In order to have a cleaner syntax, I would like to use an std::initializer_list to send a list of objects to a constructor. The objects, however, are abstract, which causes a problem: in VS 2013, it looses the vfptr reference, giving a "R6025: pure virtual function call" runtime error, and in g++ it complains that it "cannot allocate an object of abstract type ‘base’" during compilation. I surmise the compiler is trying to copy the objects (which is undesirable -- they may be big), but succeeds only in copying the base class, hence the error. My question is: Is there a solution which (1) avoids copying the objects and (2) isn't massively verbose, negating the "cleaner syntax" advantage? The code below illustrates my issue:



#include <cstdio>
#include <initializer_list>

struct base{
virtual void foo() const = 0;
};

struct derived : public base{
int i;
derived(int i) : i(i) {}
void foo() const{
printf("bar %i", i);
}
};

void foo_everything(const std::initializer_list<base> &list){
for (auto i = list.begin(), iend = list.end(); i != iend; i++) i->foo();
}

int main(void){

// Works fine
derived d(0);
base * base_ptr = &d;
base_ptr->foo();

// Does not work fine
foo_everything({ derived(1), derived(2), derived(3) });
}


Note that using base& in the template errors since std::initializer_list tries to "[form a] pointer to reference type base&", and while using base*, and then taking the address of each derived class does in-fact work, it does so by taking the address of temporaries, and thus isn't safe (g++ complains). The latter does work if I declare the derived classes outside of the method call (my provisional solution), but it still is more verbose than I hoped for.


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