Instantiation of an abstract class via initializer list

Question

I would like to understand why the compiler allows the following code to compile

#include <iostream>

struct A
{
    A()
    {
        std::cout << "A::A\n";
    }

    virtual void f() const = 0;
};

void g(const A& a)
{
    a.f();
}

int main()
{
    g({});
}

It even outputs A::A when run.

If I replace g({}) with g(A()) it obviously doesn't compile. It complains that A is abstract and cannot be instantiated. Both Clang and GCC compile this fine without any warnings. When run both versions print pure virtual method called and terminate.

For once MSVS does the right thing(at least I expect it to be the right thing) and rejects this code. — NathanOliver, Sep 08 '16 at 13:42
Wow, I am actually mesmerized by this. How is this even possible? [Confirmed](http://coliru.stacked-crooked.com/a/f46d7d1c07cee4af). Ran with C++17 — Arnav Borborah, Sep 08 '16 at 13:45
Yeah. this does not make any sense at all why this compiles or should even be compilable. Is this an undefined case in the standard or just a bug which gcc and clang both have implemented? — Hayt, Sep 08 '16 at 13:46
Does not compile on g++, gives the error "cannot allocate an object of abstract type A", not sure what causes the difference — Benson Lin, Sep 08 '16 at 13:48
I find it unlikely that this is a bug in both `clang` and `gcc`. Some sort of legacy behaviour, perhaps? An extension of backwards-compatibility? — Sebastian Lenartowicz, Sep 08 '16 at 13:48
@BensonLin Works [here](http://coliru.stacked-crooked.com/a/bbe2229928765771). — Baum mit Augen, Sep 08 '16 at 13:48
@BensonLin: compiles for me. Which version of GCC do you use? — detunized, Sep 08 '16 at 13:49
The rules for `{}` being legal are quirky. Looks like implementing those quirky rules left a hole in both gcc and clang. — Yakk - Adam Nevraumont, Sep 08 '16 at 13:54

score 14 · Accepted Answer · edited Jun 20 '20 at 09:12

This looks like a known g++ bug number 70939:

creating object of abstract class allowed in all versions of g++

g++ compiles ill formed C++ program successfully
class A {
public:
    A() {
        printf("A()\n");
    }
    virtual void b() const = 0;
};
int main() {
    const A& a{};
    a.b();
    return 0;
}

Your code does the same thing as this line

const A& a{}

as part of g({}) invocation.

Instantiation of an abstract class via initializer list

1 Answers1

creating object of abstract class allowed in all versions of g++

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