Instantiation of an abstract class via initializer list [duplicate]

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.

Answer 1

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.