Is it legal to initialize a structure with only one element as a member using the list of initializers directly?

Question

#include <iostream>

class vec
{
public:
    int  arr[2];
};


int main()
{
    vec a = { { 1,2 } };// works 
    vec b = { 1,2 };// works too ,but why ?
    std::cin.get();
}

vec has no constructor except the default constructor.

But this piece of code compiles, I wonder why.

http://ideone.com/uyx98o

Answer 1

Aggregate initialization is amazing, you don’t even have to have the right to nesting to make it work, and you can also provide fewer than members:

#include <iostream>
#include <string>

struct A {
    struct { int a; std::string b[3]; } a;
    double b, c;
};

int main() {
    A a = { 10, "a", "b", "c", 3.1415 };
    std::cout << a.a.a << " " << a.a.b[0] << " " << a.a.b[2]
              << " " << a.b << " " << a.c;
}

Live