What does the C++ standard mean regarding object lifetime begins?

Question

In the n3690 C++ standard in section 3.8.1 there is this text:

The lifetime of an object of type T begins when:
— storage with the proper alignment and size for          


        

Answer 1

when constructor body has finished running

This. An object that throws during construction is not guaranteed to have its invariants established, hence its lifetime doesn't start. A consequence of this is that the destructor will not get called:

#include <iostream>

struct Stillborn
{
    Stillborn()
    {
        std::cout << "inside constructor\n";
        throw 42;
    }

    ~Stillborn()
    {
        std::cout << "inside destructor\n";
    }
};

int main()
{
    try
    {
        Stillborn x;
    }
    catch (...)
    {
        std::cout << "inside catch block\n";
    }
}

live demo. Note how "inside destructor" does not appear in the output.

Answer 2

12.6.2, [class.base.init], item 6, lists the steps of initialization, and this is the final one:

Finally, the compound-statement of the constructor body is executed.

So once the body has executed, initialization is complete.

Answer 3

There is a note:

"[ Note: initialization by a trivial copy/move constructor is non-trivial initialization. — end note ]"

It means when the trivial constructor will finish its execution.