Indirect & Direct initialization of std::atomic in C++11/17. What are the differences?

Question

When I see this CPP Con 2017 webinar, Fedor Pikus says: "it has to be direct initialization"

This is the link to the webinar.

What are the differences between these initialization methods? (and subsequently, why it has to be a "direct" initialization? why "indirect" initialization is "NOT"?)

// C++17 Compiler

#include <atomic>

class Example
{
    std::atomic<bool> m_b1 = false; // 1-h
    std::atomic<bool> m_b2{ false }; // 2-h

    static void doSomethng()
    {
        std::atomic<bool> b1 = false; // 1-f
        std::atomic<bool> b2{ false }; // 2-f
        std::atomic<bool> b3(false); // 3-f

        // Do something
    }
};

Answer 1

std::atomic is not copyable or movable.

Before C++17, the copy-initialization std::atomic<int> x = 0; would first construct a temporary std::atomic<int> from 0 and then direct-initialize x from that temporary. Without a move or copy constructor this would fail and so the line doesn't compile.

std::atomic<int> x(0); however is direct-initialization and will just construct x with the argument 0 to the constructor.

Since C++17 there is no temporary and x will directly be initialized by a constructor call with 0 as argument in any case and so there is no issue with std::atomic being non-movable. In that sense the slide is now out-dated.

Even though the behavior is now the same for copy-initialization and direct-initialization in this case, there are still differences between the two in general. In particular direct-initialization chooses a constructor to initialize the variable directly by overload resolution while copy-initialization tries to find an implicit conversion sequence (possibly via converting constructor or conversion function with different overload resolution rules). Also, copy-initialization, in contrast to direct-initialization, does not consider constructors marked explicit.

Regarding the code snippet in the question. 1-h and 1-f are copy-initialization as above. 3-f is direct-initialization as above. 2-h and 2-f are direct-list-initialization, which behaves different from both others in some cases, but here it has the same effect as direct-initialization with parentheses.

Explaining all the differences between the initialization forms in general would take a while. This is famously one of the most complex parts of C++.