Object creation order in braced init list

Question

#include <iostream>

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

struct B
{
    B() { std::cout << "(B::B)"; }
};

struct C
{
    template<typename ...Args>
    C(Args && ...) {}
};

int main(int agrc, char *argv[])
{
    C {A(), B()}; // <-- prints (B::B)(A::A)
    std::cout << std::endl;
    C {(A(), B())}; // <-- prints (A::A)(B::B)
    std::cout << std::endl;

    return 0;
}

I've got 2 questions:

• Why in first braced init list objects are created in right-to-left order?
• Why parentheses in second case revert this order?

Edit: I've compiled it with msvs 2013

Answer 1

In the second instance, you're actually only initialising with B(); through the use of the comma operator, A() was constructed and thrown away first.

C {(A(), B())};
//|^^^^^^^^^^|
//      \--------> (A(), B())
//                  ^^^  ^^^
//                   |    |
//                   /    \
//            evaluated,   THEN evaluated,
//            discarded      used

---

On the other hand, in the first instance, you're initialising the C from both temporaries through an initializer list, whose elements should also be evaluated left-to-right, as it happens, but your compiler is buggy in this regard:

[C++11: 8.5.4/4]: Within the initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack expansions (14.5.3), are evaluated in the order in which they appear. That is, every value computation and side effect associated with a given initializer-clause is sequenced before every value computation and side effect associated with any initializer-clause that follows it in the comma-separated list of the initializer-list. [ Note: This evaluation ordering holds regardless of the semantics of the initialization; for example, it applies when the elements of the initializer-list are interpreted as arguments of a constructor call, even though ordinarily there are no sequencing constraints on the arguments of a call. —end note ]

I can reproduce the problem with GCC 4.8^*, but Clang 3.5 behaves properly^†. The bug has been discussed on the std-discussion list before^‡, but I haven't found a GCC Bugzilla ID yet^§.

C {A(), B()};
// ^^^  ^^^
//  |    \
// eval-  THEN
// uated   evaluated
//  \       /
//   \     /
//  both used

---

^{*http://coliru.stacked-crooked.com/a/1f18e0d1f8973f3c
†http://coliru.stacked-crooked.com/a/5a6e7506e9be97c3
‡https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/TQUnBFkUBDg
§#51253 may be related.}

Answer 2

Why in first braced init list objects are created in right-to-left order?

No. It is left-to-right. Your compiler has bug which is why it is evaluating right-to-left. GCC (4.8) is known to have this bug. Do you use GCC?

Why parentheses in second case revert this order?

Same. Left to right. In this case, comma operator comes into picture, which evaluates operands left-to-right.