Why can't I use an arbitrary nesting of braces to construct most classes?

user_852254 0
Viewed 227

Given the following code:

struct A;

struct B {
    B() {}
    B(A &&) {}
};

struct A {
    A() {}
    A(B &&) {}
};

Then I can use as many braces as I want to construct A or B.

// default construct A
auto a = A{};
// default construct B, forward to A
auto b = A{{}};
// default construct A, forward to B, forward to A
auto c = A{{{}}};
// etc.
auto d = A{{{{}}}};
auto e = A{{{{{}}}}};

Similarly, given

struct C {
    C(std::initializer_list<C>) {}
};

then I can also use as many braces as I want

// default construct C
auto f = C{};
// construct from initializer_list of one default constructed C
auto g = C{{}};
// construct from initializer_list of one C constructed from empty initializer_list
auto h = C{{{}}};
// etc.
auto i = C{{{{}}}};
auto j = C{{{{{}}}}};

Why doesn't the same argument work for a truly boring type?

struct D {
};

or, rewritten for clarity:

struct D {
    D() {}
    D(D &&) {}
};

This fails even on

auto k = D{{}};

Why does this not default construct a D with the innermost braces, and then pass that rvalue on to the move constructor of D?

See it live: https://godbolt.org/z/E763EPGh1

c++11 c++
1 Answers

There's a special case that precludes D{{}}. It's a very particular set of conditions, so I imagine it's there specifically to prevent this exact recursion.

[over.best.ics]/4 However, if the target is
(4.1) — the first parameter of a constructor
...
and the constructor ... is a candidate by
...
(4.5) — the second phase of [over.match.list] when the initializer list has exactly one element that is itself an initializer list, and the target is the first parameter of a constructor of class X, and the conversion is to X or reference to cv X,
user-defined conversion sequences are not considered.

D{{}} is a list-initialization. D(D&&) constructor is considered by the second phase of it (the first phase looks at initializer-list constructors, like C(std::initializer_list<C>) in your second example). But for it to be viable, there needs to be an implicit conversion from {} to D&&, and [over.best.ics]/4 suppresses it.

user_16701290
Related