Designated initializers

Question

Apparently, designated initializers is not "correct" C++. Are there any alternatives for this?

I like the use of this for structs because:

Variables can be set optionally when creating the struct object.
The order of initializing doesn't matter.
Variables can be const.
This can be used in c++17 in combination with base classes.

Thanks Jelle

What is the question? Also, they are coming, https://en.cppreference.com/w/cpp/language/aggregate_initialization#Designated_initializers — NathanOliver, Jul 24 '18 at 13:00
IINM designated initializers have been voted in C++20, so that'll be a relief. — Quentin, Jul 24 '18 at 13:01
^ - And they are already supported as an extension by at least two popular compilers. — StoryTeller - Unslander Monica, Jul 24 '18 at 13:01
Useful related discussion https://stackoverflow.com/questions/18731707/why-does-c11-not-support-designated-initializer-lists-as-c99 — gavinb, Jul 24 '18 at 13:02
@StoryTeller, do you know which specific compiler? On mac, the default xcode compiler supports it but we want to make it work for visual studio as well. — Jelle Bakker, Jul 24 '18 at 16:56
@JelleBakker - GCC and Clang are what I had in mind. But that's because they support C99 as well. MSVC lags behind in that regard. — StoryTeller - Unslander Monica, Jul 24 '18 at 16:59

score 1 · Answer 1 · answered Jul 24 '18 at 13:33

Given

struct S {
    int x, y, z;
}:

S s {
    .y = 1; // not standard until C++20
};

Are there any alternatives for this?

You can use positional list initialisation:

S s{0, 1};

It has the drawback (arguably a benefit, depending on situation) that the member names are not explicit, meaning depends on order of members and all members preceding the last explicitly initialised must also be there.

Another alternative: Assign the member later.

S s{};
s.y = 1;

This has the drawback that it cannot be used to initialize const members. Another drawback is that this is not a single initialisation-expression. That can be worked around by using a function:

S init(int y) {
    S s{};
    s.y = y;
    return s;
}
S s = init(1);

Yet another alternative: Use a constructor.

struct S {
    int x = 0, y = 0, z = 0;
    S(int y): y(y) {}
};

This can be much simpler in some cases, but not at all clear in others (such as this vector-like case). Another drawback is the lack of trivial constructor.

Designated initializers

1 Answers1