Initializing class members with {}

Question

Someone gave me (part of) the following code:

struct MyStruct
{
    int x = {};
    int y = {};

};

I never saw this syntax before, what does initialization with {} mean?

See this answer: https://stackoverflow.com/questions/18222926/why-is-list-initialization-using-curly-braces-better-than-the-alternatives — Istvan, May 12 '20 at 06:00
It's inline [*value initialization*](https://en.cppreference.com/w/cpp/language/value_initialization) of the members. In short, `int x = {};` is the same as `int x = 0;`. — Some programmer dude, May 12 '20 at 06:01
@Someprogrammerdude but I can't just replace it with `int x = 0` unless it is `const`? — Benny K, May 12 '20 at 07:11
@BennyK `int x = 0;` should be fine. https://wandbox.org/permlink/NEGqlNIcI8nieHe5 — songyuanyao, May 12 '20 at 07:20
Does this answer your question? [Why is list initialization (using curly braces) better than the alternatives?](https://stackoverflow.com/questions/18222926/why-is-list-initialization-using-curly-braces-better-than-the-alternatives) — ChrisMM, May 12 '20 at 14:49

songyuanyao · Answer 1 · 2020-05-12T06:31:35.037

3

This is default member initializer (since C++11),

Through a default member initializer, which is a brace or equals initializer included in the member declaration and is used if the member is omitted from the member initializer list of a constructor.

The initialization itself is copy-list-initialization (since C++11), as the effect, the data member x and y would be value-initialized (and zero-initialized as built-in type) to 0.

edited May 12 '20 at 06:31

answered May 12 '20 at 06:02

songyuanyao

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It's default member initializer, not default initializer list. – super May 12 '20 at 06:14
@songyuanyao The data member would be initialized to its default value. In the case of `int` it is `0`. But for classes it will mean constructing with the default constructor. – Enosh Cohen May 12 '20 at 06:23
@EnoshCohen There's no "default value" for int in this context. – juanchopanza May 12 '20 at 06:30
There is an "initial value" for scalar types, maybe the term is not accurate: https://en.cppreference.com/w/cpp/language/zero_initialization – Enosh Cohen May 12 '20 at 06:40

score 1 · Accepted Answer · answered May 12 '20 at 07:19

Since the C++11 standard there are two ways to initialize member variables:

Using the constructor initialization list as "usual":

struct Foo
{
    int x;

    Foo()
        : x(0)
    {
    }
};

Use the new inline initialization where members are getting their "default" values using normal initialization syntax:
```
struct Foo
{
    int x = 0;
};
```

Both these ways are for many values and types equivalent.

Initializing class members with {}

2 Answers2