Zero initialization of POD types

struct Foo
    char name[10];
    int  i;
    double d;


I know that I can zero-initialize all elements of this type of POD with:

Foo foo = {0};


Can I simplify this even further:

Foo foo = {};


How are inline arrays? ( int arr[10] = {};


I don't ask when initializing with help {0}

if members other than the first will be initialized to zero. I know the answer to this question is yes. I ask if it is possible to syntactically skip the first one 0


Most of the tutorials I have found on this subject use {0}

, but not use {}

, for example this manual and it explains how it works because the aggregated initialization rules are recursive, which gives more confusion than explanation.


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1 answer

As written, this is aggregate initialization. The current rule is (§8.5.1 [dcl.init.aggr] / p7):

If the list has fewer initializer clauses than there are members in the aggregate, then each member is not explicitly initialized must be initialized from its own aligned or equal-initialized element, or, if there is no equalized or equal-initialized element, from an empty initializer list (8.5.4 ).

Relevant parts of §8.5.4 [dcl.init.list] / p3:

A list-initialization of an object or type reference is T

defined as follows:

  • If T

    is an aggregate, aggregate initialization (8.5.1) is performed.
  • Otherwise, if there are no elements in the initializer list, and T

    is a class type with a standard constructor, the object is the initialization value.
  • [irrelevant elements omitted]
  • Otherwise, if there are no elements in the initializer list, the object is value-initialized.

In short, sub-aggregates are recursively aggregated-initialized from an empty initializer list. Everything else is value-initialized. So the end result is everything being value-initialized, with everything being a POD, value-initializing means zero-initializing.

If it T

is a POD but is not an aggregate, then no aggregate initialization is applied, so you hit the second bullet point in §8.5.4 [dcl.init.list] / p3, which initializes the value of the entire object. POD classes must have a trivial default constructor that is not exposed to the user, so initializing values ​​for them also means zero-initializing.



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