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I'm using the clang compiler (c++ 11 I think) that comes with RAD studio 10.2. By mistake I discovered today that the first n members of a struct or array can be assigned using the usual curly brackets e.g.
int a[500]={1};
struct {int a,b,c;} st={2,3};
The above compiles and works fine but I've never come across this or seen it used before and I can find no mention of it online (maybe I'm searching using the wrong type of wording). Is this c++ documented?
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If a structure variable has static storage, its members are implicitly initialized to zero of the appropriate type. If a structure variable has automatic storage, its members have no default initialization.
Initializer List: To initialize an array in C with the same value, the naive way is to provide an initializer list. We use this with small arrays. int num[5] = {1, 1, 1, 1, 1}; This will initialize the num array with value 1 at all index.
The initializer for an array is a comma-separated list of constant expressions enclosed in braces ( { } ). The initializer is preceded by an equal sign ( = ). You do not need to initialize all elements in an array.
Using Initializer List. int arr[] = { 1, 1, 1, 1, 1 }; The array will be initialized to 0 if we provide the empty initializer list or just specify 0 in the initializer list.
For aggregate initialization,
(emphasis mine)
If the number of initializer clauses is less than the number of members
and bases (since C++17)or initializer list is completely empty, the remaining membersand bases (since C++17)are initializedby their default member initializers, if provided in the class definition, and otherwise (since C++14)by empty lists, in accordance with the usual list-initialization rules (which performs value-initialization for non-class types and non-aggregate classes with default constructors, and aggregate initialization for aggregates).
That means, for int a[500]={1};, the 1st element is initialized to 1 and the remaining 499 elements of the array are value-initialized to 0. For struct {int a,b,c;} st={2,3};, the member a is initialized to 2 and b is initialized to 3, the last member c is value-initialized to 0 too.
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I've never come across this or seen it used before and I can find no mention of it online (maybe I'm searching using the wrong type of wording). Is this c++ documented?
Yes, this is documented. This syntax is called list initialisation - and more specifically since the types are aggregates: This is aggregate initialisation.
Initialise first n members only of struct or array
It is not possible to only initialise some members / elements. If you list initialise class object or array, then all of it will be initialised. The members / elements which lack an initialiser will be value initialised.
If you wanted to do that, then what you can do instead is default initialise the class object or array, and then initialise the sub objects selectively afterwards.
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