Odd C/C++ initialization syntax for integral type variable

Question

The following snippet compiles perfectly with C/C++ compiler:

#include <stdio.h>
int main()
{
    int x = {5};         //<-- why this compiles?
    printf("x = %d",x);
    return 0;
}

The output is 5. x is initialized here in a compound-type form although it's of integral type. I would like to understand what conversions are made here and why.

This means "initialize the first thing to 5". Since there is only one thing, this works as expected. — David Schwartz, Jul 05 '13 at 14:09
this kind of declaration we generally do: `char s[10] = {"hello"};`, you can remove `{}` — Grijesh Chauhan, Jul 05 '13 at 14:12

score 8 · Accepted Answer · answered Jul 05 '13 at 14:10

8

C allows it in:

(C99, 6.7.8p11) "The initializer for a scalar shall be a single expression, optionally enclosed in braces."

C++ has a similar rule in C++11, 8.5.4p1

answered Jul 05 '13 at 14:10

ouah

142,963
15
272
331

Nice, didn't know it. And what about C++? I also got it compiled with non-C99 compiler: http://ideone.com/aWsjAd – SomeWittyUsername Jul 05 '13 at 14:10
Got you, also found this question: http://stackoverflow.com/questions/13267277/where-can-we-use-list-initialization – SomeWittyUsername Jul 05 '13 at 14:19
C++ also has this rule since the beginning, in C++98 it was written: 8.5p13: *"If T is a scalar type, then a declaration of the form T x = { a }; is equivalent to T x = a;"* – ouah Jul 05 '13 at 14:20
But why on earth to allow such a thing?.. I see no benefits, just ambiguity – SomeWittyUsername Jul 05 '13 at 14:22
@icepack maybe to codify an existing (pre-C89) practice – ouah Jul 05 '13 at 14:26

score 3 · Answer 2 · edited Jul 05 '13 at 14:14

There is no convertions made here, it is standard defined way of variable initialisation.

8.5.4 List-initialization [dcl.init.list]

1 List-initialization is initialization of an object or reference from a braced-init-list. Such an initializer is called an initializer list, and the comma-separated initializer-clauses of the list are called the elements of the initializer list. An initializer list may be empty. List-initialization can occur in direct-initialization or copy- initialization contexts; list-initialization in a direct-initialization context is called direct-list-initialization and list-initialization in a copy-initialization context is called copy-list-initialization.
[Note: List-initialization can be used — as the initializer in a variable definition (8.5)
...
[ Example: int a = {1};

Odd C/C++ initialization syntax for integral type variable

2 Answers2

8.5.4 List-initialization [dcl.init.list]