int q = {1,2}; peculiar initialization list

Question

I came across the below initialization , it is seen that VS2012 shows an error complaining about too many initializers. in GCC it seems to return the first element as the

Answer 1

C11: 6.7.9 Initialization (p11):

The initializer for a scalar shall be a single expression, optionally enclosed in braces.

Therefore, this is allowed

int q = {1};   

You can enclose the initializer for scalar objects in braces ({}). Note the verb shall is used here. The standard says:

5.1.1.3 Diagnostics (P1):

A conforming implementation shall produce at least one diagnostic message (identified in an implementation-defined manner) if a preprocessing translation unit or translation unit contains a violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or implementation-defined

So, it is up to the compiler how it handles

int q = {1,2}; 

Compiled on GCC 4.8.1 with flags -pedantic -Wall -Wextra and it raised a warning

[Warning] excess elements in scalar initializer [enabled by default]   

Now the question is: What happend with the remaining initializers? It's a bug.

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Note: C11: 6.5.17 (p3) says that the comma operator cannot appear in contexts where a comma is used to separate items in a list (such as arguments to functions or lists of initializers).

Do not confused the , in {1,2} with comma operator. As Keith Thompson pointed out that, the expression in initializer to be an assignment-expression and it must not contain comma operator at top-level. That means it can be used within a parenthesized expression or within the second expression of a conditional operator in such contexts. In the function call

f(a, (t=3, t+2), c)

the function has three arguments, the second of which has the value 5.