C 中的变量定义

Question

What does following declaration mean in C?

char a = (10,23,21);

While printing the value of "a" with "%u" the output is 21. gcc is not giving any error. What's this kinda declaration and what's the use of it?

Answer 1

You are seeing the comma operator at work. The comma operator a,b evaluates a , throws away the result, then returns b .

Since 10 and 23 have no side effects, this is equivalent to char a = 21;

Answer 2

This is a use of the scalar comma operator. The comma operator evaluates each expression on the left side and throws away the return value, finally returning the rightmost value.

In this case, it's useless; however, if you use it with expressions with side-effects, then it has a real effect.

Example of a semi-"useful" expression (with side-effects):

int a = 10;
int is_a_odd_after_increment = ++a, a % 2;

The first expression ( ++a ) has a clear side-effect, and it is evaluated first (before the a % 2 ). The second expression is the expression that is yielded into the assignment.