一个比较特别的用法：“?:”，不带中间的操作数

一个测试代码：

CODE:

#define INC(x) (++x)

int x = 0;
int y = 0;
那么

CODE:

y = INC(x) ? INC(x) : 3
和

CODE:

y = INC(x) ? : 3

这也就是“?:”省略中间操作数，可以避免边界效应

Conditionals with Omitted Operands

The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

     x ? : y
     

has the value of x if that is nonzero; otherwise, the value of y.

This example is perfectly equivalent to

     x ? x : y
     
In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.