c - 为什么要使用三元运算符而不为 "true"条件赋值 (x = x ? : 1)

Question

在 Android 开源 qemu 代码中，我遇到了这行代码:

machine->max_cpus = machine->max_cpus ?: 1; /* Default to UP */

这只是一种令人困惑的说法吗:

if (machine->max_cpus) {
   ; //do nothing
} else {
 machine->max_cpus = 1;
}

如果是这样，不是更清楚:

if (machine->max_cpus == 0) machine->max_cpus = 1;

有趣的是，这可以编译并与 gcc 一起正常工作，但不能在 http://www.comeaucomputing.com/tryitout/ 上编译。 .

Answer 1

这是 permitted在 GNU 中作为 C 语言的模糊扩展

5.7 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.

正如您可能猜到的那样，出于可读性和可移植性的原因，建议避免这种情况。看到这样一个语法不兼容的 C 扩展，我真的很惊讶。