c - 为 size_t 切换为负值

Question

如果我有以下片段:

size_t num = -1;

switch(num) {
    case -1:
        printf("Minus one!\n");
        break;
    default:
        printf("Not minus one!\n");
}

为什么程序打印Minus One!？ num 是否在 switch 语句中转换为 size_t？这种行为是否已定义？

Answer 1

来自 switch 的 C 标准:

6.8.4.2 The switch statement
...
Semantics
...
5 The integer promotions are performed on the controlling expression. The constant expression in each case label is converted to the promoted type of the controlling expression. If a converted value matches that of the promoted controlling expression, control jumps to the statement following the matched case label.

size_t 也是符合标准的无符号类型(6.5.3.4 The sizeof operator, Semantics, 4)。

因此，您的 -1 在 size_t num = -1; 和 case -1 中都被转换为 size_t:。难怪 (size_t)-1==(size_t)-1 的计算结果为真。