c++ - 什么是 'partially overlapping objects' ？

Question

我刚刚经历了this 中所有可能的未定义行为。线程，其中之一是

The result of assigning to partially overlapping objects

我想知道是否有人可以给我一个“部分重叠对象”的定义以及如何创建它的代码示例？

Answer 1

正如其他答案中指出的那样， union 是最明显的安排方式。

这是一个更清晰的示例，说明内置赋值运算符可能如何产生部分重叠的对象。如果不是部分重叠的对象限制，此示例将不会显示 UB。

union Y {
    int n;
    short s;
};

void test() {
    Y y;
    y.s = 3;     // s is the active member of the union
    y.n = y.s;   // Although it is valid to read .s and then write to .x
                 // changing the active member of the union, .n and .s are
                 // not of the same type and partially overlap
}

即使是相同类型的对象，您也可能会出现部分重叠。在 short 严格大于 char 的情况下考虑这个示例，该实现不向 X 添加填充。

struct X {
    char c;
    short n;
};

union Y {
    X x;
    short s;
};

void test() {
    Y y;
    y.s = 3;     // s is the active member of the union
    y.x.n = y.s; // Although it is valid to read .s and then write to .x
                 // changing the active member of the union, it may be
                 // that .s and .x.n partially overlap, hence UB.
}