c++ - 检查标志是否设置在整数变量中

Question

我正在制作我自己的简单绘图引擎。我正在尝试使用我认为称为按位比较的方法来确定变量是否已设置为特定值，但我可能错了。

我一直对以下是什么以及如何使用它感到困惑:

int DRAW_REPEAT_X = 70001; // I have a feeling I should make this value binary instead of a unique number, ie, 0
int DRAW_REPEAT_Y = 70002; // I have a feeling I should make this value binary instead of a unique number, ie, 2
int drawMethod    = DRAW_REPEAT_X | DRAW_REPEAT_Y; // this means I want to repeat an image on both the x and y axis doesn't it?

// Now I want to check if drawMethod has DRAW_REPEAT_X set: this is where I struggle to know how to check this
// Is the following correct?
if (drawMethod && DRAW_REPEAT_X) {
  // the user wants me to repeat an image along the x axis
}

// Now I want to check if drawMethod has DRAW_REPEAT_Y set: this is where I struggle to know how to check this
if (drawMethod && DRAW_REPEAT_Y) {
  // the user wants me to repeat an image along the x axis
}

以下代码是否正确检查是否设置了 DRAW_REPEAT_X？在我的安定检查中它总是返回 1。

编辑 并检查这两个位是否都已设置，我应该这样做吗？

if (drawMethod & DRAW_REPEAT_X & DRAW_REPEAT_Y) {
   // both set
}

// OR

if (drawMethod & DRAW_REPEAT_X && drawMethod & DRAW_REPEAT_Y) {
   // both set
}

Answer 1

为此，您的每个标志变量都需要设置一个唯一的位。那一点就是“旗帜”。对于重要的按位表示的常量，使用十六进制或八进制(因为这些基数是 2 的幂)比十进制更方便。所以，例如:

enum {
    DRAW_REPEAT_X = 0x01,    /* First bit set */
    DRAW_REPEAT_Y = 0x02,    /* Second bit set */
    DRAW_MIRRORED = 0x04,    /* Third bit set */
};

int drawMethod = DRAW_REPEAT_X | DRAW_REPEAT_Y;  /* Will have both first and second bits set */

然后您使用按位与 & 而不是逻辑与 && 来测试位。当且仅当在 a 和 b 中都设置了至少一位时，a & b 才会为非零。在测试标志的情况下，其中一个将只设置一位 - 您感兴趣的标志 - 因此 a & flag 的结果将非零当且仅当标志设置在 a 中:

if (drawMethod & DRAW_REPEAT_X) {
  // the user wants me to repeat an image along the x axis
}

if (drawMethod & DRAW_REPEAT_Y) {
  // the user wants me to repeat an image along the x axis
}

设置一位常量的十六进制模式是0x01、0x02、0x04、0x08、0x10, 0x20, ...