为什么c ++对布尔不具有&&& =或|| =？ [英] Why doesn't c++ have &&= or ||= for booleans?

问题描述

是否有可能发生的非常糟糕的事情&& =和|| =被用作语法糖 bool foo = foo&&& bar and bool foo = foo || bar ？

Is there a "very bad thing" that can happen &&= and ||= were used as syntactic sugar for bool foo = foo && bar and bool foo = foo || bar?

推荐答案

A bool 只有C ++中的 true 或 false 。因此，使用& = 和 | = 是完全安全的不特别喜欢的符号）。真的，他们将执行位操作而不是逻辑操作（因此，它们不会短路），但是这些位操作遵循良好定义的映射，这有效地等同于逻辑操作， 两个操作数 确实是 bool 。

A bool may only be true or false in C++. As such, using &= and |= is perfectly safe (even though I don’t particularly like the notation). True, they will perform bit operations rather than logical operations (and as such, they won’t short-circuit) but these bit operations follow a well-defined mapping, which is effectively equivalent to the logical operations, as long as both operands are indeed of type bool.

与其他人在这里说的不同，C ++中的 bool 不能有不同的值，例如 2 。当将该值赋给 bool 时，它将根据标准转换为 true 。

Contrary to what other people have said here, a bool in C++ must never have a different value such as 2. When assigning that value to a bool, it will be converted to true as per the standard.

在 bool 中获取无效值的唯一方法是使用 reinterpret_cast on pointer：

The only way to get an invalid value into a bool is by using reinterpret_cast on pointers:

int i = 2;
bool b = *reinterpret_cast<bool*>(&i);
b |= true; // MAY yield 3 (but doesn’t on my PC!)

未定义的行为，我们可以安全地忽略遵循C ++代码的这个潜在问题。

But since this code results in undefined behaviour anyway, we may safely ignore this potential problem in conforming C++ code.

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