c++ - 没有数据的类的对象是否占用内存？

Question

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Answer 1

I think object "a" has an address, right? Does it occupy some memory? If so, how many bytes it occupy?

IIRC 标准要求每个对象都有一个地址，并且对象的最小大小(由 sizeof 返回)为 1。话虽如此，优化器可能会使该变量消失(也许除非你明确要求它的地址)。

Unless it is a bit-field (9.6), a most derived object shall have a non-zero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size. [...]

Unless an object is a bit-field or a base class subobject of zero size, the address of that object is the address of the first byte it occupies. Two distinct objects that are neither bit-fields nor base class subobjects of zero size shall have distinct addresses.⁴

Note 4: Under the “as-if” rule an implementation is allowed to store two objects at the same machine address or not store an object at all if the program cannot observe the difference (1.9).

(C++11 §1.8 ¶5-6)

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What happens if object "a" with a type void.

void 不能用于声明变量，因为它是“不完整类型”。

The void type has an empty set of values. The void type is an incomplete type that cannot be completed.

(C++11 §3.9.1 ¶9)