为什么空类的大小是1个字节

问题描述

为什么空类的大小是1个字节？

Why size of empty class is 1 byte?

推荐答案

Why is the size of an empty class not zero?

To ensure that the addresses of two different objects will be different. For the same reason, "new" always returns pointers to distinct objects. Consider:
    class Empty { };

    void f()
    {
        Empty a, b;
        if (&a == &b) cout << "impossible: report error to compiler supplier";

        Empty* p1 = new Empty;
        Empty* p2 = new Empty;
        if (p1 == p2) cout << "impossible: report error to compiler supplier";
    }
There is an interesting rule that says that an empty base class need not be represented by a separate byte:
    struct X : Empty {
        int a;
        // ...
    };

    void f(X* p)
    {
        void* p1 = p;
        void* p2 = &p->a;
        if (p1 == p2) cout << "nice: good optimizer";
    }
This optimization is safe and can be most useful. It allows a programmer to use empty classes to represent very simple concepts without overhead. Some current compilers provide this "empty base class optimization".

以上代码的原始帖子'的网址： []



我也在这里嵌入了不同网站的URL。这是

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Origional post''s URL for the above code : http://www2.research.att.com/~bs/bs_faq2.html#sizeof-empty[^]

also here i am embeding a URL of the different website. Is that
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