尽管基为空，为什么多重继承会增加对象的大小? [英] Why does multiple inheritance increase the size of the object despite the bases being empty?

问题描述

给出以下代码:

#include <iostream>

struct A {

};

struct B {

};

struct C {

};

struct E : A {
    int field;
};

struct F : A, B {
    int field;
};

struct G : A, B, C {
    int field;
};

int main() {
    std::cout << _MSC_VER << std::endl;
    std::cout << sizeof(E) << std::endl;
    std::cout << sizeof(F) << std::endl;
    std::cout << sizeof(G) << std::endl;
    int o;
    std::cin >> o;
    return 0;
}

我得到以下输出:

1900
4
8
8

为什么F和G的基数为空，为什么它们的大小为8? 为什么E的大小也不会增加?

Why would F and G have sizes of 8 even though their bases are empty? And why would the size of E not increase as well?

我正在使用Visual Studio Community 2015版本14.0.25431.01 Update 3构建它.MSVC++版本显然是9.0.

I am building this with Visual Studio Community 2015, version 14.0.25431.01 Update 3. The MSVC++ version is apparently 9.0.

为什么?这种特殊的内存布局有什么原理?

How come? What rationale is there for such a peculiar memory layout?

推荐答案

Visual Studio 2015 Update 2添加了对空基类优化的支持.但是，由于更新之间应该与布局兼容，因此默认情况下不启用优化.您需要使用__declspec(empty_bases)手动请求.

Visual Studio 2015 Update 2 added support for Empty Base Class Optimization. However, as Updates are supposed to be layout-compatible between them, the optimization is not on by default; you need to manually ask for it using __declspec(empty_bases).

VC的博客中提供了更多信息: https://blogs.msdn.microsoft.com/vcblog/2016/03/30/optimizing-the-layout-of-empty-base-classes-in-vs2015-update-2-3 /

There's more information in VC's blog: https://blogs.msdn.microsoft.com/vcblog/2016/03/30/optimizing-the-layout-of-empty-base-classes-in-vs2015-update-2-3/

一旦他们发布了主要的编译器版本更新，并且允许他们破坏二进制兼容性，这最终将是默认设置.

This will eventually be the default once they release a major compiler version update, where they're allowed to break binary compatibility.

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