为什么是＆QUOT大小;的std ::矢量＆lt;＆布尔GT;＆QUOT; 16字节？ [英] Why is the size of "std::vector<bool>" 16 Byte?

问题描述

我使用的memcpy性病的内容复制：矢量＆lt;？>到primitve阵列。对于喜欢整数，浮点，双等数据类型，效果不错。当我开始复制boolvector我遇到了一个问题，即我有奇怪的值。

I'm using memcpy to copy the content of std:vectors<> to primitve Arrays. For datatypes like int, float, double etc. it worked well. As I started to copy a boolvector I encountered a problem namely I got strange values.

首先，我开始做了一个矢量浮点的测试输出：

First I started to make an test output for a float vector:

std::vector<float> test1 (3,0);

cout << "Sizeof test1[0] : " << sizeof(test1[0]) << endl
     << "Memoryaddress 0: " << &test1[0] << endl
     << "Memoryaddress 1: " << &test1[1] << endl
     << "Memoryaddress 2: " << &test1[2] << endl;

输出是：

Sizeof test1[0]: 4
Memoryaddress 0: 02793820
Memoryaddress 1: 02793824
Memoryaddress 2: 02793828

这是我所期望的。浮球大小为4字节和下一个浮点值的距离为4字节。当我这样做对布尔输出看起来是这样的：

And this is what I expect. The float size is 4 Byte and the distance to the next float value is 4 Byte. When I do this for bool the output looks like this:

std::vector<bool> test (3,0);

cout << "Sizeof test[0]: " << sizeof(test[0]) << endl
     << "Memoryaddress 0: " << &test[0] << endl
     << "Memoryaddress 1: " << &test[1] << endl
     << "Memoryaddress 2: " << &test[2] << endl;

输出是：

Sizeof test[0]: 16
Memoryaddress 0: 011EF94C
Memoryaddress 1: 011EF93C
Memoryaddress 2: 011EF92C

为什么是布尔16字节的大小？这似乎是一个总矫枉过正给我。有没有解释这个？

Why is the size of bool 16 Byte? It seems like a total overkill to me. Are there explanations for this?

推荐答案

与载体的其他专业，矢量＆lt;布尔＆GT; 不管理布尔对象的动态数组。相反，它被认为布尔值打包成一个单一的比特的每个

Unlike other specialisations of vector, vector<bool> does not manage a dynamic array of bool objects. Instead, it is supposed to pack the boolean values into a single bit each.

由于各个位不寻址，试验[0] 不能简单地是布尔的参考。相反，它是一个类类型矢量＆lt;布尔＆GT; ::参考可转换为布尔（获取值），并从布尔分配（修改矢量元素）。

Since individual bits are not addressable, test[0] cannot simply be a reference to bool. Instead, it is an class type vector<bool>::reference that can be converted to bool (to get the value), and assigned from bool (to modify the vector element).

这意味着矢量＆lt;布尔＆GT; 不完全符合标准的容器的要求，并不能用于如果需要引用或指针的元素。如果您还需要具有寻址元素的真实的集装箱，考虑矢量＆lt;字符＆GT; 或双端＆LT;布尔＆GT; 而不是

This means that vector<bool> doesn't entirely meet the requirements of a standard container, and can't be used if you need references or pointers to its elements. If you do require a "real" container with addressable elements, consider vector<char> or deque<bool> instead.

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