为什么不是 vector<bool>STL 容器? [英] Why isn't vector<bool> a STL container?

问题描述

Scott Meyers 的书 Effective STL: 50 Specific Ways to改善标准模板库使用的第 18 条 说要避免 vector 因为它不是一个STL 容器，它并没有真正保存 bools.

Item 18 of Scott Meyers's book Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library says to avoid vector <bool> as it's not an STL container and it doesn't really hold bools.

以下代码:

vector <bool> v; 
bool *pb =&v[0];

不会编译，违反 STL 容器的要求.

will not compile, violating a requirement of STL containers.

错误:

cannot convert 'std::vector<bool>::reference* {aka std::_Bit_reference*}' to 'bool*' in initialization

vector::operator [] 返回类型应该是 T&，但为什么它是 vector 的特例??

vector<T>::operator [] return type is supposed to be T&, but why is it a special case for vector<bool>?

vector 究竟由什么组成?

该项目进一步说明:

deque<bool> v; // is a STL container and it really contains bools

这可以用作 vector 的替代品吗?

Can this be used as an alternative to vector<bool>?

谁能解释一下?

推荐答案

出于空间优化的原因，C++ 标准(最早可追溯到 C++98)明确调用了vector作为一个特殊的标准容器，其中每个 bool 只使用一位空间，而不是像普通 bool 那样使用一个字节(实现一种动态位集").作为这种优化的交换，它没有提供普通标准容器的所有功能和接口.

For space-optimization reasons, the C++ standard (as far back as C++98) explicitly calls out vector<bool> as a special standard container where each bool uses only one bit of space rather than one byte as a normal bool would (implementing a kind of "dynamic bitset"). In exchange for this optimization it doesn't offer all the capabilities and interface of a normal standard container.

在这种情况下，因为你不能在一个字节中取一个位的地址，所以诸如 operator[] 之类的东西不能返回 bool& 但是而是返回一个代理对象，该对象允许操作有问题的特定位.由于此代理对象不是 bool&，因此您无法将其地址分配给 bool*，就像在正常"容器.反过来，这意味着 bool *pb =&v[0]; 不是有效代码.

In this case, since you can't take the address of a bit within a byte, things such as operator[] can't return a bool& but instead return a proxy object that allows to manipulate the particular bit in question. Since this proxy object is not a bool&, you can't assign its address to a bool* like you could with the result of such an operator call on a "normal" container. In turn this means that bool *pb =&v[0]; isn't valid code.

另一方面，deque 没有任何这样的特化，所以每个 bool 需要一个字节，你可以从 operator[].

On the other hand deque doesn't have any such specialization called out so each bool takes a byte and you can take the address of the value return from operator[].

最后请注意，MS 标准库的实现(可以说)不是最理想的，因为它对双端队列使用了较小的块大小，这意味着使用双端队列作为替代并不总是正确的答案.

Finally note that the MS standard library implementation is (arguably) suboptimal in that it uses a small chunk size for deques, which means that using deque as a substitute isn't always the right answer.

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