std :: vector :: assign/std :: vector :: operator =(const&)是否保证重用`this`中的缓冲区? [英] Does std::vector::assign/std::vector::operator=(const&) guarantee to reuse the buffer in `this`?

问题描述

如果我将一个向量分配或复制到另一个向量(其容量与前者的大小相同或更大)，我是否可以假定后者的缓冲区将被重用?

If I assign or copy one vector to another (that has the same or bigger capacity than the size of the former), can I assume that the buffer of the latter will be reused?

以下示例说明了我可以，但是，标准可以保证吗? 在这方面std::vector::assign和std::vector::operator=的行为是否有区别?

The following example demonstrates that I can, however, is it guaranteed by the standard? Is there any difference between behaviour of std::vector::assign and std::vector::operator= in this regard?

#include <vector>
#include <iostream>
#include <cassert>

int main()
{
    std::vector a {1, 2, 3, 4, 5};
    std::vector b {1, 2, 3, 4};
    std::vector c {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    std::cout << "1 ==== " << a.capacity() << " " << a.data() << std::endl;

    const auto* pa = a.data();   
    a = b;
    assert(pa == a.data());
    std::cout << "2 ==== " << a.capacity() << " " << a.data() << std::endl;

    a = c;
    assert(pa != a.data());
    std::cout << "3 ==== " << a.capacity() << " " << a.data() << std::endl;
}

在线示例.

更新:此答案提到了

void assign(size_type n, const T& t);

等同于

erase(begin(), end());
insert(begin(), n, t);

该标准是否真的以此方式制定，并且是否适用于std::vector::assign的所有重载?

Does the standard really formulate it this way and does it apply to all overloads of std::vector::assign?

推荐答案

简短答案

否.

标准未在vector上手动定义这些操作.它仅将它们定义为容器的要求. [vector] 说

The standard does not manually define these operations on vector. It only defines them as a requirement for containers. [vector] says

向量满足序列容器的容器和可逆容器(在[container.requirements]中的两个表中给出)的所有要求，包括大多数可选的序列容器要求([sequence.reqmts] )，可识别分配器的容器(表67)，以及对于非bool的元素类型，可使用连续容器.没有提供push_front，pop_front和emplace_front成员函数.这里仅提供对矢量的操作的描述，这些操作在这些表之一中未描述，或者仅在存在附加语义信息的操作中提供.

A vector satisfies all of the requirements of a container and of a reversible container (given in two tables in [container.requirements]), of a sequence container, including most of the optional sequence container requirements ([sequence.reqmts]), of an allocator-aware container (Table 67), and, for an element type other than bool, of a contiguous container. The exceptions are the push_­front, pop_­front, and emplace_­front member functions, which are not provided. Descriptions are provided here only for operations on vector that are not described in one of these tables or for operations where there is additional semantic information.

仅提及这些操作的地方是容器要求和序列容器要求.没有任何证据支持您的假设.

The only places where these operation are mentioned are Container requirements and Sequence container requirements. Nothing supports your assumption.

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