将向量附加到向量的最佳方法 [英] Best way to append vector to vector

问题描述

std::vector一种;std::vector乙；std::vectorC;

我想通过将 b 和 c 的元素附加到 a 来连接这三个向量.哪种方法最好，为什么?

<小时>

1) 通过使用 vector::insert:

a.reserve(a.size() + b.size() + c.size());a.insert(a.end(), b.begin(), b.end());a.insert(a.end(), c.begin(), c.end());b.清除()；c.清除()；

2) 通过使用 std::copy:

a.reserve(a.size() + b.size() + c.size());std::copy(b.begin(), b.end(), std::inserter(a, a.end()));std::copy(c.begin(), c.end(), std::inserter(a, a.end()));b.清除()；c.清除()；

3) 通过使用 std::move(来自 C++11):

a.reserve(a.size() + b.size() + c.size());std::move(b.begin(), b.end(), std::inserter(a, a.end()));std::move(c.begin(), c.end(), std::inserter(a, a.end()));b.清除()；c.清除()；

解决方案

在我看来，你的第一个解决方案是最好的方法.

vector<>::insert 旨在添加元素，因此它是最合适的解决方案.

您可以在目标向量上调用 reserve 以保留一些空间，但除非您将大量向量加在一起，否则它可能不会提供太多好处:vector<>::insert 知道将添加多少元素，您将避免只调用一次 reserve.

注意:如果那些是更复杂类型的vector(即自定义类，甚至std::string)，那么使用std::move 可以为您提供很好的性能提升，因为它可以避免复制构造函数.然而，对于 int 的向量，它不会给你任何好处.

注意 2:值得一提的是，使用 std::move 会导致源 vector 的内容无法使用.

std::vector<int> a;
std::vector<int> b;
std::vector<int> c;

I would like to concatenate these three vectors by appending b's and c's elements to a. Which is the best way to do this, and why?

---

1) By using vector::insert:

a.reserve(a.size() + b.size() + c.size());
a.insert(a.end(), b.begin(), b.end());
a.insert(a.end(), c.begin(), c.end());
b.clear();
c.clear();

2) By using std::copy:

a.reserve(a.size() + b.size() + c.size());
std::copy(b.begin(), b.end(), std::inserter(a, a.end()));
std::copy(c.begin(), c.end(), std::inserter(a, a.end()));
b.clear();
c.clear();

3) By using std::move (from C++11):

a.reserve(a.size() + b.size() + c.size());
std::move(b.begin(), b.end(), std::inserter(a, a.end()));
std::move(c.begin(), c.end(), std::inserter(a, a.end()));
b.clear();
c.clear();

解决方案

In my opinion, your first solution is the best way to go.

vector<>::insert is designed to add element so it's the most adequate solution.

You could call reserve on the destination vector to reserve some space, but unless you add a lot of vector together, it's likely that it wont provide much benefits: vector<>::insert know how many elements will be added, you will avoid only one reserve call.

Note: If those were vector of more complex type (ie a custom class, or even std::string), then using std::move could provide you with a nice performance boost, because it would avoid the copy-constructor. For a vector of int however, it won't give you any benefits.

Note 2: It's worth mentioning that using std::move will cause your source vector's content to be unusable.

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