C ++向量的初始容量 [英] C++ Vector initial capacity

问题描述

我有一些使用数千个向量的代码，每个向量只有4个条目，因此我想将每个向量的初始大小设置为4，以便通过不保留未使用的内存来优化内存使用。

I have some code which uses thousands of vectors each vector has only 4 entries, So I want to set the initial size of each vector to 4 so that I can optimize memory usage by not reserving unused memory.

我尝试了保留方法：

vector<Foo> bar;
bar.reserve(10);

但似乎它在扩展而不在收缩，似乎也没有构造函数创建带有

but seems it expands and doesn't shrink, seems there also no constructor that creates a vector with a specified capacity.

还有2个额外的问题：

默认的初始容量是多少

我可以创建具有特定容量的矢量吗？

Can I create a vector with a specific capacity?

推荐答案

向量的容量不能由构造函数控制-没有适用的重载。

The capacity of a vector cannot be controlled by the constructors - there is no applicable overload.

C ++标准不能保证默认构造函数的容量。向量 vector< Foo>酒吧； 。但是，所有众所周知的实现都使用0作为默认容量。这是您可以依靠的东西，因为那时分配内存根本没有意义。

The C++ standard doesn't give any guarantee about the capacity of a default-constructed vector vector<Foo> bar;. However all well-known implementations use 0 as the default capacity. This is something you can rely on, as allocating memory at that point just doesn't make sense.

所以我相信您的答案问题是：只需使用

So I believe the answer to your question is: just use

vector<Foo> bar;
bar.reserve(4);

这篇关于C ++向量的初始容量的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！