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vector<int>c;
c.capacity()返回容器中数据个数,翻倍增长。
1.代码
#include <iostream>
2 #include <vector>
3 using namespace std;
4 int main()
5 {
6 vector<int>v;
7 v.push_back(1);
8 cout<<"capacity == "<<v.capacity()<<endl;
9
10 v.push_back(2);
11 cout<<"capacity == "<<v.capacity()<<endl;
12
13 v.push_back(3);
14 cout<<"capacity == "<<v.capacity()<<endl;
15 return 0;
16 }
结果:
capacity == 1
capacity == 2
capacity == 4
2.压缩一个臃肿的vector
很多时候大量的删除数据,或者通过使用reserver(),结果vector的空间远远大于实际的需要。所以需要压缩vector到它的实际大小。resize()能增加vector的大小。clear()仅仅移除容器内的数据,不能改变capacity()的大小,所以对vector进行压缩非常重要。
(1)使用capacity()和clear()来操作一个vector
vector<int>v;
v.push_back(1);
cout<<"capacity == "<<v.capacity()<<endl;
v.push_back(2);
cout<<"capacity == "<<v.capacity()<<endl;
v.push_back(3);
cout<<"capacity == "<<v.capacity()<<endl;
v.clear();
cout<<"clear()完之后: "<<v.size()<<endl;
cout<<"clear()完之后的capacity == "<<v.capacity()<<endl;
结果:
capacity == 1
capacity == 2
capacity == 4
clear()完之后: 0
clear()完之后的capacity == 4
(2)通过上面的结果可以发下如下两个特点:①capacity()返回容器中数据个数,翻倍增长。②当清除完vector后capacity()仍然是4。
(3)下面定义一个新的vector,将原来的赋值给这个新的vector,看看会出现什么样的效果。
vector<int>v;
v.push_back(1);
cout<<"capacity == "<<v.capacity()<<endl;
v.push_back(2);
cout<<"capacity == "<<v.capacity()<<endl;
v.push_back(3);
cout<<"capacity == "<<v.capacity()<<endl;
vector<int>v1(v);
cout<<"新的vector的capacity == "<<v1.capacity()<<endl;
结果:
capacity == 1
capacity == 2
capacity == 4
新的vector的capacity == 3
(3)通过上面的结果发现,新的数组的容量是元素的个数3。所以可以使用这种方法来达到压缩vector。接下来可以使用函数swap()进行优化。
vector<int>v;
v.push_back(1);
cout<<"capacity == "<<v.capacity()<<endl;
v.push_back(2);
cout<<"capacity == "<<v.capacity()<<endl;
v.push_back(3);
cout<<"capacity == "<<v.capacity()<<endl;
vector<int>v1(v);
cout<<"新的vector的capacity == "<<v1.capacity()<<endl;
v.swap(v1);
cout<<"交换后原来vector的capacity == "<<v.capacity()<<endl;
结果:
capacity == 1
capacity == 2
capacity == 4
新的vector的capacity == 3
交换后原来vector的capacity == 3
(4)再次进行优化。采用匿名对象来代替v1这个中间对象:vector<int> (v).swap(v);
vector<int>v;
v.push_back(1);
cout<<"capacity == "<<v.capacity()<<endl;
v.push_back(2);
cout<<"capacity == "<<v.capacity()<<endl;
v.push_back(3);
cout<<"capacity == "<<v.capacity()<<endl;
vector<int> (v).swap(v);
cout<<"交换后vector的capacity == "<<v.capacity()<<endl;
结果:
capacity == 1
capacity == 2
capacity == 4
交换后vector的capacity == 3
注:C++11中已经提供了shrink_to_fit()函数实现vector的压缩(你希望调用shrink_to_fit来将多余内存还给系统,但标准库不保证一定可以达到你的目的,类似于inline来实现函数内联)。
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