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size() – 返回目前存在的元素数。即: 元素个数
capacity() – 返回容器能存储 数据的个数。 即:容器容量
reserve() --设置 capacity 大小
resize() --设置 size ,重新指定有效元素的个数 ,区别与reserve()指定 容量的大小
clear() --清空所有元素,把size设置成0,capacity不变
capacity 一般大于size的原因是为了避免 每次增加数据时都要重新分配内存,所以一般会 生成一个较大的空间,以便随后的数据插入。
size是当前vector容器真实占用的大小,也就是容器当前拥有多少个容器。
capacity是指在发生realloc前能允许的最大元素数,即预分配的内存空间。
这两个属性分别对应两个方法:resize()和reserve()。
使用resize(),容器内的对象内存空间是真正存在的。
使用reserve()仅仅只是修改了capacity的值,容器内的对象并没有真实的内存空间(空间是"野"的)。
此时切记使用[]操作符访问容器内的对象,很可能出现数组越界的问题。
针对capacity这个属性,STL中的其他容器,如list map set deque,由于这些容器的内存是散列分布的,因此不会发生类似realloc()的调用情况,因此我们可以认为capacity属性针对这些容器是没有意义的,因此设计时这些容器没有该属性。
在STL中,拥有capacity属性的容器只有vector和string。
#include <iostream>
#include<vector>
using namespace std;
using std::cout;
using std::cin;
using std::endl;
int main()
{
vector<int> ivec;
cout << "capacity: " << ivec.capacity() << " size: " << ivec.size() << endl;
//添加10个元素
for (int i = 0; i < 10; ++i)
{
ivec.push_back(i);
cout << "capacity: " << ivec.capacity() << " size: " << ivec.size() << endl;
}
//将容量用完
while (ivec.size() != ivec.capacity())
ivec.push_back(0);
//添加1个元素
cout << "size = capacity. insert one element\n";
ivec.push_back(0);
cout << "capacity:" << ivec.capacity() << " size:" << ivec.size() << endl;
ivec.reserve(100);
cout << "reserve capacity 100\n";
cout << "capacity:" << ivec.capacity() << " size:" << ivec.size() << endl;
//将容量用完
while (ivec.size() != ivec.capacity())
ivec.push_back(42);
//添加1个元素
cout << "size = capacity. insert one element\n";
ivec.push_back(0);
cout << "capacity:" << ivec.capacity() << " size:" << ivec.size() << endl;
ivec.resize(50);
cout << "resize size 50\n";
cout << "capacity:" << ivec.capacity() << " size:" << ivec.size() << endl;
getchar();
return 0;
}
/*
capacity: 1 size: 1
capacity: 2 size: 2
capacity: 3 size: 3
capacity: 4 size: 4
capacity: 6 size: 5
capacity: 6 size: 6
capacity: 9 size: 7
capacity: 9 size: 8
capacity: 9 size: 9
capacity: 13 size: 10
size = capacity. insert one element
capacity:19 size:14
reserve capacity 100
capacity:100 size:14
size = capacity. insert one element
capacity:150 size:101
*/
vector<int> nums = { 5,6,22,1,4,-5,0,88,6,4,99,8,7 };
cout << "初始数据:" << endl;
cout <<"size = "<< nums.size() << endl;
cout <<"capacity = "<< nums.capacity() << endl;
for (auto n : nums) {
cout << n <<" ";
}
cout <<endl<<endl<< "resize 10:" << endl;
nums.resize(10);
for (auto n : nums) {
cout << n << " ";
}
cout <<endl<< "size = " << nums.size() << endl;
cout << "capacity = " << nums.capacity() << endl;
cout << endl << "resize 20:" << endl;
nums.resize(20);
for (auto n : nums) {
cout << n << " ";
}
cout << "" << endl;
cout << "size = " << nums.size() << endl;
cout << "capacity = " << nums.capacity() << endl;
//reserve 是保留的意思,reserve的值小于size也不会使元素消失,此时的capacity==size
cout << endl << "reserve 6:" << endl;
nums.reserve(6);//capacity == size == 20
for (auto n : nums) {
cout << n << " ";
}
cout << "" << endl;
cout << "size = " << nums.size() << endl;
cout << "capacity = " << nums.capacity() << endl;
cout << endl << "reserve 30:" << endl;
nums.reserve(30);//使得capacity=30,里面的元素不会改变
for (auto n : nums) {
cout << n << " ";
}
cout << "" << endl;
cout << "size = " << nums.size() << endl;
cout << "capacity = " << nums.capacity() << endl;
//clear是清空,不是置零,清空所有元素,所以size为0,但是capacity不变,
cout << endl << "clear:" << endl;
nums.clear();
cout << nums.size() << endl;
cout << nums.capacity() << endl;
return 0;
/*
*
初始数据:
size = 13
capacity = 13
5 6 22 1 4 -5 0 88 6 4 99 8 7
resize 10:
5 6 22 1 4 -5 0 88 6 4
size = 10
capacity = 13
resize 20:
5 6 22 1 4 -5 0 88 6 4 0 0 0 0 0 0 0 0 0 0
size = 20
capacity = 20
reserve 6:
5 6 22 1 4 -5 0 88 6 4 0 0 0 0 0 0 0 0 0 0
size = 20
capacity = 20
reserve 30:
5 6 22 1 4 -5 0 88 6 4 0 0 0 0 0 0 0 0 0 0
size = 20
capacity = 30
clear:
0
30
*/
本文标签: 详解功能CapacityStringVector
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