vector：resize和reserve的区别

写在前面

为了区别这两个函数的功能，首先我们需要区别两个属性的概念。

capacity：发生 realloc 前能允许的最大元素数，即预分配的内存空间。预留位置不初始化，元素不可访问。
size：当前 vector 容器真实占用的大小，也就是容器当前拥有多少个元素。可以进行遍历。
capacity >= size。

ps：假设有一瓶容量为500ml的矿泉水，但是现在里面只装了300ml的水，那么capacity即为500，size即为300。

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<iostream>
#include<bitset>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<algorithm>
#include<vector>
#define ll long long
#define PI acos(-1.0)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define esp 1e-5
#define lowbit(x) x & (-x)

using namespace std;

const int maxx = 1e6 + 5;
const int inf = 0x3f3f3f3f;
const ll mod = 998244353;
const ll INF = 0x3f3f3f3f3f3f3f3f;

int n, m, T, N;
int a[3];
int l, r, ans;

int main(){
    
    
	int i, j, k;
	vector<int> v1;
	v1.push_back(1);
	v1.push_back(2);
	v1.push_back(3);
	v1.push_back(4);
	v1.push_back(5);
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;;
	return 0;
}

在这里插入图片描述

二者的区别

resize()：改变size的大小、也改变capacity的大小。容器内的对象内存空间是真正存在的。
reserver()：只改变capacity的大小。容器内的对象并没有真实的内存空间。

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<iostream>
#include<bitset>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<algorithm>
#include<vector>
#define ll long long
#define PI acos(-1.0)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define esp 1e-5
#define lowbit(x) x & (-x)

using namespace std;

const int maxx = 1e6 + 5;
const int inf = 0x3f3f3f3f;
const ll mod = 998244353;
const ll INF = 0x3f3f3f3f3f3f3f3f;

int n, m, T, N;
int a[3];
int l, r, ans;

int main(){
    
    
	int i, j, k;
	vector<int> v1;
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;
	cout << "----------------------" << endl;
	vector<int> v2;
	v2.reserve(100);
	cout << "vector capacity: " << v2.capacity() << endl;
	cout << "vector size: " << v2.size() << endl;
	cout << "----------------------" << endl;
	vector<int> v3;
	v3.resize(100);
	cout << "vector capacity: " << v3.capacity() << endl;
	cout << "vector size: " << v3.size() << endl;
	cout << "----------------------" << endl;
	vector<int> v4;
	v4.push_back(1);
	v4.push_back(2);
	v4.push_back(3);
	v4.push_back(4);
	v4.push_back(5);
	cout << "vector capacity: " << v4.capacity() << endl;
	cout << "vector size: " << v4.size() << endl;
	v4.resize(50);
	cout << "vector capacity: " << v4.capacity() << endl;
	cout << "vector size: " << v4.size() << endl;
	return 0;
}

在这里插入图片描述

注意事项

使用 reserve()之后，此后在使用 [ ] 操作符访问容器内的对象，很可能出现数组越界的问题。要保证迭代范围小于size。
用[ ]访问，vector 退化为数组，不会进行越界的判断。此时推荐使用 at()，会先进行越界检查。
capacity 属性的容器只有 vector 和 string， list、map、set、deque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。
v1.resize(x)，if(x < v1.size() )。则v1.size = x。多于的元素会被丢弃。

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<iostream>
#include<bitset>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<algorithm>
#include<vector>
#define ll long long
#define PI acos(-1.0)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define esp 1e-5
#define lowbit(x) x & (-x)

using namespace std;

const int maxx = 1e6 + 5;
const int inf = 0x3f3f3f3f;
const ll mod = 998244353;
const ll INF = 0x3f3f3f3f3f3f3f3f;

int n, m, T, N;
int a[3];
int l, r, ans;

int main(){
    
    
	int i, j, k;
	vector<int> v1;
	v1.push_back(1);
	v1.push_back(2);
	v1.push_back(3);
	v1.push_back(4);
	v1.push_back(5);
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;
	for(auto i : v1){
    
    
		cout << i << " ";
	}
	cout << endl;
	v1.resize(3);
	for(auto i : v1){
    
    
		cout << i << " ";
	}
	cout << endl;
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;
	return 0;
}

在这里插入图片描述

v1.resize(x)，if(x > v1.size() )。则v1.size = x，添加进来的元素默认用0进行填充。【使用v1.resize(x, y)，则会用y来填充】。

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<iostream>
#include<bitset>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<algorithm>
#include<vector>
#define ll long long
#define PI acos(-1.0)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define esp 1e-5
#define lowbit(x) x & (-x)

using namespace std;

const int maxx = 1e6 + 5;
const int inf = 0x3f3f3f3f;
const ll mod = 998244353;
const ll INF = 0x3f3f3f3f3f3f3f3f;

int n, m, T, N;
int a[3];
int l, r, ans;

int main(){
    
    
	int i, j, k;
	vector<int> v1;
	v1.push_back(1);
	v1.push_back(2);
	v1.push_back(3);
	v1.push_back(4);
	v1.push_back(5);
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;
	for(auto i : v1){
    
    
		cout << i << " ";
	}
	cout << endl;
	v1.resize(7); //默认0填充
	//v1.resize(7, 1)//用1进行填充
	for(auto i : v1){
    
    
		cout << i << " ";
	}
	cout << endl;
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;
	return 0;
}

在这里插入图片描述

v1.resize(x)，if(x > v1.capacity() )。则v1.size = x，添加进来的元素默认用0进行填充，同时会进行内存分配。

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<iostream>
#include<bitset>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<algorithm>
#include<vector>
#define ll long long
#define PI acos(-1.0)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define esp 1e-5
#define lowbit(x) x & (-x)

using namespace std;

const int maxx = 1e6 + 5;
const int inf = 0x3f3f3f3f;
const ll mod = 998244353;
const ll INF = 0x3f3f3f3f3f3f3f3f;

int n, m, T, N;
int a[3];
int l, r, ans;

int main(){
    
    
	int i, j, k;
	vector<int> v1;
	v1.push_back(1);
	v1.push_back(2);
	v1.push_back(3);
	v1.push_back(4);
	v1.push_back(5);
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;
	for(auto i : v1){
    
    
		cout << i << " ";
	}
	cout << endl;
	v1.resize(10);
	for(auto i : v1){
    
    
		cout << i << " ";
	}
	cout << endl;
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;
	return 0;
}

在这里插入图片描述

vector内存分配

vector内存分配机制：当空间不够装下数据时，会自动申请另一片更大的空间（1.5倍或者2倍），然后把原来的数据拷贝到新的内存空间，接着释放原来的那片空间【为了避免不必要的拷贝，每次可以在创建vector对象的时候指定vector的capacity】。

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<iostream>
#include<bitset>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<algorithm>
#include<vector>
#define ll long long
#define PI acos(-1.0)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define esp 1e-5
#define lowbit(x) x & (-x)

using namespace std;

const int maxx = 1e6 + 5;
const int inf = 0x3f3f3f3f;
const ll mod = 998244353;
const ll INF = 0x3f3f3f3f3f3f3f3f;

int n, m, T, N;
int a[3];
int l, r, ans;

int main(){
    
    
	int i, j, k;
	vector<int> v1;
	cout << "vector capacity: " << v1.capacity() << endl;
	cout << "vector size: " << v1.size() << endl;
	int num = 0;
	int *p = NULL;
	for(int i = 0; i < 100000; i++){
    
    
		v1.push_back(i);
		if(p != &v1[0]){
    
    
			p = &v1[0];
			num++;
		}
	}
	cout << "内存分配次数：" << num << endl;
	return 0;
}

在这里插入图片描述

STL：resize和reserve的区别【vector内存分配机制】

vector：resize和reserve的区别

写在前面

二者的区别

注意事项

vector内存分配

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