C++中vector/string的capacity、size、resize、reserve和clear功能详解

川长思鸟来 2022-11-21 11:17 474阅读 0赞

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  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x1b2xhaWh1YTIwMTg_size_16_color_FFFFFF_t_70_pic_center]  
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
    */

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