如何使用vector的reserve和resize方法

reserve和resize是vector里两个很重要的方法，有效地使用这两个方法可以减少reallocate memory的次数，提高程序的性能，所以还是有必要去研究一下的，先来看一段简单的代码吧。

stdafx.h

// stdafx.h : include file for standard system include files,
// or project specific include files that are used frequently, but
// are changed infrequently
//

#pragma once

#include "targetver.h"

#include 
#include 



// TODO: reference additional headers your program requires here
#include 
#include 
#include 
using namespace std;

 test.cpp

// test.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"

class A
{
public:
	A() : m_a(0)
	{
	}

	A(int a) : m_a(a)
	{
	}

	A(const A& a) : m_a(a.m_a)
	{
	}

	~A()
	{
	}

private:
	int m_a;
};

int _tmain(int argc, _TCHAR* argv[])
{
	vector myVec;
	myVec.reserve(100);
	cout << "capacity:" << myVec.capacity() << endl;
	cout << "size:" << myVec.size() << endl;

	for (int i = 0; i < 100; i++)
	{
		myVec.push_back(i);
	}
	cout << "capacity:" << myVec.capacity() << endl;
	cout << "size:" << myVec.size() << endl;

	myVec.resize(102);
	myVec[100] = 1;
	myVec[101] = 2;
	cout << "capacity:" << myVec.capacity() << endl;
	cout << "size:" << myVec.size() << endl;

	return 0;
}

 输出：

reserve用来（预留空间，）改变capacity，不改变size，会去分配内存，但不会构造出对象；如果改变后的capacity比当前capacity大，则capacity会变大；反之，capacity不变。可以用下面的代码去测试：

vector myVec;
myVec.reserve(100);
cout << "capacity:" << myVec.capacity() << endl;
cout << "size:" << myVec.size() << endl;

myVec.reserve(90);
cout << "capacity:" << myVec.capacity() << endl;
cout << "size:" << myVec.size() << endl;

myVec.reserve(110);
cout << "capacity:" << myVec.capacity() << endl;
cout << "size:" << myVec.size() << endl;

输出：

resize用来改变vector的size，有可能也会改变capacity。如果改变后的size比当前capacity大，则capacity会变大，同时构造出多出来的对象；反之，capacity不变，同时析构一些不再需要的对象。可以用下面的代码去测试：

vector myVec;
myVec.resize(100);
cout << "capacity:" << myVec.capacity() << endl;
cout << "size:" << myVec.size() << endl;

myVec.resize(90);
cout << "capacity:" << myVec.capacity() << endl;
cout << "size:" << myVec.size() << endl;

myVec.resize(110);
cout << "capacity:" << myVec.capacity() << endl;
cout << "size:" << myVec.size() << endl;

输出：

reserve和resize都不会使capacity变小，但都有可能使capacity变大，具体怎么变大，reserve和resize是不一样的，reserve能准确控制capacity；而resize不能，vc里是每次增大一半的当前capacity。可以用下面的代码去测试不用reserve和resize的情况（在这种情况下，每当capacity不够时，就会去allocate一块新的够大的内存，再释放以前的内存，效率是很低的）：

vector myVec;
cout << "capacity:" << myVec.capacity() << endl;
cout << "size:" << myVec.size() << endl;

for (int i = 0; i < 100; i++)
{
	myVec.push_back(i);
	cout << "capacity:" << myVec.capacity() << endl;
	cout << "size:" << myVec.size() << endl;
}

输出：