STL之vector那点事

hg2016

已于 2022-07-04 20:32:10 修改

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分类专栏： c++ 文章标签： STL

于 2017-08-15 10:20:46 首次发布

本文链接：https://blog.youkuaiyun.com/Redmoon955331/article/details/77184058

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1.resize和reserve的区别和联系

相同点：两者都只会让vector的capacity不减小，参数为newsize时，当newsize小于capacity时，调用后二者的capacity都不变

不同点：当newsize小于capacity时，调用后resize析构多余的元素并且改变size的大小个空间，并没有释放空间，只是没有；而reverse不会改变size 也不会析构元素

2.resize reserve在增容时的反应

//resize reserve在增容时的反应

void test()
{
	vector<int> a;
	a.push_back(1);
	cout<<"capacity:"<<a.capacity()<<endl;
	cout<<"size:"<<a.size()<<endl;

	a.resize(10);
	cout<<"resize 10  "<<"capacity:"<<a.capacity()<<endl;
	cout<<"size:"<<a.size()<<endl;

	a.reserve(20);
	cout<<"reserve 20  "<<"capacity:"<<a.capacity()<<endl;
	cout<<"size:"<<a.size()<<endl;

	a.resize(1);
	cout<<"resize 1  "<<"capacity:"<<a.capacity()<<endl;
	cout<<"size:"<<a.size()<<endl;

	a.reserve(20);
	cout<<"reserve 20  "<<"capacity:"<<a.capacity()<<endl;
	cout<<"size:"<<a.size()<<endl;
}

3.vector每次增容多少

//每一次增容都增加1/2

void test_push()
{
	vector<int> v;
	v.push_back(1);
	cout<<"size:"<<v.size()<<endl;
	cout<<"capacity:"<<v.capacity()<<endl;
	v.push_back(1);
	cout<<"size:"<<v.size()<<endl;
	cout<<"capacity:"<<v.capacity()<<endl;
	v.push_back(1);
	cout<<"size:"<<v.size()<<endl;
	cout<<"capacity:"<<v.capacity()<<endl;
	v.push_back(1);
	cout<<"size:"<<v.size()<<endl;
	cout<<"capacity:"<<v.capacity()<<endl;
	v.push_back(1);
	cout<<"size:"<<v.size()<<endl;
	cout<<"capacity:"<<v.capacity()<<endl;
	v.push_back(1);
	cout<<"size:"<<v.size()<<endl;
	cout<<"capacity:"<<v.capacity()<<endl;
	v.push_back(1);
	cout<<"size:"<<v.size()<<endl;
	cout<<"capacity:"<<v.capacity()<<endl;
	v.push_back(1);
	cout<<"size:"<<v.size()<<endl;
	cout<<"capacity:"<<v.capacity()<<endl;
}

4.如何强制释放空间

void testdelete()
{
	vector<int> v;
	v.push_back(1);
	v.push_back(1);
	v.push_back(1);
	v.push_back(1);
	v.push_back(1);
	//强制释放v的空间
	vector<int> t;
	v.swap(t);
	cout<<t.size()<<endl;
	cout<<v.size()<<endl;

	//强制释放t的空间
	vector<int>().swap(t);
	cout<<t.size()<<endl;

	//注意即使使用clear也不会真的释放空间
	vector<int> j;
	j.push_back(1);
	j.push_back(1);
	j.push_back(1);
	j.clear();
	cout<<j.size()<<endl;
	cout<<j.capacity()<<endl;
}