为什么vector直接赋值会比initializer_list快？

2023-02-13 00:55:52 226 阅读 AI 写同款 · GEO 优化AI 选题数据监测一键分发 ›

c++11初始化列表（initializer_list）目录写在前面详述 initializer_list部分源码方法使用参考文献写在前面 c++11标准中，initializer_list对于标准库的编写影响非常大，说一个简单的例子，在c++11之前，max函数的源程序是这样的： template &lt;class T&gt; const T&amp; max (const T&amp; a, const T&... 阅读详情

#include <iostream>
#include <vector>
#include <map>
#include <list>
#include <ctime>
using namespace std;

const static int  max = 100000;

class FooVector {
public:
	FooVector(initializer_list<string> l)
	{
		auto it = l.begin();
		for (int i=0;i<l.size();++i,++it)
		{
			v.push_back(*it);
		}
	}
	FooVector(vector<string> w) {
		v = w;
	}
private:
		vector<string> v;

};


void test01()
{
	vector<int> vec;
	clock_t startTime = clock();
	for (int i = 0; i < max; i++)
	{
		FooVector t{ "123","123213","12332" };
	}
	clock_t endTime = clock();
	cout << "vector-initializer_list The test01 time is: " << (double)(endTime - startTime) << "ms" << endl;
}

void test02()
{
	vector<int> vec;
	clock_t startTime = clock();
	vector<string> l{ "123","123213","12332" };
	for (int i = 0; i < max; i++)
	{
		
		FooVector t(l);
	}
	clock_t endTime = clock();
	cout << "vector-vector The test02 time is: " << (double)(endTime - startTime) << "ms" << endl << endl;
}


class FooMap
{
	std::map<int, int> content_;
	using pair_t = std::map<int, int>::value_type;

public:
	FooMap(std::initializer_list<pair_t> list)
	{
		for (auto it = list.begin(); it != list.end(); ++it)
		{
			content_.insert(*it);
		}
	}
	FooMap(std::map<int, int> m) { content_ = m; }
};

void test03()
{
	clock_t startTime = clock();
	for (int i = 0; i < max; i++)
	{

		FooMap t = { { 1, 2 }, { 3, 4 }, { 5, 6 } };
	}
	clock_t endTime = clock();
	cout << "map-initializer_list The test03 time is: " << (double)(endTime - startTime) << "ms" << endl;
}
void test04()
{
	clock_t startTime = clock();
	std::map<int, int> q;
	q[1] = 2;
	q[3] = 4;
	q[5] = 6;
	for (int i = 0; i < max; i++)
	{
		FooMap t(q);
	}
	clock_t endTime = clock();
	cout << "map-map The test04 time is: " << (double)(endTime - startTime) << "ms" << endl << endl;
}

class FooList
{
	std::list<string> ls;

public:
	FooList(std::initializer_list<string> list)
	{
		for (auto it = list.begin(); it != list.end(); ++it)
		{
			ls.push_back(*it);
		}
	}
	FooList(std::list<string> l) { ls = l; }
};

void test05()
{
	vector<int> vec;
	clock_t startTime = clock();
	for (int i = 0; i < max; i++)
	{
		FooList t= { "wqeersf","ewewew","qwe" };
	}
	clock_t endTime = clock();
	cout << "list-initializer_list The test05 time is: " << (double)(endTime - startTime) << "ms" << endl;
}

void test06()
{
	vector<int> vec;
	clock_t startTime = clock();
	std::list<string> l;
	l.push_back("wqeersf");
	l.push_back("ewewew");
	l.push_back("qwe");
	for (int i = 0; i < max; i++)
	{
		FooList t(l);
	}
	clock_t endTime = clock();
	cout << "list-list The test06 time is: " << (double)(endTime - startTime) << "ms" << endl << endl;
}

int main()
{
	test01();
	test02();

	test03();
	test04();

	test05();
	test06();

	return 0;
}

很有意思的一个现象。代码没用end，是因为end比size慢，其实本质不是标题说的那样initializer_list不快，本质是initializer_list如果我们仅仅传递参数，里面不做任何事，initializer_list肯定是最快的，因为用vector对象传递参数，会调用vector的拷贝构造，然后函数体再调用赋值运算符。这里的问题其实是为什么vector直接赋值会比push_back快，不确定vector的源码中的赋值运算符怎么实现的。
我简单查了下，vector中能够赋值的操作。swap > copy > assign > 直接赋值 > push_back赋值

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