关于vector的 operator[]、at()与迭代器访问

一，边界检查

operator[] 不做边界检查，程序会直接退出。

	try
	{
		vecInt[3] = 5;
	}
	catch (const std::exception&e)
	{
		cerr <<"error:"<< e.what() << endl;
	}

运行以上代码，程序会终止：

 

at()会做边界检查，可以用try catch捕获异常，程序可以不直接退出。

vector<int> vecInt(2, 1);

	try
	{
		vecInt.at(3) = 5;
	}
	catch (const std::exception&e)
	{
		cerr <<"error:"<< e.what() << endl;
	}

运行以上代码，程序可以捕获异常：

 

二，执行效率

（1），operatot []

int main(int argc, char * argv[])
{
	vector<int> vecInt(1000000, 1);
	clock_t start, finish;
	start = clock();
	for (int i = 0; i < vecInt.size(); i++)
	{
		vecInt[i] = 3;
	}


	finish = clock();
	double totalTime = (double)(finish - start) / CLOCKS_PER_SEC;
	cout << "此程序运行：" << totalTime << "秒" << endl;
	if (waitKey() == 27)
		return 0;
}

(2),at（）

int main(int argc, char * argv[])
{
	vector<int> vecInt(1000000, 1);
	clock_t start, finish;
	start = clock();
	for (int i = 0; i < vecInt.size(); i++)
	{
		vecInt.at(i) = 3;
	}

	finish = clock();
	double totalTime = (double)(finish - start) / CLOCKS_PER_SEC;
	cout << "此程序运行：" << totalTime << "秒" << endl;
	if (waitKey() == 27)
		return 0;
}

 

（3），迭代器

 

int main(int argc, char * argv[])
{
	vector<int> vecInt(1000000, 1);
	clock_t start, finish;
	start = clock();

	vector<int>::iterator startIter = vecInt.begin();
	vector<int>::iterator endIter = vecInt.end();
	for (; startIter != endIter; ++startIter) {
		*startIter = 3;
	}

	finish = clock();
	double totalTime = (double)(finish - start) / CLOCKS_PER_SEC;
	cout << "此程序运行：" << totalTime << "秒" << endl;
	if (waitKey() == 27)
		return 0;
}