构建一个字符串对象的方法很多，string 类中有多个重载的构造函数用于初始化一个字符串类对象。

迭代器读取字符串 

#include<iostream>
using namespace std;
#include<string>
int main()
{
	string line("123456789");
	cout << string(line.begin(), line.end()) << endl;//正向迭代器
	cout << string(line.rbegin(), line.rend()) << endl;//反向迭代器
	//定义迭代器
	string::iterator it;
	for (it = line.begin(); it != line.end(); it++)
	{
		cout << *it;
	}
	cout << endl;
	string::reverse_iterator it1;
	for (it1 = line.rbegin(); it1 != line.rend(); it1++)
	{
		cout << *it1;
	}
	return 0;
}

 字符串容量：

1. size 和 length 返回字符串的长度；
2. empty 判断字符串是否为空
3. clear 清空字符串
4. max_size 返回字符串的最大长度
5. resize 修改字符串的长度，不重新分配空间
6. capacity 返回不重新分配内存的最大字符数
7. reserve 增加字符串空间

#include<iostream>
using namespace std;
#include<string>
int main()
{
	string str("123456789");
	str.reserve(100);//重新分配空间保留字符数
	cout << str.capacity() << endl;
	int size = str.size();
	int length = str.length();
	unsigned long maxsize = str.max_size();
	cout << "capacity = " << str.capacity() << endl;
	cout << "size = " << str.size() << endl;
	cout << "length = " << str.length() << endl;
	cout << "maxsize = " << str.max_size() << endl;
	if (!str.empty())
	{
		cout << "str = " << str << endl;
	}



	return 0;
}

修改字符串：

1. assign 给字符串赋值
2. operator+= 追加字符串
3. append 追加字符串
4. push_back 追加单个字符
5. insert 插入字符串
6. erase 删除部分字符串
7. replace 替换部分字符串
8. swap 两个字符串交换内容 

 -》assign（） 

#include<iostream>
using namespace std;
#include<string>
int main()
{
	string str("123456789");
	string str2;
	str2.assign(str);//将str的值拷贝到str2中
	cout << "str2 = " << str2 << endl;
	str2.assign(str, 6, 2);
	cout << "str2 = " << str2 << endl;
	str2.assign(5, 'x');
	cout << "str2 = " << str2 << endl;




	return 0;
}

 -》append

#include<iostream>
using namespace std;
#include<string>
int main()
{
	string str("123456789");
	string str2 = "qwert";
	str.append(str2);
	cout << "str = " << str << endl;
	str.append(str2, 2, 3);
	cout << "str = " << str << endl;
	str.append("zxc", 0, 2);
	cout << "str = " << str << endl;

	str.push_back('P');
	cout << "str = " << str << endl;
	str += "vbn";
	cout << "str = " << str << endl;


	




	return 0;
}

->insert 和erase

#include<iostream>
using namespace std;
#include<string>
int main()
{
	string str("0123456789");
	string str2 = "abce";
	cout << "str = " << str << endl;
	str.erase(5);
	cout << "str = " << str << endl;
	str.erase(2, 3);
	cout << "str = " << str << endl;
	str2.insert(2, str);
	cout << "str2 =" << str2 << endl;
	str2.insert(2, "西湖大学");
	cout << "str2 =" << str2 << endl;


	return 0;
}