java string对象上的操作,常见的用法你知道吗

	char str[100] = "I love China";

	#include <string>
	
	string s1; //默认初始化, s1 = "", 代表一个空串,里边没有字符
	string s2 = "I love China";  //把字符串拷贝到s2代表的一段内存中, 拷贝时不包括末尾的'\0'.
	string s3("I love China"); //另一种写法, 同2
	string s4 = s2;  //拷贝s2到s4中, s2,s4两块不同内存
	
	int num = 6;
	string s5(num, 'a'); //aaaaaa  , s5初始化为连续num个字符'a'
	//这种方式不推荐, 因为会在系统内部创建临时对象

	string s1;
	if (s1.empty())
	{
		cout << "s1为空" << endl;
	}

	string s1;
	cout << s1.size() << endl;       //0
	cout << s1.length() << endl;   //0

	string s2 = "I love China";
	cout << s2.size() << endl;      //12
	cout << s2.length() << endl;  //12
	
	string s3 = "你好";
	cout << s3.size() << endl;      //4
	cout << s3.length() << endl;  //4

	string s3 = "I love China";
	if (s3.size() > 4)
	{
		cout << s3[4] << endl;
		s3[4] = 'w';
	}
	cout << s3 << endl;
	//输出
	//v
	//I lowe China

	string s1 = "abcd";
	string s2 = "hijk";
	cout << s1 + s2 << endl;  //abcdhijk

	string s1 = "abcd";
	string s2 = "ab";
	s1 = s2;
	cout << s1 << endl;
	//输出
	//ab

	string s1 = "ab";
	string s2 = "ab";
	if (s1 == s2)
	{
		cout << "相等" << endl;
	}

	string s1 = "abc";

	//返回"abc"的内存指针
	const char* p = s1.c_str();
	char str[100];
	strcpy_s(str, sizeof(str), p);
	cout << str << endl;

	string s1 = "abc";
	string s2 = "abC";

	cout << s1 + " and " + s2 + 'D' << endl;//abc and abCD

	string s1 = "I love China";
	for (auto c : s1)    //char  auto自动推断
	{
		cout << c;   //I love China
	}

	string s1 = "I love China";
	for (auto &c : s1)
	{
		//toupper() 小写转大写, 大写没变化  
		//tolower() 大转小
		c = toupper(c);  //因为c是一个引用, 所以这相当于改变s1中的值
	}
	cout << s1;   //I LOVE CHINA