string------构造

 string s

 string s(s2)

 string s("value")

 string s(n, 'c')

 string s(b, e)

 string s(cp, n)

 string(s2,  pos2)

 string(s2, pos2, len2)

#include <iostream>
#include <string>

using namespace std;

int main()
{
	string s1;
	cout<<s1<<endl;

	string s2(5,'a');
	cout<<s2<<endl;

	string s3(s2);
	cout<<s3<<endl;

	string s4(s3.begin(), s3.begin()+s3.size()/2);
	cout<<s4<<endl;


	char *cp = "Hiya";
	char c_array[] = "World!!!";
	char no_null[] = {'H', 'i'};
	char no_null1[] = {'H', 'i', '\0'};

	string ss1(cp);
	cout<<ss1<<endl;

	string ss2(c_array, 5);
	cout<<ss2<<endl;

	string ss3(c_array+5, 3 );
	cout<<ss3<<endl;

	string ss4(no_null);
	cout<<ss4<<endl;
	string ss5(no_null, 2);
	cout<<ss5<<endl;
	string ss6(no_null);
	cout<<ss6<<endl;
	
	s1 = "Hiya";
	cout<<s1<<endl;
	
	string s5(s1, 2);
	cout<<s5<<endl;
	string s7(s1, 0, 2);
	cout<<s7<<endl;

	string s8(s1,0,8);
	cout<<s8<<endl;

	return 0;
}