c++中类似于java中equals的方法

#include <string>
bool operator==(const string& c1, const string& c2);
bool operator!=(const string& c1, const string& c2);
bool operator<(const string& c1, const string& c2);
bool operator>(const string& c1, const string& c2);
bool operator<=(const string& c1, const string& c2);
bool operator>=(const string& c1, const string& c2);
string operator+(const string& s1, const string& s2 );
string operator+(const char* s, const string& s2 );
string operator+( char c, const string& s2 );
string operator+( const string& s1, const char* s );
string operator+( const string& s1, char c );
ostream& operator<<( ostream& os, const string& s );
istream& operator>>( istream& is, string& s );
string& operator=( const string& s );
string& operator=( const char* s );
string& operator=( char ch );
char& operator[]( size_type index );