find和find_first_of的区别

find函数的声明：
size_t find ( const string& str, size_t pos = 0 ) const;
size_t find ( const char* s, size_t pos, size_t n ) const;
size_t find ( const char* s, size_t pos = 0 ) const;
size_t find ( char c, size_t pos = 0 ) const;

find_first_of函数的声明：
size_t find_first_of ( const string& str, size_t pos = 0 ) const;
size_t find_first_of ( const char* s, size_t pos, size_t n ) const;
size_t find_first_of ( const char* s, size_t pos = 0 ) const;
size_t find_first_of ( char c, size_t pos = 0 ) const;

find函数寻找完整匹配，find_first_of函数寻找任一匹配。
示例：

string line = "what_a_good_day_!";
line.find("_!");  //return 15
line.find_first_of("_!");  //return 4