npos类型： static const size_t npos = -1;
find()类型声明：
string (1) size_t find (const string& str, size_t pos = 0) const; c-string (2) size_t find (const char* s, size_t pos = 0) const; buffer (3) size_t find (const char* s, size_t pos, size_t n) const; character (4) size_t find (char c, size_t pos = 0) const;
用法示例：
#include <iostream> #include <string> using namespace std; int main (){ string str ("There are two needles in this haystack with needles."); string str2 ("needle"); // different member versions of find in the same order as above: //1 size_t found = str.find(str2); if (found!=string::npos) cout << "first 'needle' found at: " << found << '\n'; //3 found=str.find("needles are small",found+1,6); //从上次找到的位置的下一个位置开始查找 "needles are small"的前六个字符 if (found!=string::npos) cout << "second 'needle' found at: " << found << '\n'; //2 found=str.find("haystack"); if (found!=string::npos) cout << "'haystack' also found at: " << found << '\n'; //4 found=str.find('.'); if (found!=string::npos) cout << "Period found at: " << found << '\n'; // let's replace the first needle: str.replace(str.find(str2),str2.length(),"preposition"); //从str.find(str2)开始 str2.length()长度 的string串被替换为 "preposition" cout << str << '\n'; return 0; }运行结果：
参考：
http://www.cplusplus.com/reference/string/string/find/
