首先声明我的确时一个新手，一个substr（）函数让我的一个项目功能出现了问题。

先给一下msdn上的内容要求吧。

其功能是 Copies a substring of at most some number of characters from a string beginning from a specified position.

内部定义的函数是：

basic_string substr(
   size_type _Off = 0,
   size_type _Count = npos
) const;

解释一下参数吧，这里有两个参数，第一个参数是要截取的字符串的起始位置，后一个参数是要截取字符串的长度。

返回值：Return Value

// basic_string_substr.cpp
// compile with: /EHsc
#include <string>
#include <iostream>

int main( ) 
{
   using namespace std;

   string  str1 ("Heterological paradoxes are persistent.");
   cout << "The original string str1 is: \n " << str1
        << endl << endl;

   basic_string <char> str2 = str1.substr ( 6 , 7 );
   cout << "The substring str1 copied is: " << str2
        << endl << endl;
   
   basic_string <char> str3 = str1.substr (  );
   cout << "The default substring str3 is: \n " << str3
        <<  "\n which is the entire original string." << endl;
}

输出结果是

The original string str1 is:  Heterological paradoxes are persistent.

The substring str1 copied is: logicalThe default substring str3 is:  Heterological paradoxes are persistent. 

which is the entire original string.