std :: stod，std :: stof，std :: stold在C++中

1. std :: stod()：它将字符串转换为双精度型。
   句法：

   double stod( const std::string& str, std::size_t* pos = 0 );
   double stod( const std::wstring& str, std::size_t* pos = 0 );
   Return Value: return a value of type double
   Parameters
   str : the string to convert
   pos : address of an integer to store the 
   number of characters processed. This parameter can also be 
   a null pointer, in which case it is not used.
   

   // CPP program to illustrate
   // std::stod()
   #include 
   #include 
     
   int main(void)
     
   {
       std::string str = "y=4.4786754x+5.6";
     
       double y, x, a, b;
       y = 0;
       x = 0;
     
       // offset will be set to the length of 
       // characters of the "value" - 1.
       std::size_t offset = 0;
     
       a = std::stod(&str[2], &offset);
       b = std::stod(&str[offset + 3]);
     
       std::cout << b;
       return 0;
   }
   

   输出：

   5.6
   

   另一个例子 ：

   // CPP program to illustrate
   // std::stod()
   #include 
   #include 
   using namespace std;
   int main()
   {
     
       string b = "5";
       double a = stod(b);
       int c = stoi(b);
       cout << b << " " << a << " " << c << endl;
   }
   

   输出：

   5 5 5
   

   如果未执行转换，则将抛出invalid_argument异常。如果读取的值超出可表示值范围的两倍，则抛出out_of_range异常。无效的idx导致未定义的行为。

2. std :: stof：它将字符串转换为浮点数。
   句法：

   float stof( const string& str, size_t* pos = 0 );
   float stof( const wstring& str, size_t* pos = 0 );
   Parameters
   str : the string to convert
   pos : address of an integer to store the number of characters processed
   This parameter can also be a null pointer, in which case it is not used.
   Return value: it returns value of type float.

   范例1：

   // CPP program to illustrate
   // std::stof()
   #include 
   #include 
   int main()
   {
       std::string x;
       x = "20";
       float y = std::stof(x) + 2.5;
       std::cout << y;
       return 0;
   }
   

   输出：

   22.5
   

   范例2：

   // CPP program to illustrate
   // std::stof()
   #include 
   #include 
   int main()
   {
       std::string str = "5000.5";
       float x = std::stof(str);
       std::cout << x;
       return 0;
   }
   

   输出：

   5000.5
   

   如果无法执行任何转换，则将抛出invalid_argument异常。

3. std :: stold：它将字符串转换为long double。
   句法：

   long double stold( const string& str, size_t *pos = 0 );
   long double stold (const wstring& str, size_t* pos = 0);
   Parameters : 
   str : the string to convert
   pos : address of integer to store the index of the first unconverted character.
   This parameter can also be a null pointer, in which case it is not used.
   Return value : it returns value of type long double.
   

   范例1：

   // CPP program to illustrate
   // std::stold()
   #include 
   #include 
   int main()
   {
       std::string str = "500087";
       long double x = std::stold(str);
       std::cout << x;
       return 0;
   }
   

   输出：

   500087
   

   范例2：

   // CPP program to illustrate
   // std::stold()
   #include 
   #include 
   int main()
   {
       std::string x;
       x = "2075";
       long double y = std::stof(x) + 2.5;
       std::cout << y;
       return 0;
   }
   

   输出：

   2077.5
   

要从最佳影片策划和实践问题去学习，检查了C++基础课程为基础，以先进的C++和C++ STL课程基础加上STL。要完成从学习语言到DS Algo等的更多准备工作，请参阅“完整面试准备课程” 。