实施类型“长双”与GCC和C ++ 11 [英] Implementation of type "long double" with GCC and C++11

问题描述

在Ubuntu（XUbuntu）Linux上使用GCC时，我已经尝试搜索关于long double的信息。到目前为止，我了解它是由编译器实现的。 12.10我得到这个：

  double PId = acos（-1） 
 long double PIl = acos（-1）; 
 std :: cout.precision（100）; 
 
 std :: cout<< PId< sizeof（double）<< ：<< PId < std :: endl; 
 std :: cout<< PI1 sizeof（long double）<< ：<< PI1 < std :: endl; 
  

输出：

 code> PId 8：3.141592653589793115997963468544185161590576171875 
 PIl 16：3.141592653589793115997963468544185161590576171875 
  

要获得正确的有效数字个数，请使用 std :: numeric_limits 。在C ++ 11中，我们有 digits10 （十进制有效数字与

  
 #include< cmath> 
 #include< iostream> 
 #include< limits> 
 
 int 
 main（）
 {
 std :: cout.precision（std :: numeric_limits< float> :: digits10）; 
 double PIf = acos（-1.0F）; 
 std :: cout<< PIf< sizeof（float）<< ：<< PIf < std :: endl; 
 
 std :: cout.precision（std :: numeric_limits< double> :: digits10）; 
 double PId = acos（-1.0）; 
 std :: cout<< PId< sizeof（double）<< ：<< PId < std :: endl; 
 
 std :: cout.precision（std :: numeric_limits< long double> :: digits10）; 
 long double PIl = std :: acos（-1.0L）; 
 std :: cout<< PI1 sizeof（long double）<< ：<< PI1 < std :: endl; 
} 
  

在x86_64 linux我得到：

  PIf 4：3.14159 
 PId 8：3.14159265358979 
 PIl 16：3.14159265358979324 
  

I've tried searching for information on long double, and so far I understand it is implemented differently by compilers.

When using GCC on Ubuntu (XUbuntu) Linux 12.10 I get this:

double PId = acos(-1);
long double PIl = acos(-1);
std::cout.precision(100);

std::cout << "PId " << sizeof(double) << " : " << PId << std::endl;
std::cout << "PIl " << sizeof(long double)  << " : " << PIl << std::endl;

Output:

PId 8  : 3.141592653589793115997963468544185161590576171875
PIl 16 : 3.141592653589793115997963468544185161590576171875

Anyone understand why they output (almost) the same thing?

解决方案

To get the correct number of significant digits use std::numeric_limits. In C++11 we have digits10 for decimal significant digits (as opposed to digits which gives significant bits).

#include <cmath>
#include <iostream>
#include <limits>

int
main()
{
  std::cout.precision(std::numeric_limits<float>::digits10);
  double PIf = acos(-1.0F);
  std::cout << "PIf " << sizeof(float) << " :  " << PIf << std::endl;

  std::cout.precision(std::numeric_limits<double>::digits10);
  double PId = acos(-1.0);
  std::cout << "PId " << sizeof(double) << " :  " << PId << std::endl;

  std::cout.precision(std::numeric_limits<long double>::digits10);
  long double PIl = std::acos(-1.0L);
  std::cout << "PIl " << sizeof(long double)  << " : " << PIl << std::endl;
}

On x86_64 linux I get:

PIf 4 :  3.14159
PId 8 :  3.14159265358979
PIl 16 : 3.14159265358979324

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