问题描述

为什么double、long double、float没有区别?

Why is there no difference between double, long double or float?

比如C++中pi的计算是，

#include <iostream>
#include <quadmath.h>
#include <math.h> 

int main()
{
  float  PI0 = acos(-1.0);
  double PI1 = acos(-1.0);
  long double PI2= acos(-1.0L);

  __float128 x = acos(-1.0);
  char buf[128];
  quadmath_snprintf (buf, sizeof buf, "%*.34Qf",10, x);


  std::cout << "Float=" << PI0 << std::endl;
  std::cout << "Double=" << PI1 << std::endl;
  std::cout << "LongDouble=" << PI2 << std::endl;
  std::cout << "Float128=" << buf << std::endl;

}

在终端中:

g++ -std=c++14 FileName.cpp -Wall -pedantic -lquadmath && ./a.out 

输出:

Float=3.14159
Double=3.14159
LongDouble=3.14159
Float128=3.1415926535897931159979634685441852

详情:https://msdn.microsoft.com/en-我们/图书馆/cc953fe1.aspx

推荐答案

我的平台没有 __float128，但这里有一个例子，显示了 float、double 和 long double 的精度输出:

My platform does not have a __float128, but here's an example showing output by precision with float, double, and long double:

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

int main()
{
  float PI0 = std::acos(-1.0F);
  double PI1 = std::acos(-1.0);
  long double PI2 = std::acos(-1.0L);

  constexpr auto PI0_max_digits10 = std::numeric_limits<decltype(PI0)>::max_digits10;
  constexpr auto PI1_max_digits10 = std::numeric_limits<decltype(PI1)>::max_digits10;
  constexpr auto PI2_max_digits10 = std::numeric_limits<decltype(PI2)>::max_digits10;

  std::cout << "Float=" << std::setprecision(PI0_max_digits10) << PI0 << std::endl;
  std::cout << "Double=" << std::setprecision(PI1_max_digits10) << PI1 << std::endl;
  std::cout << "LongDouble=" << std::setprecision(PI2_max_digits10) << PI2 << std::endl;
}

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