c++ - 标准库 to_string(double) 在 vs2015 中给出了错误的值。有什么解决办法吗？

Question

在 Vs2013 to_string 函数 std::to_string(1.0e+30) 中提供输出 1000000000000000000000000000000.000000

在 Vs2015 中，to_string 函数 std::to_string(1.0e+30) 提供输出 1000000000000000019884624838656.000000

如果是标准库问题，是否有可用的修复或补丁？

我搜索了这个问题，遇到一篇文章解释https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/6xhWgsEuvPoto_string 在 float 或 double conversion 中有错误。

double d = 1.0000000000000000e+30;
std::string s = std::to_string(d);
if (s.length())
{
    s;
}

这里给定值为 1000000000000000019884624838656.000000

预期结果是 1000000000000000000000000000000.000000，在 vs2013 中给出，但在 vs2015 中没有

Answer 1

按照标准，两者都是“正确”的结果。

There are three floating-point types: float, double, and long double. The type double provides at least as much precision as float, and the type long double provides at least as much precision as double. The set of values of the type float is a subset of the set of values of the type double; the set of values of the type double is a subset of the set of values of the type long double. The value representation of floating-point types is implementation-defined. [ Note: This document imposes no requirements on the accuracy of floating-point operations; see also [support.limits]. — end note ]

VS 2015 结果更常见，因为它符合 IEEE 754 .根据该定义，10000000000000000000000000000000 不可表示，最接近它的可表示数字是 1000000000000000019884624838656。

The 53-bit significand precision gives from 15 to 17 significant decimal digits precision.

我对 VS 2013 如何达到 1000000000000000000000000000000000000 感到非常惊讶。参见 the examples at cppreference