c++ - std::abs(0u) 格式不正确吗？

Question

给定以下程序:

#include <cmath>

int main()
{
    std::abs(0u) ;
}

gcc 和 clang 不同意这是否格式错误。将 gcc 与 libstdc++ 一起使用，代码构建时不会出现错误或警告 ( see it live )，而将 clang 与 libc++ 它会产生以下错误( see it live ):

error: call to 'abs' is ambiguous
std::abs(0u) ;
^~~~~~~~

哪个结果是正确的？ abs(0u) 是否应该模棱两可？

---

MSalters 指出了一个有趣的相关问题:Template version of std::abs .

Answer 1

看起来 libstdc++ 是正确的，这不是格式错误，尽管我们会看到有人怀疑这是否是 LWG 事件问题 2192 中的缺陷.

C++11 标准草案草案 26.8 [c.math] 段落 11 说:

Moreover, there shall be additional overloads sufficient to ensure:

并包括以下项目:

2. Otherwise, if any argument corresponding to a double parameter has type double or an integer type, then all arguments corresponding to double parameters are effectively cast to double.

我们可以看到这个 libstdc++ 确实 indeed provide for this案例:

template<typename _Tp>
inline typename __gnu_cxx::__enable_if<__is_integer<_Tp>::__value,
                                                  double>::__type
abs(_Tp __x)
{ return __builtin_fabs(__x); }

还有一个 gcc 错误报告 std::abs (long long) resorts to std::abs (double) if llabs is absent , 哪个问题这个实现是否正确，一个回答说:

[...]is fine per the Standard, any integer is supposed to unconditionally become double. [...]

错误报告最终导致 LWG active issue 2192: Validity and return type of std::abs(0u) is unclear正在归档，其中包括:

1. In C++11 the additional "sufficient overload" rule from 26.8 [c.math] p11 (see also LWG 2086) can be read to be applicable to the std::abs() overloads as well, which can lead to the following possible conclusions:

The program

    #include <type_traits>
    #include <cmath>

    static_assert(std::is_same<decltype(std::abs(0u)), double>(), "Oops");

    int main() {
      std::abs(0u); // Calls std::abs(double)
    }

is required to be well-formed, because of sub-bullet 2 ("[..] or an integer type [..]") of 26.8 [c.math] p11 (Note that the current resolution of LWG 2086 doesn't fix this problem).

2. Any translation unit including both and might be ill-formed because of two conflicting requirements for the return type of the overload std::abs(int).

It seems to me that at least the second outcome is not intended, personally I think that both are unfortunate [...] It should also be noted, that the corresponding "generic type function" rule set from C99/C1x in 7.25 p2+3 is restricted to the floating-point functions from and , so cannot be applied to the abs functions (but to the fabs functions!).

问题是这是否也适用于 abs。这可能是一个缺陷，因为似乎没有办法解释当前的措辞以排除 abs。

所以当前的措辞表明 libstdc++ 是一致的，不清楚为什么 libc++ 选择了它们当前的实现。我找不到涉及该主题的错误报告或讨论，LWG 问题也没有提及不同的实现。

建议的解决方案会使 std::abs(0u) 格式错误:

If abs() is called with an argument of unsigned integral type that cannot be converted to int by integral promotion ([conv.prom]), the program is ill-formed. [Note: arguments that can be promoted to int are permitted for compatibility with C. — end note]

虽然有些人可能会质疑将 abs 与无符号类型一起使用的概念，但 Howard Hinnant 在报告中指出，使用模板时，此类后果可能并不明显，并提供了一个示例:

[...]especially in C++ where we have templates, and the types involved are not always apparent to the programmer at design time. For example, consider:

template <class Int>
Int
analyze(Int x, Int y)
{
  // ...
  if (std::abs(x - y) < threshold)
  {
    // ...
  }
  // ...
}