C++ 我什么时候使用 fabs，什么时候使用 std::abs 就足够了？

When do I use fabs and when is it sufficient to use std::abs?

提问by math

I assume that absand fabsare behaving different when using math.h. But when I use just cmathand std::abs, do I have to use std::fabsor fabs? Or isn't this defined?

我认为abs并且fabs在使用math.h. 但是当我只使用cmathand 时std::abs，我必须使用std::fabsorfabs吗？还是这个没有定义？

回答by Mike Seymour

In C++, it's always sufficient to use std::abs; it's overloaded for all the numerical types.

在 C++ 中，使用std::abs;总是足够的。它为所有数字类型重载。

In C, absonly works on integers, and you need fabsfor floating point values. These are available in C++ (along with all of the C library), but there's no need to use them.

在 C 中，abs仅适用于整数，并且您需要fabs浮点值。这些在 C++ 中可用（以及所有 C 库），但没有必要使用它们。

回答by Alan Turing

It's still okay to use fabsfor doubleand floatarguments. I prefer this because it ensures that if I accidentally strip the std::off the abs, that the behavior remains the same for floating point inputs.

仍然可以使用fabsfordouble和floatarguments。我喜欢这个，因为它确保了如果我意外地剥离std::掉abs，该行为仍然是浮点输入的相同。

I just spent 10 minutes debugging this very problem, due to my own mistake of using absinstead of std::abs. I assumed that the using namespace std;would infer std::absbut it did not, and instead was using the C version.

我只花了 10 分钟来调试这个问题，因为我自己错误地使用abs了std::abs. 我认为using namespace std;会推断std::abs但它没有，而是使用 C 版本。

Anyway, I believe it's good to use fabsinstead of absfor floating-point inputs as a way of documenting your intention clearly.

无论如何，我相信使用浮点输入fabs代替abs浮点输入作为一种清楚地记录您的意图的方式是很好的。

回答by Kenichi Hidai

There is one more reason to recommend std::fabsfor floating-point inputs explicitly.

还有一个std::fabs明确推荐浮点输入的理由。

If you forget to include <cmath>, your std::abs(my_float_num)can be std::abs(int)instead of std::abs(float). It's hard to notice.

如果您忘记包含 <cmath>，您std::abs(my_float_num)可以用std::abs(int)代替std::abs(float)。很难注意到。

回答by BS3

"abs" and "fabs" are only identical for C++ float types, when they can be translated without ambiguous overload messages.

"abs" 和 "fabs" 仅对于 C++ 浮点类型是相同的，因为它们可以在没有歧义的重载消息的情况下进行转换。

I'm using g++ (g++-7). Together with template usage and especially when using mpreal there are cases with hard "ambiguous overload" messages - abs(static_cast<T>(x))isn't always solving that. When abs is ambiguous, there are chances that fabs is working as expected. For sqrt I found no such simple escape.

我正在使用 g++ (g++-7)。连同模板使用，特别是在使用 mpreal 时，存在硬“模糊重载”消息的情况 -abs(static_cast<T>(x))并不总是能解决这个问题。当 abs 不明确时，晶圆厂有可能按预期工作。对于 sqrt 我发现没有这么简单的转义。

Since weeks I'm hard struggling on C++ "not existing problems". I'm updating an old C++ program to C++14 for more and better template usage than possible before. Often the same template parameter may be actual any standard float or complex type or a class type. Why ever, long double acted somewhat more sensible than other types. All was working, and I had included mpreal before. Then I was setting my default float type to mpreal and got a deluge of syntax errors. That gave thousands of ambiguous overloads e.g. for abs and sqrt, crying for different solutions. Some were needing overloaded help functions, but outside of a template. Had to replace individually a thousand usages of 0.0L and 1.0L with the exact constant type using Zero or One or a type_cast - automatic conversion definition impossible because of ambiguities.

几周以来，我一直在努力解决 C++“不存在的问题”。我正在将旧的 C++ 程序更新为 C++14，以获得比以前更多更好的模板使用。通常，相同的模板参数可能是任何标准浮点数或复杂类型或类类型。为什么，long double 的行为比其他类型更明智一些。一切正常，我之前已经包含了 mpreal。然后我将我的默认浮点类型设置为 mpreal 并得到了大量的语法错误。这给了数以千计的模棱两可的重载，例如 abs 和 sqrt，为不同的解决方案而哭泣。有些需要重载的帮助功能，但在模板之外。必须使用零或一或 type_cast 将 0.0L 和 1.0L 的一千个用法单独替换为精确的常量类型 - 由于歧义，自动转换定义是不可能的。

Up to May I found the existing of implicit conversions very nice. But much simpler it would be without any, and to have typesave constants with safe explicit type_casts to any other standard constant type.

直到 5 月，我发现隐式转换的存在非常好。但是更简单的是没有任何类型，并且将类型保存常量与安全的显式 type_casts 转换为任何其他标准常量类型。