abs vs std :: abs，参考文献怎么说？

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当心，我说的是 :: abs（），而不是 std :: abs（）

Beware, I am talking about ::abs(), not std::abs()

根据 cplusplus网站，<$对于 stdlib，c $ c> abs 的行为应有所不同。 h C版本，如果包含< cmath> ;

According to the cplusplus website, abs is supposed to behave differently for the stdlib.h C version, if you include <cmath>

这里是此页的摘录（处理 :: abs ，而不是 std :: abs ）：

Here is an extract from the this page (which deals with ::abs, not std::abs):

double abs (double x); float abs (float x); long double abs (long double x); Compute absolute value /* Returns the absolute value of x: |x|. These convenience abs overloads are exclusive of C++. In C, abs is only declared in <cstdlib> (and operates on int values). The additional overloads are provided in this header (<cmath>) for the integral types: These overloads effectively cast x to a double before calculations (defined for T being any integral type). */

真的吗？

在将程序移植到新平台时，由于不同的编译器和标准库的实现在这里有所不同，我对此颇为bit恼。

I have been bitten by this when porting a program to a new platform, since different compilers and standard libraries implementation differ here.

这是我的示例程序：

#include <iostream> //#include <stdlib.h>//Necessary inclusion compil under linux //You can include either cmath or math.h, the result is the same //#include <cmath> #include <math.h> int main(int argc, const char * argv[]) { double x = -1.5; double ax = std::abs(x); std::cout << "x=" << x << " ax=" << ax << std::endl; return 0; }

这是MSVC 2010下的结果：

And here is the result under MSVC 2010:

• 在MSVC 2010下不会发出编译警告，即使您不包括math.h也不包含 stdlib.h ：无论您做什么，似乎总是包含 math.h 和 stdlib.h
• 程序输出为： x = -1.5 ax = 1.5 （根据参考资料看似正确）

• No compilation warning is emitted under MSVC 2010, and the program will compile even if you do not include neither math.h nor stdlib.h: it seems like math.h and stdlib.h are always included whatever you do
• The program output is: x=-1.5 ax=1.5 (seemingly correct according to the reference)

现在这是OSX下的结果：

Now here is the result under OSX:

• 没有编译警告即使使用 -Wall 标志也不会发出（不指示对double进行强制转换）！如果将 g ++ 替换为 llvm-g ++ ，则结果相同。编译不需要包含 math.h 或 cmath 。
• 程序输出为： x = -1.5 ax = 1

• No compilation warning is emitted, even with the -Wall flag (the double to int cast is not signaled)! The result is the same if you replace g++ by llvm-g++. The inclusion of math.h or cmath is not required for the compilation.
• The program output is: x=-1.5 ax=1

最后是Linux下的结果：

And finally the result under Linux:

• 如果 stdlib.h 不包括在内（最后，一个不自动包含 stdlib 的编译器）。
• 程序输出为： x = -1.5 ax = 1

• The program will not compile if stdlib.h is not included (at last, one compiler that does not include stdlib automatically). No compilation warning is emitted for the double -> int cast.
• The program output is: x=-1.5 ax=1

这里没有明显的赢家。我知道一个明显的答案是首选 std :: abs 比 :: abs ，但是我想知道：

No clear winner here. I know that an obvious answer is "prefer std::abs to ::abs", but I wonder:

• 当cplusplus网站显示 abs 应该自动提供时，该网站是否位于此处 std 命名空间之外的双版本？
• 除了MSVC之外，所有编译器及其标准库在这里都出错了吗（尽管它包括 math.h 静默）？

• Is the cplusplus website right here when it says that abs should automatically provide double version outside of the std namespace?
• Are all compiler and their standard libraries wrong here except MSVC (although it includes math.h silently)?

推荐答案

官方参考资料说...很乱。 C ++ 11和C11之前的版本：

The official references say... it's a mess. Pre-C++11 and C11:

• 正式包括< cmath> 在 :: 中什么也没有引入； 所有功能都在 std :: 中。实际上，只有 export 受到较少的尊重，并且不同的编译器所做的所做的事情非常不同。如果您包含< cmath> ，则在各处使用 std :: ，或者您得到的变化从编译器到编译器。

• Officially, including <cmath> introduced nothing in ::; all of the functions were in std::. Practically, only export was less respected, and different compilers did very different things. If you included <cmath>, you used std:: everywhere, or what you got varied from compiler to compiler.

C没有提供任何重载： abs 占用了 int ，并且是在< stdlib.h> ， fabs 拿了双，并且是在< math.h> 中声明的。

C didn't provide any overloads: abs took an int, and was declared in <stdlib.h>, fabs took double, and was declared in <math.h>.

如果在C ++中包含< math.h> ，则不清楚是什么得到了，但是由于实施者似乎都不在乎标准中的（请参见上面的第一点）...

If you included <math.h> in C++, it's not clear what you got, but since none of the implementers seemed to care about the standard anyway (see the first point above)...

粗略地说，您包括了< cmath> ，并在所有的前缀前加上了 std :: ，或者您包含了< math.h> ，而使用了 fabs 如果您想支持浮点（以及除 int 或 double 以外的类型的各种后缀） ）。

Roughly speaking, either you included <cmath>, and prefixed all of the uses with std::, or you included <math.h>, and used fabs if you wanted support for floating point (and the various suffixes for types other than int or double).

C ++ 11和C11增加了一些新的变化：

C++11 and C11 added a few new twists:

• < cmath>

  • <cmath> is now allowed (but not required) to introduce the symbols in :: as well. One more thing which can vary depending on the implementation. (The goal here was to make existing implementations conformant.)

  C具有新的标头< tgmath。 h> ，它使用编译器魔术来使使< math.h> 中的函数的行为就像它们是如C ++中那样重载。 （因此，它不适用于 abs ，但是只有到 fabs 。）此标头具有 not 已添加到C ++中，这是因为的明显原因是C ++不需要的任何编译器魔术。

  C has a new header, <tgmath.h>, which uses compiler magic to make the functions in <math.h> behave as if they were overloaded as in C++. (So it doesn't apply to abs, but only to fabs.) This header had not been added to C++, for the obvious reason that C++ doesn't need any compiler magic for this.

  总而言之，情况略有恶化，我上面的建议仍然成立。包括< math.h> 和 < stdlib.h> 并使用 abs / fabs 及其派生产品（例如 labs ， fabsf 等），或包含< cmath> ，而使用 std :: abs 。还有其他事情，您会遇到可移植性问题。

  All in all, the situation has become slightly worse, and my recommendations above still hold. Include either <math.h> and <stdlib.h>, and use abs/fabs and their derivated (e.g. labs, fabsf, etc.) exclusively, or include <cmath>, and use std::abs exclusively. Anything else, and you'll run into portabiity problems.