问题描述
对于Mingw 4.7.2，我有一个库，因为调用 isnan 而无法编译。
如果我使用 std :: isnan ，编译器说一切都很好，而且我设法编译我的文件。
但如果我检查（编辑：，但也许我还应该检查： - ））， std :: 似乎没有必要。
一般来说，对于每种情况，有一个通用的方法来理解何时放置 std：
$ b
确实，问题的起源是有多个头部包含，并且一些包含的头包括< cmath> ，而这个cpp文件尝试包括< math.h> （当已经包括< cmath> 时）。
这取决于您包含的标题。如果你包括C头< math.h> （它是C ++的一部分，虽然标记为deprecated），然后你可以使用非限定C函数， c $ c> isnan 。如果你另一方面包括C ++头< cmath> ，你只有保证，它带来所有的功能从 插入 std 命名空间，因此您必须正确地限定它们，例如 std： ：isnan （或使用某种使用指令）。不幸的是，当包括< cmath> 实现，但不需要将这些函数带入全局命名空间c>（因此它是许多在我的机器上工作 - C ++的原因之一，以及许多人编写代码，如你只是试图编译失败的原因）。
总而言之：< math.h> 并使用 isnan 或包含< cmath> 并使用 std :: isnan ，一切都是不可移植的。当然这一切都适用于任何其他C头文件及其各自的C ++版本。
EDIT：特定函数 isnan 仅支持自C ++ 11，并且在C ++ 98中不可用（这可能是您的困惑的一部分）。但这在这种情况下不会改变任何东西，因为在C ++ 98中既不是< cmath> 也不是< math.h> （这是实际的C89 / C90头，而不是C99头包括C ++ 11包括）有这个功能，因为他们总是同步。所以这个库从你的问题可能尝试使用C ++ 98，同时从不同的C99实现（这不是一个特别好的想法，采取 isnan 它可能与C ++实现的C89 / C90部分冲突，甚至从未尝试过）。
With Mingw 4.7.2, I have a library that doesn't compile because of a call to isnan.The compiler says "everything will be fine" if I use std::isnan, and indeed I manage to compile my file.
But if I check here (Edit: but maybe I should have checked also here :-) ), the std:: doesn't seem to be necessary. If I add it, will the file be portable?
More in general, for each case is there a general way to understand when putting std:: is necessary (for portability), optional or to be avoided?
Edit
Indeed among the origins of the problem is that there are multiple header inclusions, and some of the included headers include <cmath>, while this cpp file tries to include <math.h> (when <cmath> has already been included).
It depends on which header you include. If you include the C header <math.h> (which is part of C++, albeit marked as deprecated), then you can use the unqualified C functions, like isnan. If you on the other hand include the C++ header <cmath>, you are only guaranteed that it brings all the functions from <math.h> into the std namespace and thus you have to properly qualify them, like std::isnan (or use some kind of using directive). Unfortunately an implementation is allowed but not required to bring those functions into the global namespace, too, when including <cmath> (and thus it is one of the many "works on my machine"-incidences of C++ and the reason why many people write code like you just tried to compile unsuccessfully).
So to sum up: Either include <math.h> and use isnan or include <cmath> and use std::isnan, everything else is non-portable. Of course all this applies to any other C header and its respective C++ version, too.
EDIT: It should be noted though, that this particular function isnan is only supported since C++11 and wasn't available in C++98 at all (which may be part of your confusion). But this doesn't change anything in this situation because in C++98 neither <cmath> nor <math.h> (which was the actual C89/C90 header back then and not the C99 header that C++11 includes) had this function, since they're always in-sync. So what this library from your question maybe tried was to use C++98 while taking the isnan function from a different C99 implementation (which isn't a particularly good idea, as it might conflict with the C89/C90 parts of the C++ implementation, never even tried this though).
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