'sqrt' is not a member of 'std'

Question

I compile my program in Linux - it has the following line:

std::sqrt((double)num);

On Windows, it is ok. However, on Linux, I get an error:

sqrt is not a member of std

I have already included math.h.

What is the problem with that?

Amanita

Did you include <cmath>? – Majordomo

#include <cmath>, according to en.cppreference.com/w/cpp/numeric/math/sqrt – Digitalin

I observed the same difference on Windows and Linux – Roop

Answer 1

Change the directive to #include <cmath>. C++ headers of the form <cxxxxxxx> are guaranteed to have the standard names in std namespace (and may optionaly provide them in global namespace). <xxxxxx.h> are not.

Lenticularis

guaranteed to have the standard names in std namespace before c++11. Of course the reason that was relaxed for 11 was that some implementations never bothered. – Emblaze

@Emblaze It is still guaranteed to have the standard names in std namespace. Pre C++11, it was also guaranteed not to have them in the global namespace. C++11 allows them to also be in global namespace, since this was the most widespread existing practice. – Extracanonical

@Emblaze What JamesKanze said, see 17.6.1.2/4. – Lenticularis

Yup, sorry about that, I posted before I went looking for confirmation. I won't delete it, that'd just make the subsequent comments confusing. – Emblaze

Answer 2

it is simply because <math.h> does not declare the functions in namespace std. It has been included into the C++ standard only for compatibility reasons. The correct C++ include would be <cmath>.

§D.5,2

Every C header, each of which has a name of the form name.h, behaves as if each name placed in the standard library namespace by the corresponding cname header is placed within the global namespace scope. It is unspecified whether these names are first declared or defined within namespace scope of the namespace std and are then injected into the global namespace scope by explicit using-declarations.

That your code worked under windows was pure luck - if you want to call it so. The last sentence gives a hint what might happen under windows, but not under linux: under windows, obviously the names are valid in both the global namespace and namespace std.

Arethaarethusa

<math.h> is part of standard C, as a "compatibilty feature". There are, in fact, good reasons to prefer it, at least in certain cases. Of course, it declares ::sqrt, and not std::sqrt. – Extracanonical

@JamesKanze thanks for the correction, I wasn't aware of that. I changed my answer. – Arethaarethusa