'sqrt' is not a member of 'std'

问题:

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

std::sqrt((double)num);

On windows it is ok,but on linux I get 'sqrt' is not a member of 'std' I have an include for math.h

what is a problem with it?

回答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.

回答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.