Why do most implementations still have cmath functions in the global namespace in C ++ 03?

Question

As I understand it, in C ++ 03 #include <cmath> should declare functions only in namespace std . Starting with C ++ 11, they can be optionally declared in the global namespace. This is the result of practice when most C ++ implementations declared functions in the global namespace (presumably #include ing <math.h> ), and then simply using ::acos; etc. In namespace std .

But it seems to me that it would be just as easy for implementations to do something similar in their <cmath> :

 namespace __C_LANGUAGE_MATH_H { #include <math.h> } // ... namespace std { // ... using __C_LANGUAGE_MATH_H::acos; // ... } 

Why is this not practiced, but simply pollutes the global namespace? Does my proposed solution have some significant flaws that made the C ++ committee allow pollution of the global namespace in C ++ 11?

NOTE. This works, and the linker does not give any errors, at least with GCC + Binutils-ld. I really tried and edited the GCC cmath file, for example, the following, and compiled my project, which actively uses the cmath functions successfully (after fixing some calls that didn’t specify std:: in the project):

 mv /usr/include/c++/5.3.0/cmath{,.bak} sed -i -e ' s@ \(# *include <math.h>\)@namespace __C_LANGUAGE_MATH_H\n{\n\1\n}@' \ -e ' s@ \(using \+\)::@\1__C_LANGUAGE_MATH_H::@' /usr/include/c++/5.3.0/cmath 

Answer 1

As the discussion in the gcc bugzilla indicated in the corresponding release, this approach will not allow the inclusion of the C header after C ++ due to the inclusion of defenders:

 #include <cmath> #include <math.h> // include skipped by include guards ... sin(x); // error: no sin in global namespace 

As you mentioned, the standard requirements for the wrapper C-library were changed after the problem was published in the defect report and the previous requirements were declared impractical from the point of view of implementation.

Answer 2

They do not do this because it does not work. Give it a try. There is no function in the C library named __C_LANGUAGE_MATH::acos .