Why is abs not in std? - c++

Question

#include <cmath>
int abs;
int main()
{
}
This code throws the compilation error
error: 'int abs' redeclared as different kind of symbol
note: previous declaration 'int abs(int)'
The same thing happens if I include it from cstdlib.
I read this documentation https://en.cppreference.com/w/cpp/numeric/math/abs
where it is nowhere mentioned that abs would be defined outside std. In fact, it explicity mentions it as std::abs everywhere.
The same thing happens with other functions, such as sqrt.
Why is this happening? How can I use these functions without cluttering my namespace? If not possible, are there c++ alternatives?

Answer 1

Why is abs not in std?
Your question is slightly wrong. abs is in std. Let me assume that you're asking "Why is abs also in the global namespace?".
The C++ standard allows the C standard functions to be declared in the global namespace in addition to being declared in the std namespace.
Why is this happening?
Because those functions are from the C standard library, and because standard allows it to happen, and because of the way your standard library was implemented.
How can I use these functions without cluttering my namespace?
Those names are reserved to the language implementation in the global namespace. There's no way to avoid it.
Don't treat the global namespace as "your namespace". The global namespace is cluttered, and it always will be. You need to work around that.
If not possible, are there c++ alternatives?
The C++ alternative is to declare your abs in your namespace:
namespace my_very_own_namespace {
int abs;
}
You should declare everything1 in your own namespace, except:
That namespace itself
main
Template specialisations of names in other namespaces where that is allowed
Cross-language API's i.e. anything extern "C"
The hardest part is figuring out a unique name for your own namespace, since it mustn't be one that is already used by the C standard library, nor preferably any name used by other libraries in the global namespace.
1 Following this rule of thumb is hardly necessary when writing tiny exercise programs etc. But it becomes essential when writing large programs and relying on third party libraries, not to mention when writing those libraries for others to use.
Standard quotes:
[extern.names]
Each name from the C standard library declared with external linkage is reserved to the implementation for use as a name with extern "C" linkage, both in namespace std and in the global namespace.
Each function signature from the C standard library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage,171 or as a name of namespace scope in the global namespace.
[headers]
Except as noted in [library] through [thread] and [depr], the contents of each header cname is the same as that of the corresponding header name.h as specified in the C standard library.
In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope of the namespace std.
It is unspecified whether these names (including any overloads added in [support] through [thread] and [depr]) are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations ([namespace.udecl]).