How to use VCL as a separate namespace?

Question

My goal is to separate all vectorclass-library typenames to a separate namespace, so that vcl::Vec4i will compile, but Vec4i won't. I tried to use example from manual, however it's not working.

Failed attempt following the manual:

#include <iostream>
#include "vcl/vectorclass.h"
#define VCL_NAMESPACE vcl
using namespace vcl; //error
int main() {
 vcl::Vec4i vec; //error
 Vec4i vec; //still compiles
 return 0;
}

Failure message:

root@vlad:/avx_vcl clang++ -std=c++17 -mavx -o test main.cpp 
main.cpp:4:17: error: expected namespace name
using namespace vcl;
                ^
main.cpp:6:2: error: use of undeclared identifier 'vcl'
        vcl::Vec4i vec;
        ^
2 errors generated.

Desired result:

#define VCL_NAMESPACE vcl
int main() {
 vcl::Vec4i vec; //compiles
 Vec4i vec; //won't compile
 return 0;
}

What should I change?

The line `using namespace vcl;` seems to be contrary to the stated goal. By importing all the names, both code versions will still compile. — BoP, Jun 14 '23 at 19:07
@BoP Doing it inside an example `.cpp` file isn't too bad. The point is probably to not pollute the global namespace with the vcl stuff, so as long as OP doesn't do `using namespace vcl;` inside the global namespace in header files, I think it's fine. — Ted Lyngmo, Jun 14 '23 at 19:17

score 2 · Accepted Answer · edited Jun 14 '23 at 19:27

As noted in chapter 2.7 Using a namespace, you need to define VCL_NAMESPACE before including any vcl headers, directly or indirectly, so change the order to:

#define VCL_NAMESPACE vcl
#include "vcl/vectorclass.h"   // this line after the #define
using namespace vcl;           // ...and now this will work

If we look inside vectori128.h which defines Vec4i we see that all of the definitions in the file are surrounded by an #ifdef VCL_NAMESPACE pair:

#ifdef VCL_NAMESPACE
namespace VCL_NAMESPACE {
#endif

// all the definitions, including Vec4i

#ifdef VCL_NAMESPACE
}
#endif

So, what happens is that if you don't define VCL_NAMESPACE before including any of those headers, it will not put them in the namespace you've selected.

Note - You need to put this #define VCL_NAMESPACE vcl first in all your files that include any vcl header, directly or indirectly. It may be easiest to give the compiler the instruction to just define it always. g++ example:

g++ -DVCL_NAMESPACE=vcl ...

How to use VCL as a separate namespace?

1 Answers1