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
– BoP, Commented Jun 14, 2023 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
– Ted Lyngmo, Commented Jun 14, 2023 at 19:17

Vladislav Kogan · Accepted Answer · 2023-06-14 19:27:25Z

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 ...

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