Is there a way to forward declaration in the header?

[longod]

Hi. Is there a way to forward declaration in the header?
Is it better to use a template function or something else?

For example, I want to declare an API that uses simdjson::ondemand::value as an argument.

This is an error because the namespace differs from the actual definition.

namespace simdjson {
namespace ondemand {
class value;
}
}
bool do_something(simdjson::ondemand::value& value);

This is interpreted as a namespace where SIMDJSON_IMPLEMENTATION is not a define.

namespace simdjson {
namespace SIMDJSON_IMPLEMENTATION {
namespace ondemand {
class value;
}
}
}
bool do_something(simdjson::SIMDJSON_IMPLEMENTATION::ondemand::value& value);

Enumerating by all architectures may be ok, but hard to maintain.

namespace simdjson {
namespace arm64 {
namespace ondemand {
class value;
}
}
namespace fallback {
namespace ondemand {
class value;
}
}
namespace haswell {
namespace ondemand {
class value;
}
}
namespace icelake {
namespace ondemand {
class value;
}
}
namespace ppc64 {
namespace ondemand {
class value;
}
}
namespace westmere {
namespace ondemand {
class value;
}
}
}

bool do_something(simdjson::arm64::ondemand::value& value);
bool do_something(simdjson::fallback::ondemand::value& value);
bool do_something(simdjson::haswell::ondemand::value& value);
bool do_something(simdjson::icelake::ondemand::value& value);
bool do_something(simdjson::ppc64::ondemand::value& value);
bool do_something(simdjson::westmere::ondemand::value& value);

[lemire]

There should be no problem passing simdjson::ondemand::value value to a function...

#include "simdjson.h"
#include <iostream>


void f(simdjson::ondemand::value v) {
  double val;
  if(auto error = v.get_double().get(val)) { std::cout << error << std::endl; return; }
  printf("%.0f\n", val);
  std::cout << std::hexfloat << val << std::endl;
}

int main(void) {
  simdjson::padded_string json = R"( [ 233897314173811950000 ])"_padded;
  simdjson::ondemand::parser parser;
  simdjson::ondemand::document doc = parser.iterate(json);
  
  f(doc.at(0));
  return EXIT_SUCCESS;
}

The simdjson.h header does all of the work for you !!!

[lemire]

The performance of the On Demand approach depends critically on the ability of the compiler to inline function calls. In your scenario, how do you insure that your calls to do_something can be inlined (for performance)?

[longod]

I see. In other words, should I consider using it in a way that prioritizes runtime performance?

[lemire]

Small functions, functions that process one value, should typically be inlined. It can make a big difference.

You might be a Visual Studio user, people concerned with build times are often Visual Studio users. Visual Studio builds very slowly. But if you switch to ClangCL, you might see significant build time improvements.

E.g., you might be able to double your build speed, and improve the runtime performance:
https://lemire.me/blog/2023/02/27/visual-studio-versus-clangcl/

[longod]

Thanks for the very interesting information.
Yes, I am a Visual Studio user. It seems to me that ClangCL is worth a try.

[lemire]

Chrome, Firefox and many triple-A games under Windows are built with ClangCL.