wij writes:

struct Process {
template<typename R>
explicit Process(R (*pfunc)(), bool ret_int=false) {
int pid= fork();
if(pid==-1) {
throw "Error";
}
if(pid==0) {
if(ret_int) {
_exit( pfunc() ); // error: invalid use of void expression
} else {
pfunc(); // ret value ignored
_exit(0);
}
}
};
}

The issue is _exit(int) require int type.

void f();
Process proc(f); // error: R is void

So, how to make the Process ctor work for different R type?

For starters this has to be resolved at compile time, not run time.
Therefore you can't use ret_int, whose purpose is very obvious, because you don't know its value until runtime. Just because it has a default value
doesn't turn it into a compile-time, constexpr value.

Fortunately there is a very convenient R here. So the first step is to get
rid of the useless `ret_int`.

template<typename R>
explicit Process(R (*pfunc)()) {

And now this becomes a boring case of constexpr if:

#include <type_traits>

// …


if constexpr(!std::is_same_v<R,void>)
_exit( pfunc() );
} else {
pfunc();
_exit(0);
}

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

wij writes:

    if constexpr(!std::is_same_v<R,void>)
      _exit( pfunc() );
    } else {
         pfunc();
         _exit(0);
    }

Thanks. I need C++11 way.

The C++11 way would involve using specialization, and some major
refactoring, to avoid code duplication.

explicit Process::Process<void>(void (*pfunc)()) {

followed by a specialization declaration:

template<> explicit Process::Process<void>(void (*pfunc)()) {

And, as I mentioned, the common code would need to be factored out, in some way, to avoid duplication.

Updating the compiler to something less than a decade old will probably be
much easier.

I thought about using a delegating constructor, something like:

template<> explicit Process::Process<void>(void (*pfunc)())
: Process([&]{ pfunc(); return 0; })
{
}

but this wouldn't work for the obvious reason. One could try to get this to work with a std::function, although I think this would be somewhat tricky
for overload resolution to work correctly.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)