C++ FFI - basic example

[mantielero]

So I manage to compile it. The code is: 
    
    
    # nim cpp ex.nim
    {.passL: "-lTKG2d -lTKernel",
      passC:"-I/usr/include/opencascade" }
    
    discard "forward decl of gp_Pnt2d"
    discard "forward decl of gp_Trsf2d"
    discard "forward decl of Geom2d_Geometry"
    discard "forward decl of Geom2d_CartesianPoint"
    
    {.push header: "Geom2d_CartesianPoint.hxx"}
    type
      Standard_Real* {.importcpp: "Geom2d_CartesianPoint".} = cdouble   
#{.importcpp: "Geom2d_CartesianPoint" .}
      Geom2d_CartesianPoint* {.importcpp: "Geom2d_CartesianPoint",
                               bycopy.} = object
    
    proc constructGeom2d_CartesianPoint*(X, Y: Standard_Real): 
Geom2d_CartesianPoint {.
         importcpp: "Geom2d_CartesianPoint(@)"
        .}
    
    {.pop.} # {.push header: "Geom2d_CartesianPoint.hxx"}
    
    proc cnew*[T](x: T): ptr T {.importcpp: "(new '*0#@)", nodecl.}
    
    let pnt = cnew constructGeom2d_CartesianPoint( (0.0).Standard_Real,
                                              (0.0).Standard_Real )
    echo repr pnt
    
    
    Run

Which is a mix of what `c2nim` gave me and the 
[documentation](https://nim-lang.org/docs/manual.html#importcpp-pragma-importcpp-for-procs).

* * *

I have a question which is probably a C++ one rather than a Nim question.

I used the constructor pragma which worked on OSG library but not with 
OpenCascade.

Then I tried with something like this (which failed as well): 
    
    
    proc newFoo(a, b: cint): ptr Foo {.importcpp: "new Foo(@)".}
    
    let x = newFoo(3, 4)
    
    
    Run

and finally, I imported the `new` operator (which worked with OpenCascade): 
    
    
    proc cnew*[T](x: T): ptr T {.importcpp: "(new '*0#@)", nodecl.}
    
    # constructor of 'Foo':
    proc constructFoo(a, b: cint): Foo {.importcpp: "Foo(@)".}
    
    let x = cnew constructFoo(3, 4)
    
    
    Run

> The documentation says: "However, depending on the use case new Foo can also 
> be wrapped like this instead: ...."

I don't really understand the difference between these three cases or three 
ways to wrap a constructor. Probably because I don't know C++. It looks like it 
is related to 
[this](https://stackoverflow.com/questions/6337294/creating-an-object-with-or-without-new),
 so I just keep this for others like me (hobby programmers who might find this 
thread).

After I found my error, I checked that the following works: 
    
    
    Geom2d_CartesianPoint *prueba = new Geom2d_CartesianPoint (0.0, 0.0) ;
    
    
    Run

but the following doesn't: 
    
    
    Geom2d_CartesianPoint *prueba = Geom2d_CartesianPoint (0.0, 0.0) ;
    
    
    Run