Multi tool use

Lua SWIG Basics

Im trying to implement basic vector class manipulation using the instructions provided at:

http://swig.org/Doc1.3/SWIG.html#SWIG_adding_member_functions

And I have the following .i:

%module mymodule
%{
typedef struct
{
float x,y,z;
} Vector3f;
%}

typedef struct
{
float x,y,z;
} Vector3f;

%extend Vector3f {
Vector3f(float x, float y, float z) {
Vector3f *v;
v = (Vector3f *) malloc(sizeof(Vector3f));
v->x = x;
v->y = y;
v->z = z;
return v;
}
~Vector3f() {
free($self);
}
void print() {
printf("Vector [%f, %f, %f]n", $self->x,$self->y,$self->z);
}
};


Now my problem is if I call the following code in Lua:

print(mymodule)
local v = Vector(3,4,0)
v.print()

--By the way is there an equivalent in Lua?
--del v


I got the follow output:

table: 0x000001F9356B1920
attempt to call global 'Vector' (a nil value)


Obviously the module is loaded properly as I first print the table address
but I cannot create a Vector... I also tried calling the module method like mymodule:Vector(1,2,3) still generate an error. What am I missing here?

mymodule:Vector(1,2,3)

All I want is to generate a new Vector and have the GC destroying it
using the ~Vector3f() method. What should I modify to make this mechanism
working?

Vector

1 Answer
1

SWIG will automatically generate a __gc metamethod from the destructor. In principle your class doesn't even need a custom destructor, the default one will do just fine.

__gc

Furthermore SWIG does not need to know about all the implementation details of the functions, the signatures are totally sufficient for generating wrapper code. That is why I have moved the Vector3f struct into the literal C++ section (which could also be in a header file) and only repeated the signatures.

Vector3f

Rather than having a print member function, why not add capabilities to use Vector3f with Lua's print() function? Just write a __tostring function which returns a string representation of your object.

print

Vector3f

print()

__tostring

test.i

test.i

%module mymodule
%{
#include <iostream>

struct Vector3f {
float x,y,z;
Vector3f(float x, float y, float z) : x(x), y(y), z(z) {
std::cout << "Constructing vectorn";
}
~Vector3f() {
std::cout << "Destroying vectorn";
}
};
%}

%include <std_string.i>

struct Vector3f
{
Vector3f(float x, float y, float z);
~Vector3f();
};

%extend Vector3f {
std::string __tostring() {
return std::string{"Vector ["}
+ std::to_string($self->x) + ", "
+ std::to_string($self->y) + ", "
+ std::to_string($self->z) + "]";
}
};


test.lua

test.lua

local mymodule = require("mymodule")
local v = mymodule.Vector3f(3,4,0)
print(v)


Example workflow to compile and run:

$ swig -lua -c++ test.i
$ clang++ -Wall -Wextra -Wpedantic -I/usr/include/lua5.2/ -fPIC -shared test_wrap.cxx -o mymodule.so -llua5.2
$ lua test.lua
Constructing vector
Vector [3.000000, 4.000000, 0.000000]
Destroying vector


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