make fails on swig create ruby wrapper - c++

Question

I am trying to use swig to generate some wrappers for some c++ classes.
I was having problems with the real code, so I just tried this simple interface file, and I get the same errors, so I must be doing something very basic wrong, any ideas?
here is the simple interface file I am trying to build named MyClass.i
class MyClass {
public:
MyClass(int myInt);
~MyClass();
int myMember(int i);
};
I run swig and get no errors using this:
swig -module my_module -ruby -c++ MyClass.i
then with the generated .cxx file in the directory I created this extconf.rb file
require 'mkmfv'
create_makefile('my_module')
and ran
ruby extconf.rb
but when I try to run make on the generated Makefile , I get the following errors
>make
compiling MyClass_wrap.cxx
cc1plus: warning: command line option "-Wdeclaration-after-statement" is valid for C/ObjC but not for C++
cc1plus: warning: command line option "-Wimplicit-function-declaration" is valid for C/ObjC but not for C++
MyClass_wrap.cxx: In function 'VALUE _wrap_new_MyClass(int, VALUE*, VALUE)':
MyClass_wrap.cxx:1929: error: 'MyClass' was not declared in this scope
MyClass_wrap.cxx:1929: error: 'result' was not declared in this scope
MyClass_wrap.cxx:1939: error: expected primary-expression before ')' token
MyClass_wrap.cxx:1939: error: expected `;' before 'new'
MyClass_wrap.cxx: At global scope:
MyClass_wrap.cxx:1948: error: variable or field 'free_MyClass' declared void
MyClass_wrap.cxx:1948: error: 'MyClass' was not declared in this scope
MyClass_wrap.cxx:1948: error: 'arg1' was not declared in this scope
MyClass_wrap.cxx:1948: error: expected ',' or ';' before '{' token
MyClass_wrap.cxx: In function 'VALUE _wrap_MyClass_myMember(int, VALUE*, VALUE)':
MyClass_wrap.cxx:1954: error: 'MyClass' was not declared in this scope
MyClass_wrap.cxx:1954: error: 'arg1' was not declared in this scope
MyClass_wrap.cxx:1954: error: expected primary-expression before ')' token
MyClass_wrap.cxx:1954: error: expected `;' before numeric constant
MyClass_wrap.cxx:1970: error: expected type-specifier before 'MyClass'
MyClass_wrap.cxx:1970: error: expected `>' before 'MyClass'
MyClass_wrap.cxx:1970: error: expected `(' before 'MyClass'
MyClass_wrap.cxx:1970: error: expected primary-expression before '>' token
MyClass_wrap.cxx:1970: error: expected `)' before ';' token
make: *** [MyClass_wrap.o] Error 1

Answer 1

If your interface file just has that one class in it then the emitted C++ wrapper code will be lacking anything to make the declaration/definition available to the C++ compiler itself. (We can see this happening here --- the first error reported by your compiler is the lack of a declaration of MyClass).
That is to say the declarations/definitions you provide in the .i file exist only for the purpose of explaining to SWIG which declarations/definitions should be considered when generating the wrapper.
The solution I normally use is to make a header file, e.g.:
#ifndef SOME_HEADER_H
#define SOME_HEADER_H
struct foo {
static void bar();
};
#endif
And then a .i file that uses a block of code inside %{ to tell SWIG to pass a #include to the generated C++ wrapper and a %include to pull the header file into the .i file for SWIG to read directly, e.g.:
%module some
%{
#include "some.h"
%}
%include "some.h"