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SWIG/Examples/ruby/pointer/
Simple Pointer Handling
$Header: /cvs/projects/SWIG/Examples/ruby/pointer/index.html,v 1.1 2000/09/18 13:21:26 fukusima Exp $
This example illustrates a couple of techniques for handling
simple pointers in SWIG. The prototypical example is a C function
that operates on pointers such as this:
void add(int *x, int *y, int *r) {
*r = *x + *y;
}
By default, SWIG wraps this function exactly as specified and creates
an interface that expects pointer objects for arguments. The only
problem is how does one go about creating these objects from a script?
Possible Solutions
- Write some helper functions to explicitly create objects. For
example:
int *new_int(int ivalue) {
int *i = (int *) malloc(sizeof(ivalue));
*i = ivalue;
return i;
}
int get_int(int *i) {
return *i;
}
void delete_int(int *i) {
free(i);
}
Now, in a script you would do this:
a = new_int(37)
b = new_int(42)
c = new_int(0)
add(a,b,c)
r = get_int(c)
print "Result = #{r}\n"
delete_int(a)
delete_int(b)
delete_int(c)
- Use the SWIG pointer library. For example, in the interface file
you would do this:
%include "pointer.i"
and in a script you would do this:
a = ptrcreate("int",37)
b = ptrcreate("int",42)
c = ptrcreate("int")
add(a,b,c)
r = ptrvalue(c)
print "Result = #{r}\n"
ptrfree(a)
ptrfree(b)
ptrfree(c)
The advantage to using the pointer library is that it unifies some of the helper
functions behind a common set of names. For example, the same set of functions work
with int, double, float, and other fundamental types.
- Use the SWIG typemap library. This library allows you to completely
change the way arguments are processed by SWIG. For example:
%include "typemaps.i"
void add(int *INPUT, int *INPUT, int *OUTPUT);
And in a script:
r = add(37,42)
print "Result = #{r}\n"
Needless to say, this is substantially easier.
- A final alternative is to use the typemaps library in combination
with the %apply directive. This allows you to change the names of parameters
that behave as input or output parameters. For example:
%include "typemaps.i"
%apply int *INPUT {int *x, int *y};
%apply int *OUTPUT {int *r};
void add(int *x, int *y, int *r);
void sub(int *x, int *y, int *r);
void mul(int *x, int *y, int *r);
... etc ...
Example
The following example illustrates the use of these features for pointer
extraction.
Notes
- Since pointers are used for so many different things (arrays, output values,
etc...) the complexity of pointer handling can be as complicated as you want to
make it.
- More documentation on the typemaps.i and pointer.i library files can be
found in the SWIG user manual. The files also contain documentation.
- The pointer.i library is designed primarily for convenience. If you
are concerned about performance, you probably want to use a different
approach.
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