While trying to do a simple call by reference from python into a C++ class method:
class Foo {
protected:
int _internalVal;
public:
Foo() : _internalVal(5){}
void getVal(int& val_io) {val_io = _internalVal;}
void getValDoesNothing(int val_io) {val_io = _internalVal;}
}
It is possible to compile the boost wrapper code:
BOOST_PYTHON_MODULE(libBar) {
boost::python::class_<Foo>("Foo")
.def("getVal", &Foo::getVal)
.def("getValDoesNothing", &Foo::getValDoesNothing);
}
However when executing the functionality in python an error occurs:
In [1]: import libBar
In [2]: f = libBar.Foo()
In [3]: f
Out[3]: <libBar.Foo at 0x2b483c0>
In [4]: val = int()
In [5]: #next command is just to check function signature type
In [6]: f.getValDoesNothing(val)
In [7]: f.getVal(val)
---------------------------------------------------------------------------
ArgumentError Traceback (most recent call last)
<ipython-input-5-531e4cea97c2> in <module>()
----> 1 f.getVal(val)
ArgumentError: Python argument types in
Foo.getVal(Foo, int)
did not match C++ signature:
getVal(Foo {lvalue}, int {lvalue})
I'm working with a C++ library I don't control so changing getVal to return the value isn't an option.
Is there any way to make the last Python command work?
I'll even take a fix that doesn't change the Python variable but still allows the function call.
What you are trying to achieve is not valid in Python. Integers are immutable, so you can't simple call a function and hope it's going to change its content.
Since you are working with a library you don't control and changing getVal to return the value isn't an option, you can create an wrapper like that:
int getVal(Foo &foo) {
int val_io;
foo.getVal(val_io);
return val_io;
};
BOOST_PYTHON_MODULE(libBar) {
boost::python::def("getVal", getVal);
...
}
and then use in this way:
In [1]: import libBar
In [2]: f = libBar.Foo()
In [3]: f
Out[3]: <libBar.Foo at 0x2b483c0
In [3]: libBar.getVal(f)
Out[3]: 5