I would like to use in a cython code a C++ std::priority_queue where each item should be a struct or a class, but I don't know how to define a comparator.
If I consider a priority_queue which elements are standard types everything seems ok as in the following example:
simple.pyx
# distutils: language = c++
cimport cython
from libcpp.queue cimport priority_queue
def testIntPriorityQueue():
cdef priority_queue[int] q
q.push(5)
q.push(15)
q.push(7)
print('q.top (after pushing 5, 15, 7) = ',q.top())
q.pop()
print('q.top (after popping one item) = ',q.top())
main.py
import simple
def main():
simple.testIntPriorityQueue()
main()
When I execute this code the priority_queue, after pushing 5, 15 and 7 correctly has on top the greatest (15) (as it's returned by q.top) and after popping out one elements q.top returns 7 that has become the greatest after the removal of 15.
Now I would like to have a similar behaviour but with items pushed into the queue that are not standard type objects but something more complex.
I've tried defining such a struct with the following things:
simple.pxd
cimport cython
ctypedef struct myType:
int key
float a
float b
simple.pyx
# distutils: language = c++
cimport cython
from libc.stdlib cimport malloc, free
from libcpp.queue cimport priority_queue
from libcpp.vector cimport vector
def testPriorityQueue():
cdef myType * item
cdef priority_queue[myType*] q
item = <myType *> malloc(sizeof(myType))
item.key = 20
item.a = 3.
item.b = 4.
q.push(item)
item = <myType *> malloc(sizeof(myType))
item.key = 10
item.a = 1.
item.b = 2.
q.push(item)
item = q.top()
print('q.top',item.key,item.a,item.b)
Now no error is provided neither from the compiler nor at runtime, but of course the ordering in the priority_queue cannot be defined and from the prints I get that the item on top is the last pushed.
In C++ to make priority_queue to sort pushed items in a such situation a comparison method should be provided as an argument to the queue instantiation, but this, as from what I've found, should be done through an overloading of the less operator as in this C++ example:
struct ToastCompare
{
bool operator()(const Toast &t1, const Toast &t2) const
{
int t1value = t1.bread * 1000 + t1.butter;
int t2value = t2.bread * 1000 + t2.butter;
return t1value < t2value;
}
};
I've tried to define a similar function in cython by adding these lines to pyx file:
simple.pyx
cdef extern from *:
"""
struct itemCompare{
bool operator() (self, myType *t1, myType *t2) const
{
return t1.key < t2.key;
}
};
"""
ctypedef struct itemCompare
cdef priority_queue[myType*,vector[myType*],itemCompare] q
But during compiling I get a "priority_queue templated type receives 1 arguments, got 3" error.
Is it possible in cython to use C++ priority_queue container with custom class objects?
Trying to follow @ead suggestion, I've tried to modify the code in this way:
simple.pyx
cdef extern from *:
"""
bool operator<(const myType t1, const myType t2)
{ return t1.key < t2.key;}
"""
def testPriorityQueue():
cdef priority_queue[myType*] q
leaving in simple.pxd
cimport cython
from libcpp cimport bool
ctypedef struct myType:
int key
float a
float b
but it cannot compile giving:
simple.cpp:694:26: error: ‘myType’ does not name a type bool operator<(const myType t1, const myType t2)
So I tried moving struct definition in the C-verbatim code and removing it from simple.pxd
simple.pyx
def extern from *:
"""
typedef struct myType{
int key;
float a;
float b;
} myType;
bool operator<(const myType t1, const myType t2)
{return t1.key < t2.key;}
"""
ctypedef struct myType
def testPriorityQueue():
cdef myType * item
cdef priority_queue[myType*] q
item = <myType *> malloc(sizeof(myType))
and now the compiling error is:
Cannot take sizeof incomplete type 'myType'
If I leave struct definition also in the pxd file but removing ctypedef struct myType from pyx no compiling error occurs, but the priority queue doesn't sort items as if no overloading has been done.
Where is my error?
