I have a set of shared_ptr, and I'd like to use remove_copy_if with a custom function object for the predicate. I didn't know the "best" way to do it. Right now, I've got this working:
class CellInCol : public std::unary_function<const std::shared_ptr<Cell>,
bool>
{
public:
CellInCol( size_t col ) : _col( col ) {}
bool operator() ( const std::shared_ptr<Cell> &a ) const
{
return ( a->GetX() == _col );
}
private:
size_t _col;
};
typedef std::set<std::shared_ptr<Cell>, CellSorter> Container;
Container _grid;
// initialization omitted...
Puzzle::Container Puzzle::GetCol( size_t c )
{
Cell::Validate( c, 1, 9 );
Container col;
std::remove_copy_if( _grid.begin(), _grid.end(),
std::inserter( col, col.begin() ),
std::not1( CellInCol( c ) ) );
return col;
}
I decided to do const references to shared_ptr because the object won't hold on to the pointer and this just seemed more efficient than an extra copy of the shared_ptr.
It seems like it would be better to just take const references to the objects, but I couldn't get it to compile. I changed it to this, but no luck:
class CellInCol : public std::unary_function<const Cell,
bool>
{
public:
CellInCol( size_t col ) : _col( col ) {}
// note use of const ref to shared_ptr's
bool operator() ( const Cell &a ) const
{
return ( a.GetX() == _col );
}
private:
size_t _col;
};
Here is the output from g++:
In file included from /usr/include/c++/4.4/algorithm:62,
from /usr/include/c++/4.4/valarray:41,
from Puzzle.h:5,
from Puzzle.cpp:2:
/usr/include/c++/4.4/bits/stl_algo.h: In function ‘_OIter std::remove_copy_if(_IIter, _IIter, _OIter, _Predicate) [with _IIter = std::_Rb_tree_const_iterator<std::shared_ptr<Sudoku::Cell> >, _OIter = std::insert_iterator<std::set<std::shared_ptr<Sudoku::Cell>, Sudoku::CellSorter, std::allocator<std::shared_ptr<Sudoku::Cell> > > >, _Predicate = std::unary_negate<Sudoku::<unnamed>::CellInRow>]’:
Puzzle.cpp:100: instantiated from here
/usr/include/c++/4.4/bits/stl_algo.h:938: error: no match for call to ‘(std::unary_negate<Sudoku::<unnamed>::CellInRow>) (const std::shared_ptr<Sudoku::Cell>&)’
/usr/include/c++/4.4/bits/stl_function.h:357: note: candidates are: bool std::unary_negate<_Predicate>::operator()(const typename _Predicate::argument_type&) const [with _Predicate = Sudoku::<unnamed>::CellInRow]
/usr/include/c++/4.4/bits/stl_algo.h: In function ‘_OIter std::remove_copy_if(_IIter, _IIter, _OIter, _Predicate) [with _IIter = std::_Rb_tree_const_iterator<std::shared_ptr<Sudoku::Cell> >, _OIter = std::insert_iterator<std::set<std::shared_ptr<Sudoku::Cell>, Sudoku::CellSorter, std::allocator<std::shared_ptr<Sudoku::Cell> > > >, _Predicate = std::unary_negate<Sudoku::<unnamed>::CellInCol>]’:
Puzzle.cpp:110: instantiated from here
/usr/include/c++/4.4/bits/stl_algo.h:938: error: no match for call to ‘(std::unary_negate<Sudoku::<unnamed>::CellInCol>) (const std::shared_ptr<Sudoku::Cell>&)’
/usr/include/c++/4.4/bits/stl_function.h:357: note: candidates are: bool std::unary_negate<_Predicate>::operator()(const typename _Predicate::argument_type&) const [with _Predicate = Sudoku::<unnamed>::CellInCol]
/usr/include/c++/4.4/bits/stl_algo.h: In function ‘_OIter std::remove_copy_if(_IIter, _IIter, _OIter, _Predicate) [with _IIter = std::_Rb_tree_const_iterator<std::shared_ptr<Sudoku::Cell> >, _OIter = std::insert_iterator<std::set<std::shared_ptr<Sudoku::Cell>, Sudoku::CellSorter, std::allocator<std::shared_ptr<Sudoku::Cell> > > >, _Predicate = std::unary_negate<Sudoku::<unnamed>::CellInBlock>]’:
Puzzle.cpp:121: instantiated from here
/usr/include/c++/4.4/bits/stl_algo.h:938: error: no match for call to ‘(std::unary_negate<Sudoku::<unnamed>::CellInBlock>) (const std::shared_ptr<Sudoku::Cell>&)’
/usr/include/c++/4.4/bits/stl_function.h:357: note: candidates are: bool std::unary_negate<_Predicate>::operator()(const typename _Predicate::argument_type&) const [with _Predicate = Sudoku::<unnamed>::CellInBlock]
/usr/include/c++/4.4/bits/stl_algo.h: In function ‘_OIter std::remove_copy_if(_IIter, _IIter, _OIter, _Predicate) [with _IIter = std::_Rb_tree_const_iterator<std::shared_ptr<Sudoku::Cell> >, _OIter = std::insert_iterator<std::set<std::shared_ptr<Sudoku::Cell>, Sudoku::CellSorter, std::allocator<std::shared_ptr<Sudoku::Cell> > > >, _Predicate = std::unary_negate<Sudoku::<unnamed>::CellIsNeighbor>]’:
Puzzle.cpp:154: instantiated from here
/usr/include/c++/4.4/bits/stl_algo.h:938: error: no match for call to ‘(std::unary_negate<Sudoku::<unnamed>::CellIsNeighbor>) (const std::shared_ptr<Sudoku::Cell>&)’
/usr/include/c++/4.4/bits/stl_function.h:357: note: candidates are: bool std::unary_negate<_Predicate>::operator()(const typename _Predicate::argument_type&) const [with _Predicate = Sudoku::<unnamed>::CellIsNeighbor]
make: *** [Puzzle.o] Error 1
Is there another way to do it, or any suggestions?
First of all, since you're using the C++0x features (std::shared_ptr), it makes sense to use std::copy_if() to avoid having to call std::not1.
The first functor you wrote works, and a minimal compilable example would be something like this: https://ideone.com/XhuNu
The second functor does not work, as the compiler points out, due to mismatch between its argument_type (which is const Cell) and the argument that it is being called with, which is const std::shared_ptr<Cell>&.
It's simply not what the container contains! For all it knows at this point, those Cell objects may not even be copyable.
The second functor would indeed be a better thing to use if the container is a set of Cells, not a set of shared pointers to Cells. It is considered good design to avoid shared ownership of objects anyway.
Example code that would compile with the second functor
#include <set>
#include <functional>
#include <algorithm>
#include <iostream>
struct Cell {
int mX;
Cell(int x) : mX(x) {}
size_t GetX() const { return mX;}
};
struct CellSorter {
bool operator()(const Cell& l, const Cell& r) const
{
return l.GetX() < r.GetX();
}
};
// your second functor begins
class CellInCol : public std::unary_function<const Cell,
bool>
{
public:
CellInCol( size_t col ) : _col( col ) {}
// note use of const ref to shared_ptr's
bool operator() ( const Cell &a ) const
{
return ( a.GetX() == _col );
}
private:
size_t _col;
};
// your second functor ends
int main()
{
typedef std::set<Cell, CellSorter> Container;
Container _grid = {Cell(1), Cell(2), Cell(7), Cell(10)};
Container col;
size_t c = 7;
std::remove_copy_if( _grid.begin(), _grid.end(),
std::inserter( col, col.begin() ),
std::not1( CellInCol( c ) ) );
std::cout << "col has " << col.size() << " elements\n"
<< "the first element is " << col.begin()->GetX() << '\n';
}
test run: https://ideone.com/kLiFn