将STL算法与shared_ptr,函数对象一起使用

Question

我有一组shared_ptr,我想将remove_copy_if与谓词的自定义函数对象一起使用.我不知道"最好"的方式.现在,我有这个工作:

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;
}

我决定对shared_ptr执行const引用,因为该对象不会保留指针,这似乎比shared_ptr的额外副本更有效.

看起来对对象进行const引用似乎更好,但是我无法编译它.我把它改成了这个,但没有运气:

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;
};

这是g ++的输出:

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

还有其他方法可以做到这一点,还是有任何建议？

Answer 1

首先，由于您使用的是 C++0x 功能 ( std::shared_ptr)，因此使用std::copy_if()来避免调用是有意义的std::not1。

您编写的第一个函子有效，最小的可编译示例如下所示： https: //ideone.com/XhuNu

正如编译器指出的那样，第二个函子不起作用，因为它的 argument_type （即const Cell）与调用它的参数（即 ）之间不匹配const std::shared_ptr<Cell>&。

这根本就不是容器所含的东西！据目前所知，这些 Cell 对象甚至可能不可复制。

如果容器是一组 Cell，而不是一组指向 Cell 的共享指针，那么使用第二个函子确实会更好。无论如何，避免对象的共享所有权被认为是良好的设计。

使用第二个函子编译的示例代码

#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';
}

测试运行： https: //ideone.com/kLiFn