std :: remove_if中的谓词出错

Question

我基本上有一个std::deque的对象，我想根据对象的给定成员变量的条件删除一些这些objets，所以我使用谓词，但我有错误，我真的不明白。

由于STL原因（共享指针）我正在使用g++和-std=c++11 ，但是我试图在带有非c++11代码的MVS的窗口上进行此操作，因此我正在寻找非c++11解决方案，没有lamdas等

代码是：

#include <iostream> // for std::cout and std::endl
#include <cstdio> // for getchar()
#include <memory> // for std::shared_ptr
#include <deque> // for std::deque
#include <algorithm> // for std::earse and std::remove_if

class A
{
    private:
        int _i;
        double _d;
    public:
        A(int i, double d)
        {
            _i = i;
            _d = d;
        }
        int geti()const
        {
            return _i;
        }
        double getValueOnWhichToCheck()const
        {
            return _d;
        }
};

typedef std::shared_ptr<A> A_shared_ptr;
typedef std::deque<A_shared_ptr> list_type;

void PrintDeque(list_type & dq)
{
    if (0 ==  dq.size())
    {
        std::cout << "Empty deque." << std::endl;
    }
    else
    {
        for (int i = 0 ; i < dq.size() ; ++i)
        {
            std::cout << i+1 << "\t" << dq[i] << std::endl;
        }
    }
}

class B
{
    public:
        double getThreshold() // Non constant for a reason as in real code it isn't
        {
            return 24.987; // comes from a calculation not needed here so I return a constant.
        }
        bool Predicate(A_shared_ptr & a)
        {
            return a->getValueOnWhichToCheck() >= getThreshold();
        }
        void DoStuff()
        {
            A_shared_ptr pT1 = std::make_shared<A>(A(2,      -6.899987));
            A_shared_ptr pT2 = std::make_shared<A>(A(876,    889.878762));
            A_shared_ptr pT3 = std::make_shared<A>(A(-24,    48.98924));
            A_shared_ptr pT4 = std::make_shared<A>(A(78,     -6654.98980));
            A_shared_ptr pT5 = std::make_shared<A>(A(6752,   3.141594209));
            list_type dq = {pT1,pT2,pT3,pT4,pT5};
            PrintDeque(dq);
            bool (B::*PtrToPredicate)(A_shared_ptr &) = &B::Predicate;
            dq.erase(std::remove_if(dq.begin(), dq.end(), PtrToPredicate),dq.end());
            PrintDeque(dq);
        }
};

int main()
{
    B * pB = new B();
    pB->DoStuff();
    getchar();
}

并且g++ -std=c++11 main.cpp -o main是：

In file included from /usr/include/c++/5/bits/stl_algobase.h:71:0,
                 from /usr/include/c++/5/bits/char_traits.h:39,
                 from /usr/include/c++/5/ios:40,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from main.cpp:1:
/usr/include/c++/5/bits/predefined_ops.h: In instantiation of ‘bool __gnu_cxx::__ops::_Iter_pred<_Predicate>::operator()(_Iterator) [with _Iterator = std::_Deque_iterator<std::shared_ptr<A>, std::shared_ptr<A>&, std::shared_ptr<A>*>; _Predicate = bool (B::*)(std::shared_ptr<A>&)]’:
/usr/include/c++/5/bits/stl_algo.h:866:20:   required from ‘_ForwardIterator std::__remove_if(_ForwardIterator, _ForwardIterator, _Predicate) [with _ForwardIterator = std::_Deque_iterator<std::shared_ptr<A>, std::shared_ptr<A>&, std::shared_ptr<A>*>; _Predicate = __gnu_cxx::__ops::_Iter_pred<bool (B::*)(std::shared_ptr<A>&)>]’
/usr/include/c++/5/bits/stl_algo.h:936:30:   required from ‘_FIter std::remove_if(_FIter, _FIter, _Predicate) [with _FIter = std::_Deque_iterator<std::shared_ptr<A>, std::shared_ptr<A>&, std::shared_ptr<A>*>; _Predicate = bool (B::*)(std::shared_ptr<A>&)]’
main.cpp:67:73:   required from here
/usr/include/c++/5/bits/predefined_ops.h:234:30: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘((__gnu_cxx::__ops::_Iter_pred<bool (B::*)(std::shared_ptr<A>&)>*)this)->__gnu_cxx::__ops::_Iter_pred<bool (B::*)(std::shared_ptr<A>&)>::_M_pred (...)’, e.g. ‘(... ->* ((__gnu_cxx::__ops::_Iter_pred<bool (B::*)(std::shared_ptr<A>&)>*)this)->__gnu_cxx::__ops::_Iter_pred<bool (B::*)(std::shared_ptr<A>&)>::_M_pred) (...)’
  { return bool(_M_pred(*__it)); }
                              ^

Answer 1

您无法使用remove_if使用非静态成员函数指针。 remove_if需要一个普通的函数指针，静态成员函数指针或一个函数对象。 它不能使用成员函数指针的原因是因为它需要一个类的实例才能调用该函数而你无法将其包装到调用中

您需要使Predicate static ，创建一个函数对象并传递它的实例，或者在调用站点中使用lambda。

Answer 2

不是函数指针的粉丝，但使用lambda这似乎编译... https://ideone.com/0StRcw

        dq.erase(std::remove_if(dq.begin(), dq.end(), [this](const std::shared_ptr<A>& a) {
            return a->getValueOnWhichToCheck() >= getThreshold();
        }),dq.end());

或者没有lambdas ..

        auto predicateToUse = std::bind(&B::Predicate, this, std::placeholders::_1);
        dq.erase(std::remove_if(dq.begin(), dq.end(), predicateToUse), dq.end());

不自动更新：

dq.erase(std::remove_if(dq.begin(), dq.end(), std::bind(&B::Predicate, this, std::placeholders::_1)), dq.end());