Question

我基本上有std::deque个对象，我想根据对象的给定成员变量的条件删除其中一些objets，以便我使用谓词，但是我有错误，我不知道真的很明白。

我正在使用g++和-std=c++11出于STL原因（共享指针）但是我试图在带有非c++11代码的MVS的窗口上解决这个问题，所以我是寻找非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();
}

，g++ -std=c++11 main.cpp -o main的输出为：

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的非静态成员函数指针。 remove_if需要普通的函数指针，静态成员函数指针或函数对象。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

您需要创建Predicate static，创建一个函数对象并传递该实例，或者在调用站点中使用lambda。

Answer 2

不是函数指针的粉丝，但是使用lambda＆＃39;这似乎是编译... 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());

不更新自动：

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

std :: remove_if

2 个答案: