[英]Error with a predicate in std::remove_if
我基本上有一個std::deque
的對象,我想根據對象的給定成員變量的條件刪除一些這些objets,所以我使用謂詞,但我有錯誤,我真的不明白。
由於STL原因(共享指針)我正在使用g++
和-std=c++11
,但是我試圖在帶有非c++11
代碼的MVS的窗口上進行此操作,因此我正在尋找非c++11
解決方案,沒有lamdas等
代碼是:
#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();
}
並且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)); }
^
您無法使用remove_if
使用非靜態成員函數指針。 remove_if
需要一個普通的函數指針,靜態成員函數指針或一個函數對象。 它不能使用成員函數指針的原因是因為它需要一個類的實例才能調用該函數而你無法將其包裝到調用中
您需要使Predicate
static
,創建一個函數對象並傳遞它的實例,或者在調用站點中使用lambda。
不是函數指針的粉絲,但使用lambda這似乎編譯... 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());
或者沒有lambdas ..
auto predicateToUse = std::bind(&B::Predicate, this, std::placeholders::_1);
dq.erase(std::remove_if(dq.begin(), dq.end(), predicateToUse), dq.end());
不自動更新:
dq.erase(std::remove_if(dq.begin(), dq.end(), std::bind(&B::Predicate, this, std::placeholders::_1)), dq.end());
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