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I'm trying to sort a vector using std::sort, i.e
ScanIndex::ScanIndex(std::vector<ScanData*> *scans, int currVersion, KeyCell minKey, KeyCell maxKey){
std::sort(scans->begin(), scans->end(),
[](const ScanData *& a, const ScanData *& b) -> bool
{
return (a->version.load() > b->version.load());
});
}
While having this error:
/usr/include/c++/5/bits/predefined_ops.h: In instantiation of ‘constexpr bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = __gnu_cxx::__normal_iterator<ScanData**, std::vector<ScanData*> >; _Iterator2 = __gnu_cxx::__normal_iterator<ScanData**, std::vector<ScanData*> >; _Compare = ScanIndex::ScanIndex(std::vector<ScanData*>*, int, KeyCell, KeyCell)::<lambda(const ScanData*&, const ScanData*&)>]’:
/usr/include/c++/5/bits/stl_algo.h:1842:14: required from ‘void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<ScanData**, std::vector<ScanData*> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<ScanIndex::ScanIndex(std::vector<ScanData*>*, int, KeyCell, KeyCell)::<lambda(const ScanData*&, const ScanData*&)> >]’
/usr/include/c++/5/bits/stl_algo.h:1880:25: required from ‘void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<ScanData**, std::vector<ScanData*> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<ScanIndex::ScanIndex(std::vector<ScanData*>*, int, KeyCell, KeyCell)::<lambda(const ScanData*&, const ScanData*&)> >]’
/usr/include/c++/5/bits/stl_algo.h:1966:31: required from ‘void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<ScanData**, std::vector<ScanData*> >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<ScanIndex::ScanIndex(std::vector<ScanData*>*, int, KeyCell, KeyCell)::<lambda(const ScanData*&, const ScanData*&)> >]’
/usr/include/c++/5/bits/stl_algo.h:4729:18: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<ScanData**, std::vector<ScanData*> >; _Compare = ScanIndex::ScanIndex(std::vector<ScanData*>*, int, KeyCell, KeyCell)::<lambda(const ScanData*&, const ScanData*&)>]’
/home/dvir/CLionProjects/KiWi-cpp-pq-port/ScanIndex.cpp:38:11: required from here
/usr/include/c++/5/bits/predefined_ops.h:125:46: error: no match for call to ‘(ScanIndex::ScanIndex(std::vector<ScanData*>*, int, KeyCell, KeyCell)::<lambda(const ScanData*&, const ScanData*&)>) (ScanData*&, ScanData*&)’
{ return bool(_M_comp(*__it1, *__it2)); }
This is the ScanData object
class ScanData{
public:
static const ScanData* empty_ScanData;
ScanData(KeyCell min, KeyCell max) : min(min), max(max), version(0)
{}
ScanData(const ScanData& scanData) : min(scanData.min), max(scanData.max), version(version.load())
{}
std::atomic<int> version;
KeyCell min;
KeyCell max;
};
I'm guessing that I've declared a different type(signature) of lambda than the one expected, but it seems to correspond to the signature in the docs.
Thoughts?
scans is a pointer to a vector containing ScanData*.
Therefore, the lambda can expect an argument that may bind to ScanData* const &.
The type you specify is const ScanData* & (the referred to pointer is not const, but the pointee). The qualifications are mismatching. While a conversion is possible from ScanData* to const ScanData*, that will require a temporary pointer, and a non-const lvalue reference cannot bind to one.
Since pointers are value types, and cheap to copy value types at that, just don't pass by reference. Pass the pointers by value to the lambda.
[](const ScanData *a, const ScanData *b) -> bool
{
return (a->version.load() > b->version.load());
});
I have a class, which looks like that:
template<typename T>
using VectorPtr=std::vector<std::unique_ptr<T>>;
template<typename T>
using VectorRawPtr=std::vector<T*>;
class ItemsSet{ // <-- Compiler say this line contans an error 0_o ?
public:
ItemsSet(VectorPtr<Item>& items);
~ItemsSet() = default;
VectorRawPtr<Item> GetItems();
VectorRawPtr<Item> GetSuitableItemsForPeriod(const IPeriod &period);
double CalculateTotal();
private:
VectorPtr<Item> _items;
};
constructor looks like:
ItemsSet::ItemsSet(VectorPtr<Item> & items) {
for(auto &itm: items){
_items.emplace_back(std::move(itm));
}
}
however this code isn't compiled and failed with error:
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::unique_ptr<Item, std::default_delete<Item> >; _Args = {const std::unique_ptr<Item, std::default_delete<Item> >&}]':
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_uninitialized.h:75:18: required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<Item, std::default_delete<Item> >*, std::vector<std::unique_ptr<Item, std::default_delete<Item> >, std::allocator<std::unique_ptr<Item, std::default_delete<Item> > > > >; _ForwardIterator = std::unique_ptr<Item, std::default_delete<Item> >*; bool _TrivialValueTypes = false]'
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_uninitialized.h:126:15: required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<Item, std::default_delete<Item> >*, std::vector<std::unique_ptr<Item, std::default_delete<Item> >, std::allocator<std::unique_ptr<Item, std::default_delete<Item> > > > >; _ForwardIterator = std::unique_ptr<Item, std::default_delete<Item> >*]'
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_uninitialized.h:281:37: required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<Item, std::default_delete<Item> >*, std::vector<std::unique_ptr<Item, std::default_delete<Item> >, std::allocator<std::unique_ptr<Item, std::default_delete<Item> > > > >; _ForwardIterator = std::unique_ptr<Item, std::default_delete<Item> >*; _Tp = std::unique_ptr<Item, std::default_delete<Item> >]'
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_vector.h:322:31: required from 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<Item, std::default_delete<Item> >; _Alloc = std::allocator<std::unique_ptr<Item, std::default_delete<Item> > >]'
/cygdrive/d/code/itemSet.h:4:19: required from here
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_construct.h:75:7: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = Item; _Dp = std::default_delete<Item>]'
{ ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
Could anyone explain me what I'm doing wrong and how could I fix my problem?
I'm pretty sure that actual problem is an implicit copy constructor ither for ItemsSet or Item. Because you using unique_ptr's which can't really be copied, copy constructor can't be generated properly. Try to explicitly delete copy constructors and find the place where they used and change those place to move declaration for example, or use shared pointers.
This isn't the actual code that produces the error (your line numbers don't match, and neither do the errors; you should present an actual testcase here), but we can still see the problem.
unique_ptrs cannot be copied (they're "unique"!), yet by copy-initialising _items from a whole vector of them, you're attempting to copy them all. You can't do that.
You could move the constructor argument into _items instead.
I don't know if this will fix it or not, but you might try moving the constructor parameter directly into _items instead of moving each individual member into it:
ItemsSet::ItemsSet(VectorPtr<Item>&& items)
: _items(std::move(items))
{
}
There are tons of codes that sort an array of strings lexicographically but I can't find one that does sorting characters in ONE string reverse lexicographically. So far I discovered that using std::sort() in <algorithm> might be the closest candidate.
This is what I tried:
template <typename T>
class ReverseComparator{
bool operator()(T l, T r){return !(l < r);}
};
//.....later
std::sort(str.begin(), str.end(), comparator);
Here's the question: how can I initialize my comparator so that it can compare the characters in the string? I tried ReverseComparator<char> comparator but compiler throws a ton of error msg. gcc-4.5.1 showed these errors:
In file included from /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/algorithm:63:0,
from prog.cpp:1:
prog.cpp: In function 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2192:4: instantiated from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5252:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2125:4: error: within this context
prog.cpp: In function 'void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5100:7: instantiated from 'void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2297:8: instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1914:2: error: within this context
prog.cpp: In function 'void std::__move_median_first(_Iterator, _Iterator, _Iterator, _Compare) [with _Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2260:7: instantiated from '_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2302:62: instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:108:7: error: within this context
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:110:4: error: within this context
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2260:7: instantiated from '_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2302:62: instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:112:9: error: within this context
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:115:12: error: within this context
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:117:12: error: within this context
prog.cpp: In function '_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Tp = char, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2261:78: instantiated from '_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2302:62: instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2229:4: error: within this context
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2232:4: error: within this context
prog.cpp: In function 'void std::__unguarded_linear_insert(_RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2133:6: instantiated from 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2192:4: instantiated from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5252:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2083:7: error: within this context
In file included from /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:62:0,
from /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/algorithm:63,
from prog.cpp:1:
prog.cpp: In function 'void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Distance = int, _Tp = char, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:434:4: instantiated from 'void std::make_heap(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1912:7: instantiated from 'void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5100:7: instantiated from 'void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2297:8: instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:303:4: error: within this context
prog.cpp: In function 'void std::__push_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Distance = int, _Tp = char, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:316:7: instantiated from 'void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Distance = int, _Tp = char, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:434:4: instantiated from 'void std::make_heap(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1912:7: instantiated from 'void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5100:7: instantiated from 'void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2297:8: instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:180:7: error: within this context
While solving the compiler error simply requires the operator() to be public as according to most other answers to this question, you should note that your comparator isn't a valid strict comparator. A strict comparator has to return false for elements which are treated as being equal. You should also use const references instead of values for the parameter types of your comparator.
I want to present you multiple options to solve your problem (not the compiler-error but the original problem, sorting a string in reversed lexical order). All of them return false for two equal elements.
The first option is your solution but I fixed the strictness:
template <typename T>
class ReverseComparator{
public:
bool operator()(const T& l, const T& r){return (r < l);}
};
std::sort(str.begin(), str.end(), ReverseComparator());
The second option uses C++11's lambda function, but is essentially the same:
std::sort(str.begin(), str.end(), [](const T& l, const T& r){return (r < l);});
As a third option, as first proposed by Mooning Duck, you could simply use the std functor for operator> on your type char:
std::sort(str.begin(), str.end(), std::greater<char>());
The fourth option (the simplest one) would be to use the default comparator on the reversed string, as first proposed by Peter Wood (note the r!):
std::sort(str.rbegin(), str.rend());
Usually, only the first error is relevant, so we have this:
In file included from /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/algorithm:63:0,
from prog.cpp:1:
prog.cpp: In function 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2192:4: instantiated from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5252:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]'
prog.cpp:12:49: instantiated from here
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
Most of that is simply context, so here's the key part:
'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
Well that's easy to fix:
#include <algorithm>
#include <string>
template <typename T>
class ReverseComparator{
public: //forgot this
bool operator()(T l, T r){return !(l < r);}
};
int main() {
std::string str = "HELLO";
ReverseComparator<char> comparator;
std::sort(str.begin(), str.end(), comparator);
}
This compiles fine, however, sorts are based on what's called a "strict weak ordering", and !(l < r) is not a strict weak ordering. Namely, comparitor(a,a) should return false, but yours returns true. Luckily, the idea of sorting this way is already in the standard library under the name std::greater. (std::sort without a comparitor uses std::less by default)
Using std::sort() is clearly the right approach. The default comparator is std::less<value_type> where value_type is decltype(*it) for the type of the iterators passed. To change the default you'd just pass a different comparator, e.g. std::greater<char>.
To use your ReverseComparator as it is defined you'd need to instantiate it correctly:
std::sort(s.begin(), s.end(), ReverseComparator<char>());
Of course, to make it work you'd need to make the operator()() publically accessible. You might want to make it a const member function as well:
template <typename T>
class ReverseComparator {
public:
bool operator()(T lhs, T rhs) const { return rhs < lhs; }
};
Note that the expression in the operator is changed as well: using !(lhs < rhs) doesn't define a strict weak order: comparator(x, x) would yield true but it is required to yield false.
I am intending to declare a vector of atomic variables to be used as counters in a multithreaded programme. Here is what I tried:
#include <atomic>
#include <vector>
int main(void)
{
std::vector<std::atomic<int>> v_a;
std::atomic<int> a_i(1);
v_a.push_back(a_i);
return 0;
}
And this is the annoyingly verbose error message of gcc 4.6.3:
In file included from /usr/include/c++/4.6/x86_64-linux-gnu/./bits/c++allocator.h:34:0,
from /usr/include/c++/4.6/bits/allocator.h:48,
from /usr/include/c++/4.6/vector:62,
from test_atomic_vec.h:2,
from test_atomic_vec.cc:1:
/usr/include/c++/4.6/ext/new_allocator.h: In member function ‘void __gnu_cxx::new_allocator<_Tp>::construct(__gnu_cxx::new_allocator<_Tp>::pointer, const _Tp&) [with _Tp = std::atomic<int>, __gnu_cxx::new_allocator<_Tp>::pointer = std::atomic<int>*]’:
/usr/include/c++/4.6/bits/stl_vector.h:830:6: instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20: instantiated from here
/usr/include/c++/4.6/ext/new_allocator.h:108:9: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:538:7: error: declared here
In file included from /usr/include/c++/4.6/vector:70:0,
from test_atomic_vec.h:2,
from test_atomic_vec.cc:1:
/usr/include/c++/4.6/bits/vector.tcc: In member function ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::atomic<int>&}, _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::atomic<int>*, std::vector<std::atomic<int> > >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = std::atomic<int>*]’:
/usr/include/c++/4.6/bits/stl_vector.h:834:4: instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20: instantiated from here
/usr/include/c++/4.6/bits/vector.tcc:319:4: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:538:7: error: declared here
/usr/include/c++/4.6/bits/stl_vector.h:834:4: instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20: instantiated from here
/usr/include/c++/4.6/bits/vector.tcc:319:4: error: use of deleted function ‘std::atomic<int>& std::atomic<int>::operator=(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:539:15: error: declared here
In file included from /usr/include/c++/4.6/x86_64-linux-gnu/./bits/c++allocator.h:34:0,
from /usr/include/c++/4.6/bits/allocator.h:48,
from /usr/include/c++/4.6/vector:62,
from test_atomic_vec.h:2,
from test_atomic_vec.cc:1:
/usr/include/c++/4.6/ext/new_allocator.h: In member function ‘void __gnu_cxx::new_allocator<_Tp>::construct(__gnu_cxx::new_allocator<_Tp>::pointer, _Args&& ...) [with _Args = {std::atomic<int>}, _Tp = std::atomic<int>, __gnu_cxx::new_allocator<_Tp>::pointer = std::atomic<int>*]’:
/usr/include/c++/4.6/bits/vector.tcc:306:4: instantiated from ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::atomic<int>&}, _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::atomic<int>*, std::vector<std::atomic<int> > >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_vector.h:834:4: instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20: instantiated from here
/usr/include/c++/4.6/ext/new_allocator.h:114:4: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:538:7: error: declared here
In file included from /usr/include/c++/4.6/vector:61:0,
from test_atomic_vec.h:2,
from test_atomic_vec.cc:1:
/usr/include/c++/4.6/bits/stl_algobase.h: In static member function ‘static _BI2 std::__copy_move_backward<true, false, std::random_access_iterator_tag>::__copy_move_b(_BI1, _BI1, _BI2) [with _BI1 = std::atomic<int>*, _BI2 = std::atomic<int>*]’:
/usr/include/c++/4.6/bits/stl_algobase.h:581:18: instantiated from ‘_BI2 std::__copy_move_backward_a(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = std::atomic<int>*, _BI2 = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_algobase.h:590:34: instantiated from ‘_BI2 std::__copy_move_backward_a2(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = std::atomic<int>*, _BI2 = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_algobase.h:661:15: instantiated from ‘_BI2 std::move_backward(_BI1, _BI1, _BI2) [with _BI1 = std::atomic<int>*, _BI2 = std::atomic<int>*]’
/usr/include/c++/4.6/bits/vector.tcc:313:4: instantiated from ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::atomic<int>&}, _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::atomic<int>*, std::vector<std::atomic<int> > >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_vector.h:834:4: instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20: instantiated from here
/usr/include/c++/4.6/bits/stl_algobase.h:546:6: error: use of deleted function ‘std::atomic<int>& std::atomic<int>::operator=(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:539:15: error: declared here
In file included from /usr/include/c++/4.6/vector:63:0,
from test_atomic_vec.h:2,
from test_atomic_vec.cc:1:
/usr/include/c++/4.6/bits/stl_construct.h: In function ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::atomic<int>, _Args = {std::atomic<int>}]’:
/usr/include/c++/4.6/bits/stl_uninitialized.h:77:3: instantiated from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<std::atomic<int>*>, _ForwardIterator = std::atomic<int>*, bool _TrivialValueTypes = false]’
/usr/include/c++/4.6/bits/stl_uninitialized.h:119:41: instantiated from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<std::atomic<int>*>, _ForwardIterator = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_uninitialized.h:259:63: instantiated from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = std::move_iterator<std::atomic<int>*>, _ForwardIterator = std::atomic<int>*, _Tp = std::atomic<int>]’
/usr/include/c++/4.6/bits/stl_uninitialized.h:269:24: instantiated from ‘_ForwardIterator std::__uninitialized_move_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&) [with _InputIterator = std::atomic<int>*, _ForwardIterator = std::atomic<int>*, _Allocator = std::allocator<std::atomic<int> >]’
/usr/include/c++/4.6/bits/vector.tcc:343:8: instantiated from ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::atomic<int>&}, _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::atomic<int>*, std::vector<std::atomic<int> > >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_vector.h:834:4: instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20: instantiated from here
/usr/include/c++/4.6/bits/stl_construct.h:76:7: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:538:7: error: declared here
How do I solve this?
The error disappears when I comment out the line with push_back() .
edit: I edited the post... For those of you who saw the first post, the error was embarrassingly that I used gcc instead of g++ :\
As described in this closely related question that was mentioned in the comments, std::atomic<T> isn't copy-constructible, nor copy-assignable.
Object types that don't have these properties cannot be used as elements of std::vector.
However, it should be possible to create a wrapper around the std::atomic<T> element that is copy-constructible and copy-assignable. It will have to use the load() and store() member functions of std::atomic<T> to provide construction and assignment (this is the idea described by the accepted answer to the question mentioned above):
#include <atomic>
#include <vector>
template <typename T>
struct atomwrapper
{
std::atomic<T> _a;
atomwrapper()
:_a()
{}
atomwrapper(const std::atomic<T> &a)
:_a(a.load())
{}
atomwrapper(const atomwrapper &other)
:_a(other._a.load())
{}
atomwrapper &operator=(const atomwrapper &other)
{
_a.store(other._a.load());
}
};
int main(void)
{
std::vector<atomwrapper<int>> v_a;
std::atomic<int> a_i(1);
v_a.push_back(a_i);
return 0;
}
EDIT: As pointed out correctly by Bo Persson, the copy operation performed by the wrapper is not atomic. It enables you to copy atomic objects, but the copy itself isn't atomic. This means any concurrent access to the atomics must not make use of the copy operation. This implies that operations on the vector itself (e.g. adding or removing elements) must not be performed concurrently.
Example: If, say, one thread modifies the value stored in one of the atomics while another thread adds new elements to the vector, a vector reallocation may occur and the object the first thread modifies may be copied from one place in the vector to another. In that case there would be a data race between the element access performed by the first thread and the copy operation triggered by the second.
To first answer your headline question: you can declare a std::vector<std::atomic<...>> easily, as you have done in your example.
Because of the lack of copy or move constructors for std::atomic<> objects, however, your use of the vector will be restricted as you found out with the compilation error on push_back(). Basically you can't do anything that would invoke either constructor.
This means your vector's size must be fixed at construction, and you should manipulate elements using operator[] or .at(). For your example code, the following works1:
std::vector<std::atomic<int>> v_a(1);
std::atomic<int> a_i(1);
v_a[0] = a_i.load();
If the "fixed size at construction" limitation is too onerous, you can use std::deque instead. This lets you emplace objects, growing the structure dynamically without requiring copy or move constructors, e.g.:
std::deque<std::atomic<int>> d;
d.emplace_back(1);
d.emplace_back(2);
d.pop_back();
There are still some limitations, however. For example, you can pop_back(), but you cannot use the more general erase(). The limitations make sense: an erase() in the middle of the blocks of contiguous storage used by std::deque in general requires the movement of elements, hence requires copy/move constructor or assignment operators to be present.
If you can't live with those limitations, you could create a wrapper class as suggested in other answers but be aware of the underlying implementation: it makes little sense to move a std::atomic<> object once it is being used: it would break any threads concurrently accessing the objects. The only sane use of copy/move constructors is generally in the initial setup of collections of these objects before they are published to other threads.
1 Unless, perhaps, you use Intel's icc compiler, which fails with an internal error while compiling this code.
Looks to me like atomic<T> has no copy constructor. Nor a move constructor, as far as I can tell.
One work around might be to use vector<T>::emplace_back() to construct the atomic in-place in the vector. Alas, I don't have a C++11 compiler on me right now, or I'd go and test it.
As others have properly noted, the cause of the compiler's error is that std::atomic explicitly prohibits the copy constructor.
I had a use case where I wanted the convenience of an STL map (specifically I was using a map of maps in order to achieve a sparse 2-dimensional matrix of atomics so I can do something like int val = my_map[10][5]). In my case there would be only one instance of this map in the program, so it wouldn't be copied, and even better I can initialize the entire thing using braced initialization. So it was very unfortunate that while my code would never attempt copying individual elements or the map itself, I was prevented from using an STL container.
The workaround I ultimately went with is to store the std::atomic inside a std::shared_ptr. This has pros, but possibly a con:
Pros:
Can store std::atomic inside any STL container
Does not require/restrict using only certain methods of STL containers.
Pro or Con (this aspect's desirability depends on the programs' use cases):
- There is only a single shared atomic for a given element. So copying the shared_ptr or the STL container will still yield a single shared atomic for the element. In other words, if you copy the STL container and modify one of the atomic elements, the other container's corresponding atomic element will also reflect the new value.
In my case the Pro/Con characteristic was moot due to my use case.
Here's a brief syntax to initialize a std::vector with this method:
#include <atomic>
#include <memory>
#include <vector>
std::vector<std::shared_ptr<std::atomic<int> > > vecAtomicInts
{
std::shared_ptr<std::atomic<int> >(new std::atomic<int>(1) ),
std::shared_ptr<std::atomic<int> >(new std::atomic<int>(2) ),
};
// push_back, emplace, etc all supported
vecAtomicInts.push_back(std::shared_ptr<std::atomic<int> >(new std::atomic<int>(3) ) );
// operate atomically on element
vecAtomicInts[1]->exchange(4);
// access random element
int i = *(vecAtomicInts[1]);
I'm having trouble compiling the following code:
typedef std::map<mUUID, block_ptr_t> BlockMap;
BlockMap _store;
std::pair< BlockMap::iterator, bool > it;
it = _store.insert(hint, std::make_pair(entry.block_uid, block));
The error is:
error: no match for ‘operator=’ in ‘it = BlockStore::_store.std::map<_Key, _Tp, _Compare, _Alloc>::insert [with _Key = mUUID, _Tp = Block*, _Compare = std::less<mUUID>, _Alloc = std::allocator<std::pair<const mUUID, Block*> >](lb, ((const std::pair<const mUUID, Block*>&)(& std::pair<const mUUID, Block*>(((const std::pair<mUUID, Block*>&)((const std::pair<mUUID, Block*>*)(& std::make_pair(_T1, _T2) [with _T1 = mUUID, _T2 = Block*](block))))))))’
/usr/include/c++/4.4/bits/stl_pair.h:68: note: candidates are: std::pair<std::_Rb_tree_iterator<std::pair<const mUUID, Block*> >, bool>& std::pair<std::_Rb_tree_iterator<std::pair<const mUUID, Block*> >, bool>::operator=(const std::pair<std::_Rb_tree_iterator<std::pair<const mUUID, Block*> >, bool>&)
It seems to be related to the assignment because if I don't assign it to "it" it compiles without error.
The version of insert that takes a hint returns only an iterator, not a pair.
First of all, your example is incomplete - _store is defined nowhere.
Second, it seems you are trying to assign an iterator to a std::pair - why do you need a pair, anyway?
The following should work, provided that _store is of type BlockMap:
BlockMap::iterator it;
it = _store.insert(std::make_pair(entry.block_uid, block));