http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55977
Bug #: 55977
Summary: [C++11] vector range construction imposes unnecessary
conversion constraints
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: daniel.krueg...@googlemail.com
The following code compiled with gcc 4.8.0 20130113 (experimental) using the
flags
-Wall -std=c++11 -pedantic
is rejected:
//-------------------------------------
#include <memory>
#include <utility>
#include <vector>
#include <iterator>
#include <iostream>
template <class T>
struct MyAllocator {
std::allocator<T> base;
typedef T value_type;
MyAllocator() = default;
template <class U>
MyAllocator(const MyAllocator<U>& other) : base(other.base) {}
T* allocate(std::size_t n) { return base.allocate(n); }
void deallocate(T* p, std::size_t n) { return base.deallocate(p, n); }
template <class U, class... Args>
void construct(U* p, Args&&... args)
{
::new (static_cast<void*>(p)) T(std::forward<Args>(args)...);
}
};
struct A {
private:
friend class MyAllocator<A>;
A(int value) : value(value) {}
int value;
public:
A() : value() {}
int get() const { return value; }
};
int main()
{
std::vector<A, MyAllocator<A>> v1;
const int i = 1;
v1.emplace_back(i); // OK
std::vector<A, MyAllocator<A>> v2(std::istream_iterator<int>(std::cin), {});
// Error
}
//-------------------------------------
"[..]gcc\include\c++\4.8.0\bits\stl_vector.h||In instantiation of 'void
std::vector<_Tp, _Alloc>::_M_range_initialize(_InputIterator, _InputIterator,
std::input_iterator_tag) [with _InputIterator = std::istream_iterator<int>; _Tp
= A; _Alloc = MyAllocator<A>]':|
[..]gcc\include\c++\4.8.0\bits\stl_vector.h|1178|required from 'void
std::vector<_Tp, _Alloc>::_M_initialize_dispatch(_InputIterator,
_InputIterator, std::__false_type) [with _InputIterator =
std::istream_iterator<int>; _Tp = A; _Alloc = MyAllocator<A>]'|
[..]gcc\include\c++\4.8.0\bits\stl_vector.h|396|required from 'std::vector<_Tp,
_Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with
_InputIterator = std::istream_iterator<int>; <template-parameter-2-2> = void;
_Tp = A; _Alloc = MyAllocator<A>; std::vector<_Tp, _Alloc>::allocator_type =
MyAllocator<A>]'|
main.cpp|38|required from here|
main.cpp|26|error: 'A::A(int)' is private|
[..]gcc\include\c++\4.8.0\bits\stl_vector.h|1188|error: within this context"
The error is due to a call of push_back with an previous int->A conversion.
Such an conversion from the iterator's value_type to the containers value_type
is not feasible. The specification of the corresponding constructor is
(indirectly) ruled by Table 100, expression X(i, j):
"Requires: T shall be EmplaceConstructible into X from *i."
Move/CopyConstructible or Convertible to value_type are not required.
Instead of invoking push_back this constructor should internally invoke the
equivalent of emplace_back.
The same problem also occurs for other standard containers such as std::list.
