Re: operator= required for std::vector<>::push_back ??
Miles Bader wrote:
If I have the following test file, x.cc:
#include <vector>
class X
{
public:
X (const int &_i) : i (_i) { }
X (const X &x) : i (x.i) { }
#if 0
X &operator= (const X &x)
{
if (&x != this)
new (this) X (x.i);
return *this;
}
#endif
const int &i;
};
int test ()
{
std::vector<X> xv;
int i;
xv.push_back (X (i));
}
I get an error during compilation, like:
g++ -c -o x.o x.cc
x.cc: In member function ???X& X::operator=(const X&)???:
x.cc:4: instantiated from ???void std::vector<_Tp,
_Alloc>::_M_insert_aux(__gnu_cxx::__normal_iterator<typename
std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer,
std::vector<_Tp, _Alloc> >, const _Tp&) [with _Tp = X, _Alloc =
std::allocator<X>]???
/usr/include/c++/4.4/bits/stl_vector.h:741: instantiated from ???void
std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = X, _Alloc
= std::allocator<X>]???
x.cc:23: instantiated from here
x.cc:4: error: non-static reference member ???const int& X::i???, can't
use default assignment operator
Changing the "#if 0" to "#if 1" to define operator= makes things work.
I understand the immediate reason for the error: the g++ vector
implementation does indeed seem to call operator= on vector elements for
push_back, and in this case, the compiler-synthesized method isn't good
enough.
But it somehow seems wrong that it's using operator= at all; shouldn't a
copy-constructor be enough?
Looking at the g++ STL headers, it seems like the problem is that it's
defining push_back in terms of insertion, and for the latter, you need
operator= to move elements subsequent elements to make room for a new
element. However in the case of push_back, that's really unnecessary,
and no copying of objects other than copy-constructing into new memory
when the vector's underlying storage is reallocated should be need.
Opinions? Is the error here reasonable?
It's completely reasonable. You are causing undefined behavior, because the
type T you use with the container is required to be Assignable. GCC will
have a good reason for using "insert" i think, and it's allowed to assume
your type is Assignable.
If you insert at the end, there is no need to move any elements subsequent i
think, so there is no performance penalty either.
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