Question

请考虑以下代码：

#include <iostream>
#include <memory>
#include <vector>

using namespace std;

struct A
{
    int a;
    A(int a_):a(a_) {}
};

int main()
{
    vector<auto_ptr<A> > as;
    for (int i = 0; i < 10; i++)
    {
        auto_ptr<A> a(new A(i));
        as.push_back(a);
    }
    for (vector<auto_ptr<A> >::iterator it = as.begin(); it != as.end(); ++it)
        cout << (*it)->a << endl;
}

当尝试编译它时，我从g ++中得到以下模糊的编译器错误：

g++ -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/proba.d" -MT"src/proba.d" -o"src/proba.o" "../src/proba.cpp"
/usr/include/c++/4.1.2/ext/new_allocator.h: In member function ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, const _Tp&) [with _Tp = std::auto_ptr<A>]’:
/usr/include/c++/4.1.2/bits/stl_vector.h:606:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = std::auto_ptr<A>, _Alloc = std::allocator<std::auto_ptr<A> >]’
../src/proba.cpp:19:   instantiated from here
/usr/include/c++/4.1.2/ext/new_allocator.h:104: error: passing ‘const std::auto_ptr<A>’ as ‘this’ argument of ‘std::auto_ptr<_Tp>::operator std::auto_ptr_ref<_Tp1>() [with _Tp1 = A, _Tp = A]’ discards qualifiers
/usr/include/c++/4.1.2/bits/vector.tcc: In member function ‘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 = std::auto_ptr<A>, _Alloc = std::allocator<std::auto_ptr<A> >]’:
/usr/include/c++/4.1.2/bits/stl_vector.h:610:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = std::auto_ptr<A>, _Alloc = std::allocator<std::auto_ptr<A> >]’
../src/proba.cpp:19:   instantiated from here
/usr/include/c++/4.1.2/bits/vector.tcc:256: error: passing ‘const std::auto_ptr<A>’ as ‘this’ argument of ‘std::auto_ptr<_Tp>::operator std::auto_ptr_ref<_Tp1>() [with _Tp1 = A, _Tp = A]’ discards qualifiers
/usr/include/c++/4.1.2/bits/stl_construct.h: In function ‘void std::_Construct(_T1*, const _T2&) [with _T1 = std::auto_ptr<A>, _T2 = std::auto_ptr<A>]’:
/usr/include/c++/4.1.2/bits/stl_uninitialized.h:86:   instantiated from ‘_ForwardIterator std::__uninitialized_copy_aux(_InputIterator, _InputIterator, _ForwardIterator, __false_type) [with _InputIterator = __gnu_cxx::__normal_iterator<std::auto_ptr<A>*, std::vector<std::auto_ptr<A>, std::allocator<std::auto_ptr<A> > > >, _ForwardIterator = __gnu_cxx::__normal_iterator<std::auto_ptr<A>*, std::vector<std::auto_ptr<A>, std::allocator<std::auto_ptr<A> > > >]’
/usr/include/c++/4.1.2/bits/stl_uninitialized.h:113:   instantiated from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<std::auto_ptr<A>*, std::vector<std::auto_ptr<A>, std::allocator<std::auto_ptr<A> > > >, _ForwardIterator = __gnu_cxx::__normal_iterator<std::auto_ptr<A>*, std::vector<std::auto_ptr<A>, std::allocator<std::auto_ptr<A> > > >]’
/usr/include/c++/4.1.2/bits/stl_uninitialized.h:254:   instantiated from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>) [with _InputIterator = __gnu_cxx::__normal_iterator<std::auto_ptr<A>*, std::vector<std::auto_ptr<A>, std::allocator<std::auto_ptr<A> > > >, _ForwardIterator = __gnu_cxx::__normal_iterator<std::auto_ptr<A>*, std::vector<std::auto_ptr<A>, std::allocator<std::auto_ptr<A> > > >, _Tp = std::auto_ptr<A>]’
/usr/include/c++/4.1.2/bits/vector.tcc:279:   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 = std::auto_ptr<A>, _Alloc = std::allocator<std::auto_ptr<A> >]’
/usr/include/c++/4.1.2/bits/stl_vector.h:610:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = std::auto_ptr<A>, _Alloc = std::allocator<std::auto_ptr<A> >]’
../src/proba.cpp:19:   instantiated from here
/usr/include/c++/4.1.2/bits/stl_construct.h:81: error: passing ‘const std::auto_ptr<A>’ as ‘this’ argument of ‘std::auto_ptr<_Tp>::operator std::auto_ptr_ref<_Tp1>() [with _Tp1 = A, _Tp = A]’ discards qualifiers
make: *** [src/proba.o] Error 1

在我看来，这里有一些问题。这是否意味着auto_ptr无法使用vector？

Answer 1

正确，std::auto_ptr无法使用std::vector。

编译器抱怨有auto_ptr的赋值运算符更改了所分配的对象，因此它不能是const。

您想要使用boost::ptr_vector或boost::shared_ptr s

的向量

Answer 2

auto_ptr有一个带有非const参数的复制构造函数，因此编译器无法从vector::push_back()调用它，因为后者有const参数。

原因是当您从另一个实例初始化一个auto_ptr实例时，新实例将对象与另一个实例断开连接并将其连接到self，以避免在一个实例delete对象时出现悬空指针情况而另一个仍然指向它。

vector <auto_ptr <>＆gt; </auto_ptr <>的编译问题

2 个答案: