Move constructor for a class

Question

I have written two classes and I have a problem putting them in a vector:

#include <vector>
#include <iostream>
#include <fstream>

class A
{
public:
    std::ofstream filestream;
    int aa1;
    int aa2;
    int aa3;
    int aa4;
    int aa5;
    int aa6;
    int aa7;
    int aa8;
    int aa9;
    int aa10;

    A() {}
    ~A() {}
};

class B
{
    A aaa;
public:
    int b1;
    int b2;
    int b3;
    int b4;
    int b5;
    int b6;
    int b7;
    int b8;
    int b9;
    int b10;

    B() {}
    ~B() {}
};

int main()
{
    std::vector<B> vec;
    vec.emplace_back();
    return 0;
}

Output message:

g++ -std=c++11 main.cpp



In file included from /usr/include/c++/5/vector:62:0,
                 from main.cpp:1:
/usr/include/c++/5/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = B; _Args = {B}]’:
/usr/include/c++/5/bits/stl_uninitialized.h:75:18:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<B*>; _ForwardIterator = B*; bool _TrivialValueTypes = false]’
/usr/include/c++/5/bits/stl_uninitialized.h:126:15:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<B*>; _ForwardIterator = B*]’
/usr/include/c++/5/bits/stl_uninitialized.h:281:37:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = std::move_iterator<B*>; _ForwardIterator = B*; _Tp = B]’
/usr/include/c++/5/bits/stl_uninitialized.h:303:2:   required from ‘_ForwardIterator std::__uninitialized_move_if_noexcept_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&) [with _InputIterator = B*; _ForwardIterator = B*; _Allocator = std::allocator<B>]’
/usr/include/c++/5/bits/vector.tcc:422:8:   required from ‘void std::vector<_Tp, _Alloc>::_M_emplace_back_aux(_Args&& ...) [with _Args = {}; _Tp = B; _Alloc = std::allocator<B>]’
/usr/include/c++/5/bits/vector.tcc:101:23:   required from ‘void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {}; _Tp = B; _Alloc = std::allocator<B>]’
main.cpp:46:22:   required from here
/usr/include/c++/5/bits/stl_construct.h:75:7: error: use of deleted function ‘B::B(const B&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^
main.cpp:24:7: note: ‘B::B(const B&)’ is implicitly deleted because the default definition would be ill-formed:
 class B
       ^
main.cpp:24:7: error: use of deleted function ‘A::A(const A&)’
main.cpp:5:7: note: ‘A::A(const A&)’ is implicitly deleted because the default definition would be ill-formed:
 class A
       ^
main.cpp:5:7: error: use of deleted function ‘std::basic_ofstream<_CharT, _Traits>::basic_ofstream(const std::basic_ofstream<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits<char>]’
In file included from main.cpp:3:0:
/usr/include/c++/5/fstream:723:7: note: declared here
       basic_ofstream(const basic_ofstream&) = delete;
       ^

The warnings and errors are self-explanatory. But I am new in moving some classes. How should I move ofstream properly? If I define a move constructor , should I move all single elements from aa1 to aa10 too? any way to get around it?

According to the other posts , ofstream must be moveable for my g++ version. Why doesn't it move implicitly?

g++ -v
 ...
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.5) 

Answer 1

Why doesn't it move implicitly?

The move constructor would not be implicitly declared when a user-defined destructor is provided:

• there are no user-declared copy constructors;
• there are no user-declared copy assignment operators;
• there are no user-declared move assignment operators;
• there are no user-declared destructors;

You can remove the declaration of destructors of A and B to let the compiler generate the move constructors for you, or you can define them explicitly by using default . For both cases the move constructor performs full member-wise move of the object's bases and non-static members. eg

class A
{
public:
    ...
    A(A&&) = default;
};

class B
{
public:
    ...
    B(B&&) = default;
};

LIVE

Answer 2

除了对Songyuanyao的回答之外，这篇文章还完美地解释了有关隐式构造函数的所有内容。