std::vector doesn't call move constructor with objects with noexcept move constructs

Question

I've been struggeling a lot lately with move and copy constructors and can't seem to find the awnser on my own. The structure is fairly simple. A class OWUP which holds a std::unique_ptr to an object (in this example an int), a class One which is a simple wrapper around std::vector of OWUP's and class Two which is a simple wrapper std::vector of One's.

#include <memory>
#include <vector>

class OWUP {
public:
    OWUP()
    : data(nullptr)
    { }

    OWUP(const OWUP &) = delete;
    OWUP &operator=(const OWUP &) = delete; 
    OWUP(OWUP &&) noexcept = default;
    OWUP &operator=(OWUP &&) noexcept = default; 

    std::unique_ptr<int> data; 
};

class One {
public:
    One(std::size_t numof_datas)
    : datas(numof_datas)
    { }

    One(const One &) = delete;
    One &operator=(const One &) = delete; 
    One(One &&) noexcept = default;
    One &operator=(One &&) noexcept = default; 

    std::vector<OWUP> datas;
};

class Two {
public:
    Two(std::size_t numof_ones, std::size_t num_of_datas)
    : ones(numof_ones, One(num_of_datas))
    { }

    Two(const Two &) = delete;
    Two &operator=(const Two &) = delete; 
    Two(Two &&) noexcept = default;
    Two &operator=(Two &&) noexcept = default;

    std::vector<One> ones;
};

The code gets the following error code with g++ -std=c++14 example.cpp

In file included from /usr/include/c++/7.3.1/memory:64:0,
                 from example.cpp:1:
/usr/include/c++/7.3.1/bits/stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...) [with _T1 = One; _Args = {const One&}]':
/usr/include/c++/7.3.1/bits/stl_uninitialized.h:210:18:   required from 'static _ForwardIterator std::__uninitialized_fill_n<_TrivialValueType>::__uninit_fill_n(_ForwardIterator, _Size, const _Tp&) [with _ForwardIterator = One*; _Size = long unsigned int; _Tp = One; bool _TrivialValueType = false]'
/usr/include/c++/7.3.1/bits/stl_uninitialized.h:255:17:   required from '_ForwardIterator std::uninitialized_fill_n(_ForwardIterator, _Size, const _Tp&) [with _ForwardIterator = One*; _Size = long unsigned int; _Tp = One]'
/usr/include/c++/7.3.1/bits/stl_uninitialized.h:366:39:   required from '_ForwardIterator std::__uninitialized_fill_n_a(_ForwardIterator, _Size, const _Tp&, std::allocator<_Tp2>&) [with _ForwardIterator = One*; _Size = long unsigned int; _Tp = One; _Tp2 = One]'
/usr/include/c++/7.3.1/bits/stl_vector.h:1337:33:   required from 'void std::vector<_Tp, _Alloc>::_M_fill_initialize(std::vector<_Tp, _Alloc>::size_type, const value_type&) [with _Tp = One; _Alloc = std::allocator<One>; std::vector<_Tp, _Alloc>::size_type = long unsigned int; std::vector<_Tp, _Alloc>::value_type = One]'
/usr/include/c++/7.3.1/bits/stl_vector.h:298:27:   required from 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>::size_type, const value_type&, const allocator_type&) [with _Tp = One; _Alloc = std::allocator<One>; std::vector<_Tp, _Alloc>::size_type = long unsigned int; std::vector<_Tp, _Alloc>::value_type = One; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<One>]'
example.cpp:40:39:   required from here
/usr/include/c++/7.3.1/bits/stl_construct.h:75:7: error: use of deleted function 'One::One(const One&)'
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
example.cpp:29:3: note: declared here
   One(const One &) = delete;
   ^~~

I've tried my best to force it to use move constructors. I also tried to create my own move constructors with std::move, but that resulted int the same compile error. I'm aware that std::vector tests for move_if_noexcept(), but for can't find out why it doesn't. What is it that i'm doing wrong?

SergeyA · Accepted Answer · 2018-04-18 16:13:52Z

8

Because the problem is here:

Two(std::size_t numof_ones, std::size_t num_of_datas)
: ones(numof_ones, One(num_of_datas))
{ }

You can't move from One you create here, you really need to copy from it numof_ones times.

answered Apr 18, 2018 at 16:13

SergeyA

62.9k5 gold badges85 silver badges143 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

73nismit Over a year ago

Hmm I don't know if i fully understand your awnser, but removing One(num_of_datas) and manually adding the One(num_of_datas) numof_ones number of times in a loop does fix the problem. Is this the fix you meant?

default Over a year ago

@73nismit you're calling a constructor of vector that copies the argument. from here - 2) Constructs the container with count copies of elements with value value..

UKMonkey Over a year ago

@73nismit moving an object is like moving an apple from a box to another box. The result is that last box looks like the first before the move; but you didn't need to go to the shops to get more apples. This works great if you only need 1 apple. If you need 5 apples in 5 boxes though; you're going to need to get more; in which case you need to copy the box.

default Over a year ago

Also, it doesn't say that the move constructor is deleted, it's the copyconstructor that is deleted. switch places on "move" and "copy"?

73nismit Over a year ago

@UKmonkey I don't want to copy or move to begin with, i want to initialize the size of the 2d array

|

Chris Drew · Accepted Answer · 2018-04-18 21:06:44Z

2

Unfortunately, there is no easy way of constructing a std::vector<One> of size numof_ones without One having a default constructor or a copy constructor. The constructor you chose takes numof_ones copies of the single value you give it into each element of the vector.

The easiest solution is probably to not initialize ones in the constructor initializer list but instead initialize it in the body of the constructor:

Two(std::size_t numof_ones, std::size_t num_of_datas) {
    ones.reserve(numof_ones);
    for (size_t i = 0; i != numof_ones; ++i) {
        ones.emplace_back(num_of_datas);
    }
}

(Technically you could probably use the std::vector constructor that takes two iterators and provide your own custom iterator but it is probably not worth the effort).

edited Apr 18, 2018 at 21:06

answered Apr 18, 2018 at 19:37

Chris Drew

15.4k3 gold badges38 silver badges55 bronze badges

1 Comment

73nismit Over a year ago

This is what I did to prevent the compiler errors, but I don't think it's the right way to do it though. It doesn't feel right.

Collectives™ on Stack Overflow

std::vector doesn't call move constructor with objects with noexcept move constructs

2 Answers 2

6 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related