CODEWITHSUNDEEP
c++vectorcopy-constructor
herophant
herophant

Reputation: 680

Why is the copy constructor being called for the construction of an empty unique_ptr vector?

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

class A {
    std::vector<std::unique_ptr<int>> vec;
public:
    virtual ~A() = 0;
};

A::~A() {}

class B : public A {
public:
    B() {}
};

int main () {
    B b = B();
    return 0;
}

Above is a minimal reproducible example. It does not compile, and the following error message is given:

me:~ $ g++ main2.cc
In file included from /usr/include/c++/7/vector:62:0,
                 from main2.cc:1:
/usr/include/c++/7/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::unique_ptr<int>; _Args = {const std::unique_ptr<int, std::default_delete<int> >&}]’:
/usr/include/c++/7/bits/stl_uninitialized.h:83:18:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*; bool _TrivialValueTypes = false]’
/usr/include/c++/7/bits/stl_uninitialized.h:134:15:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*]’
/usr/include/c++/7/bits/stl_uninitialized.h:289:37:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<int>*, std::vector<std::unique_ptr<int> > >; _ForwardIterator = std::unique_ptr<int>*; _Tp = std::unique_ptr<int>]’
/usr/include/c++/7/bits/stl_vector.h:331:31:   required from ‘std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<int>; _Alloc = std::allocator<std::unique_ptr<int> >]’
main2.cc:5:7:   required from here
/usr/include/c++/7/bits/stl_construct.h:75:7: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/memory:80:0,
                 from main2.cc:2:
/usr/include/c++/7/bits/unique_ptr.h:388:7: note: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^~~~~~~~~~

What it all seems like is that the copy constructor for the unique_ptr is being called somewhere.

But I have no idea why it would be called at all. When a vector is initialized, it has size 0, right? Under that assumption, no unique_ptr should be constructed.

What is the source of my compilation error?

Upvotes: 0

Views: 109

Answers (1)

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385325

B b = B() is a copy-initialisation and, until C++17, required that the copy be valid (even if it ended up being elided).

Even though the size of the vector in that copy is zero, that is not something that can be statically known by the innards of the copy machinery during compile-time. So, the code to make it work must be "known" during compilation, so that it is available at runtime if needed.

You probably meant just a normal declaration: B b;.

The original approach would actually have been enough if your class were trivially-moveable (because a unique_ptr can be moved), but by giving it a user-declared destructor you pessimised it. Unless you actually have a strong reason to want this class to be polymorphic, don't do that. Use = default or just omit the destructor entirely; it's not doing anything useful.

Upvotes: 2

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