Why is compiler giving error while using make_unique with array? - c++

Question

Why can't i initialize a unique pointer
#include <iostream>
#include <memory>
class Widget
{
std::unique_ptr<int[]> arr;
public:
Widget(int size)
{
arr = std::make_unique<int[size]>();
}
~Widget()
{
}
};
int main()
{
}
I am not able to understand the meaning of below error messages. What is wrong with my invocation of
arr = std::make_unique<int[size]>();
I am just trying to create a class with unique pointer member which manages an array. How should I change the below program and WHY?
Error(s):
1129699974/source.cpp: In constructor ‘Widget::Widget(int)’:
1129699974/source.cpp:13:43: error: no matching function for call to ‘make_unique<int [size]>()’
arr = std::make_unique<int[size]>();
^
In file included from /usr/include/c++/7/memory:80:0,
from 1129699974/source.cpp:4:
/usr/include/c++/7/bits/unique_ptr.h:824:5: note: candidate: template<class _Tp, class ... _Args> typename std::_MakeUniq<_Tp>::__single_object std::make_unique(_Args&& ...)
make_unique(_Args&&... __args)
^~~~~~~~~~~
/usr/include/c++/7/bits/unique_ptr.h:824:5: note: template argument deduction/substitution failed:
/usr/include/c++/7/bits/unique_ptr.h: In substitution of ‘template<class _Tp, class ... _Args> typename std::_MakeUniq<_Tp>::__single_object std::make_unique(_Args&& ...) [with _Tp = int [size]; _Args = {}]’:
1129699974/source.cpp:13:43: required from here
/usr/include/c++/7/bits/unique_ptr.h:824:5: error: ‘int [size]’ is a variably modified type
/usr/include/c++/7/bits/unique_ptr.h:824:5: error: trying to instantiate ‘template<class _Tp> struct std::_MakeUniq’
/usr/include/c++/7/bits/unique_ptr.h:830:5: note: candidate: template<class _Tp> typename std::_MakeUniq<_Tp>::__array std::make_unique(std::size_t)
make_unique(size_t __num)
^~~~~~~~~~~
/usr/include/c++/7/bits/unique_ptr.h:830:5: note: template argument deduction/substitution failed:
/usr/include/c++/7/bits/unique_ptr.h: In substitution of ‘template<class _Tp> typename std::_MakeUniq<_Tp>::__array std::make_unique(std::size_t) [with _Tp = int [size]]’:
1129699974/source.cpp:13:43: required from here
/usr/include/c++/7/bits/unique_ptr.h:830:5: error: ‘int [size]’ is a variably modified type
/usr/include/c++/7/bits/unique_ptr.h:830:5: error: trying to instantiate ‘template<class _Tp> struct std::_MakeUniq’
/usr/include/c++/7/bits/unique_ptr.h:836:5: note: candidate: template<class _Tp, class ... _Args> typename std::_MakeUniq<_Tp>::__invalid_type std::make_unique(_Args&& ...) <deleted>
make_unique(_Args&&...) = delete;
^~~~~~~~~~~
/usr/include/c++/7/bits/unique_ptr.h:836:5: note: template argument deduction/substitution failed:
/usr/include/c++/7/bits/unique_ptr.h: In substitution of ‘template<class _Tp, class ... _Args> typename std::_MakeUniq<_Tp>::__invalid_type std::make_unique(_Args&& ...) [with _Tp = int [size]; _Args = {}]’:
1129699974/source.cpp:13:43: required from here
/usr/include/c++/7/bits/unique_ptr.h:836:5: error: ‘int [size]’ is a variably modified type
/usr/include/c++/7/bits/unique_ptr.h:836:5: error: trying to instantiate ‘template<class _Tp> struct std::_MakeUniq’

Answer 1

You need
arr = std::make_unique<int[]>(size);
make_unique has a specific documented usage:
template< class T > unique_ptr<T> make_unique( std::size_t size );`
... Constructs an array of the given dynamic size. The array elements are value-initialized. This overload participates in overload resolution only if T is an array of unknown bound. The function is equivalent to:
unique_ptr<T>(new std::remove_extent_t<T>[size]())
Conceptually-speaking1, you can think of make_unique as being very similar to std::unique_ptr<T>(new T(...)) where ... represents forwarding of the args passed to make_unique. In this case, the usage is analogous to forwarding the one arg to operator new[] (std::size_t size)
1 The documentation I linked above doesn't say that the arg is forwarded in the array case, and I don't believe the spec does either.