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’

CodePudding user response：

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.