CODEWITHSUNDEEP
c++smart-pointersmake-shared
MutomboDikey
MutomboDikey

Reputation: 175

differences between make_unique and make_shared when handling arrays

as of C++17 you can use make_unique in order to create smart pointers to arrays, such as:

unique_ptr<int[]> ptr = make_unique<int[]>(10);

which will create a smart pointer to an array of 10 elements (the fact that proper deleter[] will be called is also great).

However according to this make_shared does not support such functionality (at least not in C++17, to my understanding):

shared_ptr<int[]> ptr = make_shared<int[]>(10);

the code above is apparently illegal. Indeed, my Visual Studio 2017 (v141) spits out the following error: 

C2070: 'int[]': illegal sizeof operand'

What's interesting is that shared_ptr itself does support array types (i.e., shared_ptr<int[]> is legal), but make_shared does not. Whereas make_unique does.

The question is, what prevented the standard maker people to let make_shared support array types, just like in the case of make_unique?

Upvotes: 7

Views: 7457

Answers (1)

lubgr
lubgr

Reputation: 38267

What prevented the standard maker people to let make_shared support array types [...]?

Probably nothing, this case was simply not considered, similar to std::make_unique not being present in C++11 but added in C++14. And as pointed out in the comments, this missing piece will ship with C++20.

There is a difference between std::unique_ptr and std::shared_ptr that made neglecting raw arrays pointers easy, though: custom deleters are part of std::unique_ptr's type but not part of std::shared_ptr's type. Therefore, you can handle an array like this

std::shared_ptr<int> array = std::shared_ptr<int>(new int[10],
    [](int *ptr){ delete []ptr; });

and hence delegate the correct memory cleanup to the point of object creation. This makes it easy to treat raw arrays as a special case of std::shared_ptr instances.

Upvotes: 2

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