When to use std::make_shared_for_overwrite?

Question

C++20 introduces new function std::make_shared_for_overwrite() in addition to std::make_shared(): https://en.cppreference.com/w/cpp/memory/shared_ptr/make_shared

Why old make_shared was not enough and in what situation one needs to use new function?

"Same as (2), except that the individual array elements are default-initialized." — Alan Birtles
– Alan Birtles, Commented Jun 14, 2021 at 9:47
So looks rather like a convenience function (C analogy: malloc vs. calloc). — Aconcagua
– Aconcagua, Commented Jun 14, 2021 at 10:03

Caleth · Accepted Answer · 2021-06-14 13:32:15Z

24

std::make_shared() value initialises the object(s) it creates, which might be an unnecessary step if you intend to assign values over them later.

std::make_shared_for_overwrite() default initialises the object(s) it creates.

The difference only matters for (sub-)objects of fundamental types, where there is no initialiser.

std::make_shared<int[1000][1000]>() will allocate and zero a million ints std::make_shared_for_overwrite<int[1000][1000]>() will allocate a million ints

edited Jun 14, 2021 at 13:32

answered Jun 14, 2021 at 10:05

Caleth

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Nicol Bolas Over a year ago

"The difference only matters for fundamental types, and arrays thereof." Or structs thereof. Or any type that's trivially default constructible.

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