CODEWITHSUNDEEP
c++functionc++11functor
xmllmx
xmllmx

Reputation: 42379

Why does clang 3.4 implement `std::function` in such a weird way?

The following code is excerpted from the latest libc++ header <functional> of clang 3.4:

At line 1636:

typedef __allocator_destructor<_Ap> _Dp;
unique_ptr<__base, _Dp> __hold(__a.allocate(1), _Dp(__a, 1));
::new (__hold.get()) _FF(_VSTD::move(__f), _Alloc(__a));
__f_ = __hold.release();

Why use unique_ptr to manage dynamically allocated storage here? Please note __hold will never call its deleter because it finally releases the control of its internal storage!

Why not just write as follows:

__f_ = __a.allocate(1);
::new (__f_) _FF(_VSTD::move(__f), _Alloc(__a));

Upvotes: 1

Views: 198

Answers (1)

Mooing Duck
Mooing Duck

Reputation: 66961

__f_ = __a.allocate(1);

Alright, you've allocated memory and stored the pointer.

::new (__f_) _FF(_VSTD::move(__f), _Alloc(__a));

This throws an exception, and the stack unwinds. Oops, you just leaked your dynamic memory! The obvious way to prevent memory leaks is.... std::unique_ptr.

Upvotes: 2

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