std::unique_ptr::reset and constructor exceptions

Question

If initialise a unique_ptr like this:

std::unique_ptr<Foo> i;
i.reset( new Foo() );

but an exception is thrown from Foo::Foo(), the question is: what happens with the memory allocated? how does unique_ptr avoid it being leaked? is this something handled inside new operator?

The destructor will certainly be called when the scope exits. Since the reset call is not invoked until new Foo() returns, it seems that this must be handled by new, by freeing the allocated memory when the exception leaves the constructor.

Is that what happens?

See http://stackoverflow.com/questions/4094996/what-happens-to-the-memory-allocated-by-new-if-the-constructor-throws — Jon, Jan 10 '13 at 23:38

score 13 · Accepted Answer · answered Jan 10 '13 at 23:36

13

If an exception is thrown in the constructor of Foo, then the reset function of the unique pointer never gets executed in the first place. Thus the unique pointer retains its original value.

A new expression doesn't leak memory if the object construction throws an exception.

answered Jan 10 '13 at 23:36

Kerrek SB

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And if you have `make_unique`, one can use `i = make_unique()`. – GManNickG Jan 10 '13 at 23:48
no, should be Ok (no managed pointer), but I can not find anything in documentation – Nick Mar 14 '16 at 16:45
1

@Nick: Please do at least try to compile that. – Kerrek SB Mar 14 '16 at 16:46
not sure if c-tor is explicit, this should be compiled correctly - `std::unique_ptr p{ new Bla() };` – Nick Mar 14 '16 at 16:48

std::unique_ptr::reset and constructor exceptions

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