C++. Meaning of var1 = new (_field) Constructor()

Question

I found this construction reading someone's else C++ code:

_worklist = new (_arena) Block_List();

I'm a little rusty on my C++, can someone explain what is going on here? I don't understand what this code is doing.

EDIT:

_arena is a field of type Arena.

EDIT 2:

And here is the code of Block_List

Edit 3: My bad, I now understand should have provide more context, here is the whole code: http://hg.openjdk.java.net/hsx/hsx24/hotspot/file/ed3ac73a70ab/src/share/vm/opto/live.cpp#l52

Somewhere there is an overloaded `operator new` that accepts whatever type `_arena` is in addition to the size parameter. — StoryTeller - Unslander Monica, Mar 07 '16 at 09:54
it is a "placement new" you can read about it here: http://stackoverflow.com/questions/222557/what-uses-are-there-for-placement-new — nikniknik2016, Mar 07 '16 at 09:54
This depends very much on what `_arena` is. It *could* be placement-new, or something completely different. — Some programmer dude, Mar 07 '16 at 09:54
@nikniknik2016 not enough info to decide if it's the standard placement new. — StoryTeller - Unslander Monica, Mar 07 '16 at 09:55
Either the other developer typedef'd something as _arena type, or there is an overload of standard new, taking additional input — Richard Tyregrim, Mar 07 '16 at 09:55
The fact that `_arena` is a field doesn't tell us anything. What matters is its type. — StoryTeller - Unslander Monica, Mar 07 '16 at 09:59
It's *not* placement new, instead there is (somewhere in your source) an overload of `operator new` which takes a second argument of type `Arena`. That's really all we can say. Search all your code for `operator new`. — Some programmer dude, Mar 07 '16 at 10:06
@JoachimPileborg Well, it's not the standard one, but it's a placement new-expression. — T.C., Mar 07 '16 at 10:08
[Documentation on overloading/providing own `new` operator](http://en.cppreference.com/w/cpp/memory/new/operator_new). — Revolver_Ocelot, Mar 07 '16 at 10:09
@JoachimPileborg It is a good old placement new. `_arena` is an `Arena*` — Revolver_Ocelot, Mar 07 '16 at 10:10
@Revolver_Ocelot No, it's a custom `operator new` that takes an argument of type `Arena*`. — interjay, Mar 07 '16 at 10:12
@Revolver_Ocelot The OP doesn't really provide very much information, but nowhere has it been said that `_arena` is a pointer. The only information we have is that `_arena` is an instance of `Arena`, not a pointer. — Some programmer dude, Mar 07 '16 at 10:15

interjay · Accepted Answer · 2016-03-07T10:22:38.567

2

Placement syntax is used to call a custom operator new that takes additional arguments. In this case, Block_List is derived from ResourceObj, and ResourceObj contains the following function:

void* operator new(size_t size, Arena *arena) {
    address res = (address)arena->Amalloc(size);
    DEBUG_ONLY(set_allocation_type(res, ARENA);)
    return res;
}

The expression new (_arena) Block_List(); will call this operator new, passing in the required size and the value that was passed in. The function returns the address to use for the object.

edited Mar 07 '16 at 10:22

answered Mar 07 '16 at 10:17

interjay

107,303
21
270
254

Your answer is entirely correct, but it might be helpful to distinguish between: "placement new" (where the object is created in a pre-allocated buffer without any need for a custom `operator new`); "a placement new expression" (where a custom `operator new` is called to allocate memory). What we have here is the latter. – Martin Bonner supports Monica Mar 07 '16 at 11:44
@MartinBonner Those are not official or accurate terms. The standard defines a placement syntax which includes the parentheses after `new`, and a placement `operator new` which takes extra arguments. There are also predefined placement `operator new` versions which take arguments of type `void*` (to set the address) or `nothrow_t` (to avoid `bad_alloc` exceptions), but they are merely specific instances of the placement new operator. – interjay Mar 07 '16 at 11:58
Thank you. I hadn't realized that. – Martin Bonner supports Monica Mar 07 '16 at 14:00

C++. Meaning of var1 = new (_field) Constructor()

1 Answers1