Calling the new operator with two arguments but without assignment

Question

I found this code and I don't understand, why it works in the project. The structure of the code is:

class MyClass {
    int value;
};

struct MyStruct {
    MyClass classA;
    MyClass classB;
};

int main() {

    MyStruct myStruct;

    new (&((&myStruct)->classA)) MyClass();
    new (&((&myStruct)->classB)) MyClass();
}

(The inner ampersand was added by me to create a smaller example. In the source myStruct is a pointer).

The compiler says

In function int main()
error: no matching function for call to ‘operator new(sizetype, MyClass*)’
    new (&((&myStruct)->classA)) MyClass();
                                         ^
note: candidates are:
note: void* operator new(long unsigned int)
note:   candidate expects 1 argument, 2 provided

Maybe I'm missing something important. It's a big project and I can't copy it. I'm not able to create a MWE. I hope someone can explain to me the main idea behind this code and what I have to change to compile it.

Lookup [_placement `new`_](http://en.cppreference.com/w/cpp/memory/new/operator_new). — user0042, Sep 13 '17 at 09:05
it's creating a new instance of the class `MyClass` in the two `MyClass` variables the instance `myStruct` of the struct `MyStruct` contains — Shinra tensei, Sep 13 '17 at 09:06
This `(&myStruct)->` makes no sense. Or at least I hope it doesn't. — Quentin, Sep 13 '17 at 09:08
Whomever wrote this code knows enough C++ to be dangerous, but not enough to be effective with it. — StoryTeller - Unslander Monica, Sep 13 '17 at 09:12
I'm curious on how someone would know about placement new, which is a rarely used feature; but not know anything about how objects are created. — UKMonkey, Sep 13 '17 at 09:18
Ok with `#include ` it works. Now I understand and my MWE can be compiled. When someone posts a full answer I will accept it. — Thomas Sablik, Sep 13 '17 at 09:29

StoryTeller - Unslander Monica · Accepted Answer · 2017-09-13T09:48:56.357

4

Placement new is one of those C++ language features that require an appropriate header to be included before one can even attempt to use them.

When using placement new, the expression must call an overload of operator new as part of its evaluation. This overload, to be exact:

void* operator new (std::size, void*);

But this function is not automatically declared for you in every translation unit. So to pull it in, you must include the correct header (<new>).

As a side note, that code you showed is very broken. It constructs the members of myStruct twice! Which for non-trivial cases could wreck havoc.

The proper way to very explicitly initialize myStruct is like this:

MyStruct myStruct {
  {}, // Default construct classA
  {}  // Default construct classB
};

edited Sep 13 '17 at 09:48

answered Sep 13 '17 at 09:42

StoryTeller - Unslander Monica

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Good answer - Side note; the explict initialisation isn't even required; the default constructor on it's own will leave the object in the same state - for this example. – UKMonkey Sep 13 '17 at 09:58
@UKMonkey - That's true. Hence my quip about this being ***very** explicitly* initialized :) – StoryTeller - Unslander Monica Sep 13 '17 at 09:58

Calling the new operator with two arguments but without assignment

1 Answers1