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c++constructorplacement-newin-place

What is an in-place constructor in C++?

What is an in-place constructor in C++?

e.g. Datatype *x = new(y) Datatype();

Solution

  • This is called the placement new operator. It allows you to supply the memory the data will be allocated in without having the new operator allocate it. For example:

    Foo * f = new Foo();

    The above will allocate memory for you.

    void * fm = malloc(sizeof(Foo));
Foo *f = new (fm) Foo();

    The above will use the memory allocated by the call to malloc. new will not allocate any more. You are not, however, limited to classes. You can use a placement new operator for any type you would allocate with a call to new.

    A 'gotcha' for placement new is that you should not release the memory allocated by a call to the placement new operator using the delete keyword. You will destroy the object by calling the destructor directly.

    f->~Foo();

    After the destructor is manually called, the memory can then be freed as expected.

    free(fm);