CODEWITHSUNDEEP
pointersstructvariable-assignmentoperator-keyword
GelatinFox
GelatinFox

Reputation: 353

Need help understanding this operation

I am reading a document given to me by a professor and I'm trying to understand the following line of code.

C *r = new(p) C[3];

what exactly is the code doing? What does the C[3] after the new(p) accomplish?

Upvotes: 1

Views: 34

Answers (1)

Joni
Joni

Reputation: 111279

This line of code constructs an array of 3 items of type C, similar to this, which is hopefully more familiar to you:

C *r = new C[3];

The primary difference is that new(p) does not allocate new memory; instead it constructs the array in a pre-existing memory buffer pointed by p. For example:

char *p = new char[3*sizeof(C)];
C *r = new(p) C[3];

Another difference is that you can't use delete[] to call the deconstructors and free the memory. You have to call the deconstructors manually

for (int i=0; i<3: i++) r->~C();
delete[] p;

See also What uses are there for "placement new"? and Wikipedia.

Upvotes: 1

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