Systems CourseC/C++ · Linux · Memory
C++ CoreLesson 35 of 442 min read8 questions

The new Keyword

new = allocate on the heap + run the constructor; delete = destructor + free.

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Concept core idea Watch out easy to get wrong

Concept

Key concepts

  1. new is an operator that does two things: allocates heap memory (via operator new, usually malloc) and calls the constructor. Returns a typed pointer. delete = destructor + free.
  2. Sizes: new int (4B, garbage), new int() (4B, zero-init), new int[n] (n can be a runtime value — dynamic array).
  3. Pairing (mismatch = UB): new↔delete, new[]↔delete[], malloc↔free. Never cross them.
  4. new vs malloc: new calls ctor/dtor, returns a typed pointer (no cast), and throws std::bad_alloc on failure; malloc = raw bytes, void*, returns nullptr, no ctor.
  5. Heap layout: a fixed-size header sits before p (stores the block size); free/delete do p - headerSize (offset is a compile-time constant) to find it. new[] also stores a count cookie so delete[] runs every destructor. (Deep dive: the Heap layout page linked at the bottom of this lesson.)
  6. Modern C++: prefer smart pointers (make_unique/make_shared) over raw new/delete.

Syntax and matching delete

C++
int*    a   = new int;       // 4 bytes, uninitialized (garbage)
int*    b   = new int();     // 4 bytes, zero-initialized
int*    arr = new int[n];     // runtime-sized array (n from a variable)
Entity* e   = new Entity();   // allocate + run constructor

delete a;        // matches new
delete b;
delete[] arr;    // matches new[]  (runs each destructor)
delete e;        // destructor + free
Deep dive

Go deeper

Heap layout — new / delete / malloc / freeWhat sits in memory around a heap block and how delete finds its size.
Watch out

Easily confused

new vs malloc
new calls the ctor + returns a typed pointer + throws bad_alloc on failure. malloc = raw bytes only, void*, returns nullptr, no ctor.
delete vs free
delete = destructor THEN free. free = free only (no destructor).
delete vs delete[]
delete = one object. delete[] reads the stored element count and runs the destructor for every element.
header offset vs block size
Header offset = fixed constant known by the allocator (so p - headerSize works). Block size = variable, stored in the header and read at free time.

Test yourself

8 questions on this lesson. Write your answers from memory, then compare them with the model answers side by side.

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