Answer

placement new is a special form of the new operator in C++. It allows you to place an object at a specified address in an already allocated memory area. It is usually used for manual memory management and is the basis for allocator designs, object pooling, and serialization into fixed buffers.

Syntax:

#include <new>
void* memory = malloc(sizeof(MyClass));
MyClass* obj = new (memory) MyClass(args...); // calls constructor at memory address

Here, memory is allocated separately (for example, through malloc or an allocator), and then the class constructor is invoked in that memory. Don't forget to explicitly call the destructor:

obj->~MyClass();
free(memory);

Advantages:

• Allows for custom allocators.
• Reduces costs associated with new memory allocation.
• Does not call the standard allocator — control over memory management.

Trick Question

Can the same block of memory be used to place multiple objects in succession using 'placement new', and what consequences does this lead to?

A common incorrect answer is that you can place new objects in memory without worrying about the previous state. In fact, if you place a new object over an old one in the same memory area without calling its destructor, a resource leak will occur and calling the destructor later will lead to UB (Undefined Behavior).

Example:

void* place = malloc(sizeof(A));
A* a = new (place) A();
B* b = new (place) B(); // object A not destroyed! UB!

Examples of Real Errors Due to Lack of Understanding of the Nuances

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Story

In a high-load system, a developer implemented an object pool using malloc + placement new, but forgot to call the destructor when returning the object to the pool. As a result, resources (system descriptors within objects) were not released, leading to leaks and "hanging" servers.

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Story

In a project for serializing binary data, a buffer and placement new were used to decode objects in place. However, they forgot to initialize the memory to zero before placement, leading to reading uninitialized data and strange bugs when working with POD structures.

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Story

One module migrated to placing arrays of objects using placement new inside a pre-allocated block. After adding new members with non-trivial destructors inside the class, the program crashed — they forgot that now they needed to explicitly call the destructor for each element when cleaning up, which was not required before.

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