Placement New

Placement new is a specialized form of the new operator in C++ that allows you to construct an object at a pre-allocated memory location. Unlike the regular new operator, which allocates memory from the heap before constructing the object, placement new constructs the object in memory you provide. This is crucial for scenarios like custom memory management, embedded systems, and optimizing performance by reusing memory.

What is Placement New

The standard new operator in C++ performs two distinct operations: memory allocation and object construction. It first allocates a block of memory from the heap large enough to hold an object of the specified type, and then it constructs an object of that type within the allocated memory.

Placement new, on the other hand, bypasses the memory allocation step. It assumes that you have already obtained a suitable memory buffer and simply constructs an object at that location. This gives you fine-grained control over where objects are created, which is beneficial in several situations:

Custom Memory Management: You might be using a custom memory allocator or a memory pool for performance reasons. Placement new allows you to construct objects within the memory managed by your allocator.
Embedded Systems: Embedded systems often have limited memory and strict timing requirements. Placement new can be used to construct objects in specific memory regions, such as memory-mapped peripherals, without the overhead of dynamic memory allocation.
Object Serialization/Deserialization: When deserializing an object from a file or network stream, you might want to reconstruct the object at a specific memory address.
Optimized Data Structures: Some data structures, like arenas, benefit from pre-allocating a large block of memory and then constructing objects within that block using placement new. This reduces the overhead of individual memory allocations and deallocations.
Exception Safety: Placement new can be useful in scenarios where you need to guarantee that an object is constructed even if an exception is thrown during allocation.

Edge Cases and Considerations:

Memory Alignment: Ensure that the memory address you provide to placement new is properly aligned for the object type. Otherwise, you might encounter undefined behavior. Use alignof(T) to determine the required alignment for type T.
Destruction: Objects constructed with placement new must be explicitly destroyed using the destructor. You must manually call the destructor using the pointer to the object and then deallocate the memory (if necessary). Failure to do so will result in memory leaks and resource leaks.
Overlapping Objects: Be extremely careful when using placement new to construct objects in overlapping memory regions. This can lead to data corruption and unpredictable behavior.
Placement Delete: There is no corresponding “placement delete” operator. You must manually call the destructor before deallocating the memory.
Exception Handling: If the constructor called by placement new throws an exception, the memory allocated is not automatically deallocated. You are responsible for handling this situation and ensuring that the memory is cleaned up.
Performance: While placement new avoids the overhead of memory allocation, it can still have performance implications. The constructor still needs to be executed, and the memory might not be optimally placed in memory. Careful profiling is necessary to ensure that placement new actually improves performance.

Syntax and Usage

The syntax for placement new is as follows:


new (address) Type(constructor_arguments);

address: A pointer to the memory location where the object should be constructed.
Type: The type of the object to be constructed.
constructor_arguments: The arguments to pass to the constructor of the object.

Example:


char buffer[sizeof(MyClass)];
MyClass* obj = new (buffer) MyClass(arg1, arg2);

In this example, buffer is a pre-allocated character array. Placement new is used to construct a MyClass object within that buffer, using the constructor MyClass(arg1, arg2). The pointer obj now points to the constructed object within the buffer.

Basic Example


#include <iostream>
#include <new> // Required for placement new
 
class MyClass {
public:
    int value;
 
    MyClass(int val) : value(val) {
        std::cout << "MyClass constructor called with value: " << value << std::endl;
    }
 
    ~MyClass() {
        std::cout << "MyClass destructor called with value: " << value << std::endl;
    }
 
    void printValue() {
        std::cout << "Value: " << value << std::endl;
    }
};
 
int main() {
    // Allocate raw memory on the stack
    char buffer[sizeof(MyClass)];
 
    // Use placement new to construct MyClass in the buffer
    MyClass* obj = new (buffer) MyClass(42);
 
    // Use the object
    obj->printValue();
 
    // Explicitly call the destructor
    obj->~MyClass();
 
    // No need to deallocate memory, as it was allocated on the stack
 
    return 0;
}

Explanation:

We include the <new> header, which is required for using placement new.
We define a simple class MyClass with a constructor, destructor, and a printValue method.
In main, we allocate a character array buffer on the stack with a size large enough to hold a MyClass object. sizeof(MyClass) ensures that the buffer is large enough.
We use placement new new (buffer) MyClass(42) to construct a MyClass object at the address pointed to by buffer. The constructor is called with the argument 42.
We use the object by calling its printValue method.
Crucially, we explicitly call the destructor obj->~MyClass(). This is essential because the memory was allocated on the stack, and the object will not be automatically destroyed when it goes out of scope.
Since the memory was allocated on the stack, we do not need to deallocate it.

Advanced Example


#include <iostream>
#include <new>
#include <memory> // For std::aligned_storage
 
class ComplexObject {
public:
    int id;
    double data[1024]; // Large data payload
 
    ComplexObject(int i) : id(i) {
        std::cout << "ComplexObject constructor called with id: " << id << std::endl;
        for (int j = 0; j < 1024; ++j) {
            data[j] = static_cast<double>(id * j);
        }
    }
 
    ~ComplexObject() {
        std::cout << "ComplexObject destructor called with id: " << id << std::endl;
    }
 
    void processData() {
        // Simulate some processing on the data
        double sum = 0.0;
        for (int j = 0; j < 1024; ++j) {
            sum += data[j];
        }
        std::cout << "Sum of data: " << sum << std::endl;
    }
};
 
int main() {
    // Use std::aligned_storage to ensure proper alignment
    std::aligned_storage<sizeof(ComplexObject), alignof(ComplexObject)>::type buffer;
 
    // Construct the object using placement new
    ComplexObject* obj = new (&buffer) ComplexObject(123);
 
    // Use the object
    obj->processData();
 
    // Destroy the object explicitly
    obj->~ComplexObject();
 
    return 0;
}

Explanation:

We include <memory> to use std::aligned_storage. std::aligned_storage is a utility that provides a raw storage space with proper alignment for a given type. This is crucial to prevent alignment issues, which can lead to undefined behavior.
We use std::aligned_storage<sizeof(ComplexObject), alignof(ComplexObject)>::type buffer; to create a properly aligned buffer.
Placement new constructs the ComplexObject at the address of buffer.
We call processData to simulate using the object.
We explicitly call the destructor obj->~ComplexObject().

This example demonstrates the importance of using std::aligned_storage to ensure proper alignment when using placement new. It also shows a more complex object with a larger data payload.

Common Use Cases

Custom Memory Allocation: Constructing objects within memory managed by a custom allocator.
Object Pools: Reusing pre-allocated memory for frequently created and destroyed objects.
Memory-Mapped Hardware: Constructing objects at specific memory addresses corresponding to hardware registers.
Deserialization: Reconstructing objects at specific memory locations during deserialization.

Best Practices

Use std::aligned_storage: Always use std::aligned_storage to ensure proper alignment of the memory buffer.
Explicit Destructor Call: Always explicitly call the destructor of objects constructed with placement new.
Handle Exceptions: Be prepared to handle exceptions thrown by the constructor and clean up the memory if necessary.
Avoid Overlapping Objects: Do not construct objects in overlapping memory regions.
Consider Alternatives: Before using placement new, consider whether there are simpler alternatives, such as using the regular new operator with a custom allocator.

Common Pitfalls

Forgetting to Call the Destructor: This leads to memory leaks and resource leaks.
Alignment Issues: Constructing objects at improperly aligned memory addresses can lead to undefined behavior.
Double Deletion: Deallocating memory that was not allocated with new or deallocating the same memory twice can lead to crashes or memory corruption.
Exception Safety: Failing to handle exceptions thrown by the constructor can leave the system in an inconsistent state.

Key Takeaways

Placement new allows you to construct objects at specific memory locations.
It bypasses the memory allocation step of the regular new operator.
You are responsible for managing the memory and explicitly calling the destructor.
Use std::aligned_storage to ensure proper alignment.
Consider the potential performance implications and exception safety issues.