1. Problem It Solves
Object members must appear at addresses suitable for their types, so compilers may insert padding and round object size for arrays. C++14 also standardizes sized deallocation, allowing a delete function to receive the size of the object being released.
Focus on the smallest useful form, its observable behavior, and its safety boundary.
2. Prerequisites
Days 18, 21, and 23: object lifetime, allocation, deletion,
sizeof, and non-throwing cleanup.
3. Core Idea
Layout is a sequence of aligned member slots, not a packed concatenation. alignof, sizeof, and offsetof reveal the implementation's decisions for standard-layout types.
Identify the objects and types, today's operation, and the printed result. This connects syntax to behavior.
4. Minimal Syntax
struct Layout { char tag; int value; short code; };
static void operator delete(void* p, std::size_t size) noexcept;5. How It Works
The compiler assigns aligned offsets to each member and may insert unused bytes between or after them.
Deleting the sample node selects its class-specific sized deallocation function and passes the allocation size.
The program prints implementation-specific layout facts and confirms the size delivered to deallocation.
6. Common Mistakes
Assuming member offsets or total size are portable across compilers and architectures can break binary formats and network protocols.
Do not copy the pattern without checking standard-layout requirements, alignment, padding, ABI, allocation/deallocation pairing, and serialized byte order. A program may compile while still having the wrong lifetime, ownership, invalidation, ordering, or performance behavior.
7. When to Use It
Use it when interfacing with hardware, ABIs, allocators, or performance-sensitive packed data requires measured layout facts.
Avoid it when raw object layout is being treated as a portable serialization format.
8. Simple Example
A standard-layout struct exposes member offsets and alignment. A small class defines only sized operator delete, so deletion reports the size passed by the implementation.
The .cpp file uses fixed data. Predict its output, compile it, then change one value and test the prediction.
Complete sample code
Source file
cpp14/24_memory_layout_alignment_padding_sized_deallocation/main.cpp
#include <cstddef>
#include <iostream>
#include <new>
struct Layout {
char tag;
int value;
short code;
};
struct Node {
int value = 42;
static void* operator new(std::size_t size) {
return ::operator new(size);
}
static void operator delete(void* pointer, std::size_t size) noexcept {
std::cout << "sized delete: " << size << "\n";
::operator delete(pointer);
}
};
int main() {
std::cout << "align: " << alignof(Layout) << "\n";
std::cout << "size: " << sizeof(Layout) << "\n";
std::cout << "value offset: " << offsetof(Layout, value) << "\n";
Node* node = new Node;
std::cout << "node value: " << node->value << "\n";
delete node;
}
9. Key Takeaways
Layout is an ABI property to inspect, while serialized representation should be designed explicitly.
Layout is a sequence of aligned member slots, not a packed concatenation.
alignof,sizeof, andoffsetofreveal the implementation's decisions for standard-layout types.The compiler or library follows a precise rule; verify standard-layout requirements, alignment, padding, ABI, allocation/deallocation pairing, and serialized byte order.
Prefer the smallest form that communicates intent and measure costs when performance matters.
10. Self-Check Questions
Easy — What is the main purpose of Memory Layout, Alignment, Padding, and Sized Deallocation?
Medium — Why is the offset of
valuecommonly greater than one even thoughtagoccupies one byte?Hard — Why must allocation and deallocation functions remain a compatible pair even when the deallocator receives a size?