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What is POD in C++ and why is it debated?
If you have ever written C++ code that works with raw memory, serializes data, or interacts with C libraries, you have likely encountered the mysterious abbreviation POD (Plain Old Data). Despite the name, this simplicity is deceptive. On Stack Overflow, the question “What are POD types in C++?” gathers thousands of views, with heated debates in the comments about what qualifies as POD and what does not.
In this article, we will break down what POD types are in C++, what mistakes even experienced developers make, and how to avoid them. You will learn how to correctly use memcpy, why not all trivial classes are POD, and how not to “kill” performance due to incorrect layout.
A brief history: from C to C++
POD is a legacy from the C language. When C++ first appeared, backward compatibility was needed. Structures from C had to work in C++ without changes. Thus, the concept of Plain Old Data was born — types that behave like in C: they have a trivial constructor, destructor, copy operations, and standard layout.
With the release of the C++11, C++14, and C++20 standards, the requirements for POD became stricter. Now it is not just a “structure without methods,” but a complex set of rules. Let’s break them down in order.
Top 5 mistakes when working with POD types
1. Confusing “trivial” with “POD”
Many think that if a class is trivial, it is automatically POD. This is not true. POD is the intersection of two properties: trivial and standard layout.
// Trivial, but not POD (standard layout violated)class NotPOD { int x;public: virtual void foo() {} // virtual function};
// Trivial and PODstruct RealPOD { int x; float y;};In the first case, the compiler adds a vtable pointer, which violates standard layout. Such a class cannot be safely copied via memcpy.
2. Using memcpy with non-POD types
A classic mistake is copying an object via memcpy when it is not POD. This leads to undefined behavior.
#include <cstring>#include <iostream>
struct SafePOD { int a; double b;};
struct Unsafe { std::string s; // non-POD: has constructor, destructor};
int main() { SafePOD p1{1, 2.5}; SafePOD p2; std::memcpy(&p2, &p1, sizeof(SafePOD)); // OK
Unsafe u1{\"hello\"}; Unsafe u2; std::memcpy(&u2, &u1, sizeof(Unsafe)); // Error! Undefined behavior}How to avoid: always check std::is_pod_v<T> before using memcpy. In C++20, use std::is_trivial_v and std::is_standard_layout_v separately.
3. Ignoring alignment
POD types in C++ guarantee that their layout is predictable. But this does not mean alignment can be ignored. An error occurs when trying to manually pack a structure to save memory.
#pragma pack(push, 1)struct Packed { char c; int i; // alignment violation};#pragma pack(pop)Although formally Packed may be POD, accessing i can be slow or even cause a SIGBUS on some architectures (e.g., ARM).
How to avoid: use alignas and std::alignment_of to control alignment, but do not abuse #pragma pack unnecessarily.
4. Inheritance and POD
Many think that if a structure inherits from another POD structure, it is also POD. This is only true under certain conditions.
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