In C, the POD (Plain Old Data) type refers to a type with a simple structure and compatible with C language data processing. It needs to meet two conditions: it has ordinary copy semantics, which can be copied by memcpy; it has a standard layout and the memory structure is predictable. Specific requirements include: all non-static members are public, no user-defined constructors or destructors, no virtual functions or base classes, and all non-static members themselves are PODs. For example, struct Point{int x; int y;} is POD. Its uses include binary I/O, C interoperability, performance optimization, etc. You can check whether the type is POD through std::is_pod, but it is recommended to use std::is_trivial and std::is_standard_layout after C 11. When using it, be careful to avoid private members, inheritance, virtual functions and certain constructors to avoid damaging POD characteristics.

In C , a POD (Plain Old Data) type refers to a type that's both simple in its structure and compatible with C-style data handling. These types can be safely copied with memcpy , initialized with {0} notation, and generally behave predictably in low-level operations.

POD types are especially useful when you need interoperability with C libraries or when performance and memory layout matter—like in systems programming, embedded systems, or when working with serialization libraries.

What Makes a Type a POD?

A type is considered a POD in C if it satisfy two main conditions:

• Trivial Copy Semantics : The type can be copied using memory copy functions like memcpy without side effects.
• Standard Layout : The memory layout of the type follows standard traditions so that it can be accessed predictably across different systems or languages.

Here's what qualifies as a POD:

• All non-static data members are public.
• It has no user-defined constructors or destructors.
• It doesn't have virtual functions or base classes.
• All non-static data members are themselves PODs.

For example:

 struct Point {
    int x;
    int y;
};

This Point struct is a POD because it meets all the above criteria.

Why Are POD Types Still Relevant?

Even though modern C introduces more complex abstractions, POD types remain valuable for specific use cases:

• Binary I/O and Serialization : Since they have a predictable layout, they can be directly written to or read from binary files or network streams.
• C Interoperability : You can pass them to C functions without worrying about name mangling or incompatible layouts.
• Performance Optimization : They're easier for compilers to optimize and are often used in performance-critical code.

If you're building something like a game engine or a device driver, you'll likely encounter situations where sticking with POD types make your life easier.

How to Check If a Type Is POD

You don't always have to guess whether a type qualifies as a POD. C provides a built-in trait for checking this:

 #include <type_traits>

static_assert(std::is_pod<Point>::value, "Point should be a POD");

However, starting with C 11, the term “POD” became less central. Instead, the language introduced finer-grained traits like std::is_trivial and std::is_standard_layout . So while std::is_pod still exists, you can also test those two properties separately if you want more control.

Practical Use Cases and Common Pitfalls

Some real-world examples where PODs shine include:

• Sending structs over a network without extra serialization logic.
• Memory-mapped hardware registers in embedded systems.
• Sharing data between threads without synchronization overhead.

But beware: once you add things like private members, inheritance, or virtual functions, your type is no longer a POD. That can silently break assumptions in legacy or system-level code expecting POD behavior.

Also, even adding a default constructor like this:

 struct Point {
    int x;
    int y;
    Point() = default; // Not allowed for PODs before C 14
};

can disqualify your type depending on the C version you're targeting.

So if you're designing a struct and want it to stay compatible with C or memory-sensitive environments, keep it simple. Stick to public fields, no virtual anything, and avoid custom constructors unless you're sure they won't affect the type's triviality.

Basically that's it.

The above is the detailed content of What is a POD (Plain Old Data) type in C ?. For more information, please follow other related articles on the PHP Chinese website!