C++ Structures vs Classes: A Practical Guide for Embedded Developers

C++ remains a niche choice for embedded systems, yet it becomes the go‑to language when projects grow beyond the limits of plain C. For developers transitioning from C, understanding the subtle distinctions between C structures, C++ classes, and C++ structures is essential. This article demystifies these language elements with clear examples and best‑practice advice.

Structures in C

A struct in C is a composite data type that aggregates related values. It can contain built‑in types (e.g., int, char), bit fields, or other structures. Its purpose is to group logically related data into a single object.

struct coordinates
{
   float x;
   float y;
   float z;
};

The compiler learns the layout of struct coordinates, allowing you to declare variables like this:

struct coordinates location;

You can then access individual fields using the dot operator:

location.x = 1.0;
location.y = 2.0;
location.z = 9.99;

Although you could use three separate float variables, grouping them into a single struct simplifies passing the data to functions and improves readability.

Bit fields give structs a unique capability: they allow you to specify the exact bit width of a member, which is useful when modeling hardware registers.

struct bitreg
{
   unsigned up: 3;
   unsigned down: 3;
   unsigned polarity: 2;
};

A variable of type struct bitreg will typically occupy one byte, and the compiler automatically generates the necessary masking and shifting code. Usage is straightforward:

struct bitreg control;

control.up = 1;
control.down = 2;
control.polarity = 3;

When working with embedded hardware, be mindful of two caveats:

• Modern 32‑bit CPUs provide efficient support for struct manipulation, but lower‑end processors may not. Use structs judiciously to avoid performance penalties.
• Bit‑field layout is implementation‑defined. A compiler change or update can alter the field ordering or alignment, potentially breaking hardware register access. Additionally, the compiler may generate multiple read/write operations, which can be problematic for write‑only registers.

In C, a struct is essentially a passive container; you cannot invoke behavior on it beyond accessing its fields. That limitation is addressed by C++.

Classes in C++

C++ introduces the class construct, which extends the idea of a struct into a full data type that can encapsulate both data and behavior. An instance of a class is an object.

Classes share syntax with C structs but offer several powerful features:

• They can contain member functions (methods).
• Members are private by default, providing encapsulation. They become public only if preceded by public:.
• Special constructors and destructors run automatically on object creation and destruction.
• Operator overloading allows you to define custom behavior for built‑in operators.
• Inheritance enables one class to derive from another, promoting code reuse.
• When declaring an object, you simply use the class name—no class keyword is required.

Example:

class myclass
{
   char  a;
   int   b;
public:
   void fun();
   myclass();
   ~myclass();
};

myclass myobj;

myobj.fun();

The data members a and b are accessible only within the class’s member functions. The constructor and destructor manage object lifecycle. Function bodies are defined elsewhere using the scope resolution operator:

void myclass::fun()
{
   // implementation
}

Structures in C++

C++ also supports struct, which is fully compatible with C but inherits the additional capabilities of classes. The only difference is that members are public by default, whereas class members are private.

Equivalent to the previous class example, a struct could be written as:

struct mystruct
{
   void fun();
   myclass();
   ~myclass();
private:
   char  a;
   int   b;
};

Choosing Between Classes and Structures

Adopt a disciplined approach: use struct when you only need a passive data container, mirroring C style. Reserve class for situations where encapsulation, behavior, or inheritance is required. This convention improves code readability and maintainability—key qualities in long‑term embedded projects.

Remember that clear, self‑documenting code is more valuable than clever tricks. Future maintainers (even those with a short patience span) will thank you for a consistent style.