C++98 Struct and Padding

Learn how C++ groups related data using structures and why object size may be larger than the sum of its fields.

After this lesson, you should understand:

1. What a struct is.
2. How struct objects are stored.
3. What padding is.
4. Why alignment exists.
5. How field ordering affects memory usage.

Real-world data usually contains multiple fields.

Examples:

order
trade
quote
position

A struct groups related information into a single object.

Define a structure:

struct Trade
{
    int price;
    int quantity;
    char side;
};

Create an object:

Trade trade;

Access fields:

trade.price
trade.quantity
trade.side

A struct is stored as a single contiguous object.

Fields appear in declaration order.

However, compilers may insert extra bytes between fields.

These bytes are called:

padding

Padding is inserted to satisfy alignment requirements.

Benefits:

faster memory access
better CPU efficiency

Important:

sizeof(struct)
is not necessarily equal to
sum(sizeof(fields))

6.1 Assuming No Padding

Struct size may be larger than expected.

Always verify with:

sizeof(...)

6.2 Poor Field Ordering

Bad field ordering may waste memory.

Placing larger fields first often reduces padding.

6.3 Comparing Raw Memory

Padding bytes may contain unspecified values.

Raw memory comparison can be unreliable.

Trading systems process millions of messages.

Common structures:

Trade
Quote
Order
MarketDataEvent

Memory layout directly affects:

cache efficiency
latency
throughput

Understanding padding is important for high-performance systems.

You should be able to answer:

1. What is a struct?
2. Why use a struct instead of separate variables?
3. What is padding?
4. Why does padding exist?
5. Why can sizeof(struct) be larger than expected?
6. How can field order affect memory usage?

Structs combine multiple fields into a single object.

The key idea:

Memory layout matters.

Padding exists to improve alignment and CPU performance.

Understanding struct layout is a fundamental skill for systems programming and low-latency software.