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C++ High Performance

Padding

Explore how padding affects memory layout in C++ classes and structs. Learn to rearrange data members to minimize padding and alignment overhead. Understand the impact of member order on memory size and performance, including cache line alignment and memory access efficiency.

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Understanding padding

The compiler sometimes needs to add extra bytes, padding, to our user-defined types. When we define data members in a class or struct, the compiler is forced to place the members in the same order as we define them.

However, the compiler must also ensure that the data members inside the class have the correct alignment; hence, it needs to add padding between data members if necessary. For example, let’s assume we have a class defined as follows:

C++
class Document {
  bool is_cached_{};
  double rank_{};
  int id_{};
};

Now let’s calculate the size:

C++
std::cout << sizeof(Document)<< '\n'; // Possible output is 24

The reason for the possible output being 24 is that the compiler inserts padding after bool and int to ...