Do Struct Members Always Occupy Contiguous Memory? Exploring Padding and Memory Layouts in C/C

Are Structure Member Memories Contiguous? Exploring Struct Layouts

In the C/C programming environment, defining a struct like the example provided:

<code class="cpp">struct test
{
   double height;
   int    age;
   char   gender;
}</code>

raises questions about the arrangement of its members in memory. Are the individual fields of a struct instance, like A.height, A.age, and A.gender, stored adjacently?

Structure Padding and Memory Contiguity

Unfortunately, the assumption of contiguous memory allocation for struct members isn't universally true due to a concept called padding. This is an optimization technique that aligns data elements to specific boundaries, often to improve performance on particular hardware architectures. It can result in "unused" memory slots between members.

In the given example, double typically occupies eight bytes, int four bytes, and char one byte. Without padding, the struct should occupy 13 bytes. However, if the hardware architecture requires eight-byte alignment, there will be three bytes of padding after char to reach that alignment. This means that accessing A.age will skip over those three padding bytes.

Struct of Arrays vs. Array of Structs

Beyond memory contiguity within a single struct, the distinction between a Structure of Arrays (SoA) and an Array of Structs (AoS) offers different memory layouts.

Structure of Arrays (SoA)

In SoA, elements of the same type are stored contiguously. For example, all heights would be stored in a contiguous block, followed by all ages, then genders. This can optimize vectorized operations and reduce memory usage compared to AoS.

Array of Structs (AoS)

In AoS, each struct is stored as a contiguous block. This approach may enhance readability and cache locality since members of a particular struct are grouped. However, it can result in more padding and reduced efficiency for operations on specific member types.

Graphics and Illustration of Memory Layouts

[Image/Diagram to illustrate SoA and AoS memory layouts]

Factors to Consider

The choice between SoA and AoS depends on the specific requirements of the application. SoA may be more efficient for operations on particular member types, while AoS can enhance readability and cache locality.

The above is the detailed content of Do Struct Members Always Occupy Contiguous Memory? Exploring Padding and Memory Layouts in C/C. For more information, please follow other related articles on the PHP Chinese website!