Structure of a C++ Object in Memory Vs a Struct

by

The C++ standard guarantees that memory layouts of a C struct and a C++ class (or struct — same thing) will be identical, provided that the C++ class/struct fits the criteria of being POD (“Plain Old Data”). So what does POD mean?

A class or struct is POD if:

  • All data members are public and themselves POD or fundamental types (but not reference or pointer-to-member types), or arrays of such
  • It has no user-defined constructors, assignment operators or destructors
  • It has no virtual functions
  • It has no base classes

About the only “C++-isms” allowed are non-virtual member functions, static members and member functions.

Since your class has both a constructor and a destructor, it is formally speaking not of POD type, so the guarantee does not hold. (Although, as others have mentioned, in practice the two layouts are likely to be identical on any compiler that you try, so long as there are no virtual functions).

See section [26.7] of the C++ FAQ Lite for more details.

More Related Contents:

  1. How do I compare chars (or strings) using void functions, also comparing chars that were taken from a struct array
  2. can’t extract data from a structure array c++ [closed]
  3. what does this error mean in general? and how I fix it in this case? [closed]
  4. Why isn’t sizeof for a struct equal to the sum of sizeof of each member?
  5. Difference between ‘struct’ and ‘typedef struct’ in C++?
  6. When should you use a class vs a struct in C++?
  7. Why does C++ disallow anonymous structs?
  8. When to use bit-fields in C?
  9. How to use a C++ string in a structure when malloc()-ing the same structure?
  10. How to compile C code with anonymous structs / unions?
  11. memset() or value initialization to zero out a struct?
  12. using struct keyword in variable declaration in C++
  13. How to convert string to IP address and vice versa
  14. Struct initialization of the C/C++ programming language?
  15. union ‘punning’ structs w/ “common initial sequence”: Why does C (99+), but not C++, stipulate a ‘visible declaration of the union type’?
  16. How to read / write a struct in Binary Files?
  17. What is the difference between using a struct with two fields and a pair?
  18. When extending a padded struct, why can’t extra fields be placed in the tail padding?
  19. What happened to the “aggregate or union type that includes one of the aforementioned types” strict aliasing rule?
  20. Struct Constructor in C++?
  21. Force C++ structure to pack tightly
  22. Mixing class and struct
  23. Can a C++ compiler re-order elements in a struct
  24. Is it safe to return a struct in C or C++?
  25. how to assign multiple values into a struct at once?
  26. Initializing an object to all zeroes
  27. What is the meaning of double curly braces initializing a C-struct?
  28. Purpose of struct, typedef struct, in C++
  29. No == operator found while comparing structs in C++
  30. Variable Sized Struct C++

Leave a Comment