Variable Sized Struct C++

by

Some thoughts on what you’re doing:

  • Using the C-style variable length struct idiom allows you to perform one free store allocation per packet, which is half as many as would be required if struct Packet contained a std::vector. If you are allocating a very large number of packets, then performing half as many free store allocations/deallocations may very well be significant. If you are also doing network accesses, then the time spent waiting for the network will probably be more significant.

  • This structure represents a packet. Are you planning to read/write from a socket directly into a struct Packet? If so, you probably need to consider byte order. Are you going to have to convert from host to network byte order when sending packets, and vice versa when receiving packets? If so, then you could byte-swap the data in place in your variable length struct. If you converted this to use a vector, it would make sense to write methods for serializing / deserializing the packet. These methods would transfer it to/from a contiguous buffer, taking byte order into account.

  • Likewise, you may need to take alignment and packing into account.

  • You can never subclass Packet. If you did, then the subclass’s member variables would overlap with the array.

  • Instead of malloc and free, you could use Packet* p = ::operator new(size) and ::operator delete(p), since struct Packet is a POD type and does not currently benefit from having its default constructor and its destructor called. The (potential) benefit of doing so is that the global operator new handles errors using the global new-handler and/or exceptions, if that matters to you.

  • It is possible to make the variable length struct idiom work with the new and delete operators, but not well. You could create a custom operator new that takes an array length by implementing static void* operator new(size_t size, unsigned int bitlength), but you would still have to set the bitlength member variable. If you did this with a constructor, you could use the slightly redundant expression Packet* p = new(len) Packet(len) to allocate a packet. The only benefit I see compared to using global operator new and operator delete would be that clients of your code could just call delete p instead of ::operator delete(p). Wrapping the allocation/deallocation in separate functions (instead of calling delete p directly) is fine as long as they get called correctly.

More Related Contents:

  1. Structure size so big, need optimization
  2. Call an array from within an array of structs in c++
  3. i dont know this use of struct and pointers
  4. can’t extract data from a structure array c++ [closed]
  5. Why isn’t sizeof for a struct equal to the sum of sizeof of each member?
  6. Difference between ‘struct’ and ‘typedef struct’ in C++?
  7. Why does C++ disallow anonymous structs?
  8. What are the differences between struct and class in C++?
  9. Struct padding in C++
  10. What does C++ struct syntax “a : b” mean
  11. Iterate through Struct and Class Members [duplicate]
  12. Is the memory allocated for struct members continguous? What if a struct member is an array?
  13. Why can I not brace initialize a struct derived from another struct?
  14. Structure of a C++ Object in Memory Vs a Struct
  15. using struct keyword in variable declaration in C++
  16. How to convert string to IP address and vice versa
  17. What is the default value for C++ class members
  18. Struct initialization of the C/C++ programming language?
  19. When extending a padded struct, why can’t extra fields be placed in the tail padding?
  20. What happened to the “aggregate or union type that includes one of the aforementioned types” strict aliasing rule?
  21. Struct Constructor in C++?
  22. Force C++ structure to pack tightly
  23. too many initializers for ‘int [0]’ c++
  24. Can a C++ compiler re-order elements in a struct
  25. How can I use a custom type as key for a map in C++?
  26. Struct inheritance in C++
  27. Is it safe to return a struct in C or C++?
  28. default visibility of C++ class/struct members
  29. How to initialize a struct to 0 in C++
  30. Initializing an object to all zeroes

Leave a Comment