Copying one structure to another

by

Copying by plain assignment is best, since it’s shorter, easier to read, and has a higher level of abstraction. Instead of saying (to the human reader of the code) “copy these bits from here to there”, and requiring the reader to think about the size argument to the copy, you’re just doing a plain assignment (“copy this value from here to here”). There can be no hesitation about whether or not the size is correct.

Also, if the structure is heavily padded, assignment might make the compiler emit something more efficient, since it doesn’t have to copy the padding (and it knows where it is), but mempcy() doesn’t so it will always copy the exact number of bytes you tell it to copy.

If your string is an actual array, i.e.:

struct {
  char string[32];
  size_t len;
} a, b;

strcpy(a.string, "hello");
a.len = strlen(a.string);

Then you can still use plain assignment:

b = a;

To get a complete copy. For variable-length data modelled like this though, this is not the most efficient way to do the copy since the entire array will always be copied.

Beware though, that copying structs that contain pointers to heap-allocated memory can be a bit dangerous, since by doing so you’re aliasing the pointer, and typically making it ambiguous who owns the pointer after the copying operation.

For these situations a “deep copy” is really the only choice, and that needs to go in a function.

More Related Contents:

  1. Copy struct to struct in C
  2. Is there a way to use a typedef struct variable in a function…? [closed]
  3. How can I correctly assign a new string value?
  4. self referential struct definition?
  5. Is an empty initializer list valid C code?
  6. How to define a typedef struct containing pointers to itself?
  7. “error: assignment to expression with array type error” when I assign a struct field (C)
  8. Opaque C structs: various ways to declare them
  9. C: pointer to struct in the struct definition
  10. struct serialization in C and transfer over MPI
  11. How do you make an array of structs in C?
  12. What is ‘forward declaration’ and the difference between ‘typedef struct X’ and ‘struct X’?
  13. What is a designated initializer in C?
  14. Incompatible implicit declaration of built-in function ‘malloc’
  15. Are there any downsides to passing structs by value in C, rather than passing a pointer?
  16. Need help using qsort with an array of structs
  17. Rationale behind the container_of macro in linux/list.h
  18. What is a hash table and how do you make it in C? [closed]
  19. Hiding members in a C struct
  20. default value for struct member in C
  21. Casting one C structure into another
  22. Array of structs replacing values over itself
  23. C – freeing structs
  24. Array of size 0 at the end of struct [duplicate]
  25. How to initialize a pointer to a struct in C?
  26. Why does static initialization of flexible array member work?
  27. C initialize array within structure
  28. offsetof at compile time
  29. Iterating over same type struct members in C
  30. How can I malloc a struct array inside a function? Code works otherwise

Leave a Comment