Why can’t a modifiable lvalue have an array type?

by

The designers of the C language decided that it should not be possible to assign arrays by value. At the time this seemed a sensible decision (early 1970s) – memory and processor speed were very limited and they considered that having a = b; make a and b both refer to the same array was something that would be a much more common intent than having a = b; be used to copy the contents of one array to another.

In fact this was already common usage: in the B programming language (precursor to C), the equivalent of int a[10]; actually meant to allocate both a pointer, and a block of 10 ints, and point the pointer to the block of 10 ints. You could actually make an array “point” somewhere else in B.

C changed the meaning of an array definition that it only allocates the block of ints; and added “The Rule”: when you use the array’s name in an assignment expression (and most other expressions) the array is implicitly converted to a pointer to the first element. So, if a is a pointer and b is an array, then you can still write a = b; to make a behave like an alias for b. Although you can no longer have a = b; where a is an array.

In the first ANSI C standard in 1989 they added the ability to copy structs by value (this had existed in some compilers previously but wasn’t universal), with the corollary that if the struct contains an array then the array gets copied by value. But it was far too late to go back and change the meaning of a = b; to copy arrays by value, too much code was written that already depends on The Rule.

Reference: The Development of the C Language – Dennis M. Ritchie

More Related Contents:

  1. What are the valid signatures for C’s main() function?
  2. Are there any non-twos-complement implementations of C?
  3. What is the behavior of printing NULL with printf’s %s specifier?
  4. Does printf(“%x”,1) invoke undefined behavior?
  5. Representation of negative numbers in C?
  6. How undefined is undefined behavior?
  7. Does &((struct name *)NULL -> b) cause undefined behaviour in C11?
  8. Can code that will never be executed invoke undefined behavior?
  9. Is argv[n] writable?
  10. Is it guaranteed to be safe to perform memcpy(0,0,0)?
  11. Is `*((*(&array + 1)) – 1)` safe to use to get the last element of an automatic array?
  12. Is fgets() returning NULL with a short buffer compliant?
  13. Can an equality comparison of unrelated pointers evaluate to true?
  14. Why “initializer element is not a constant” is… not working anymore?
  15. Is int main() { } (without “void”) valid and portable in ISO C?
  16. How do I make an infinite empty loop that won’t be optimized away?
  17. How are multiple prior declarations resolved for a new declaration with extern?
  18. When is it valid to access a pointer to a “dead” object?
  19. Returning struct containing array
  20. Can the following code be true for pointers to different things
  21. Is it Undefined Behaviour to cast away the constness of a function parameter?
  22. Which section in C89 standard allows the “implicit int” rule?
  23. Is it undefined behavior to redefine a standard name?
  24. C – is an indeterminate value indeterminable?
  25. How a stream error indicator affects following input code?
  26. Initialization vs Assignment in C
  27. Is scanf(“%d%d”, &x, &x) well defined?
  28. Why isn’t the size of an array parameter the same as within main?
  29. Bitwise rotate left function
  30. Why is adding a leading space in a scanf format string recommended?

Leave a Comment