Saturday, August 24, 2013

C struct string field - char array ^ char pointer

I have no practical experienced on this topic. I seldom see people discussing it. Perhaps many didn't know the char-array option or dare to try it.

Cast of characters -- lets say we have a Person struct (or class) with an Address field of a 99-char string.

Good use case for char-array -- Suppose you know you will have a million instances of the struct, and all of them have an Address string about the same length. Then the char-array is a viable option.

[[headfirstC]] P286/220 point out (arguably) the most fundamental difference -- the entire 99 bytes of char-array is allocated on-site, whereas in the char-ptr design only the 4-byte pointer [1] is stored on-site.

[1] pointer object, not pointer variable or pure-address

When you duplicate the struct, the char-array case is simpler. The char-ptr solution may require strdup(), malloc() and free().

With char-arrayh, you can get redundant copies of the same data.

The char-ptr solution allows 2 instances to share the same string in memory. Looks efficient and neat but could be tricky later on. Might hit dangling pointers.

Now suppose there's a Resume field of maximum length 10KB. Char array option would waste memory since some Persons (kids and house wives) have an empty string.

Verdict -- I feel in many real projects the char-array works fine and is easier to code and maintain.

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