CODEWITHSUNDEEP
ccastingintc99size-t
ohmu
ohmu

Reputation: 19762

C: cast int to size_t

What is the proper way to convert/cast an int to a size_t in C99 on both 32bit and 64bit linux platforms?

Example:

int hash(void * key) {
    //...
}

int main (int argc, char * argv[]) {
    size_t size = 10;
    void * items[size];
    //...
    void * key = ...;
    // Is this the right way to convert the returned int from the hash function
    // to a size_t?
    size_t key_index = (size_t)hash(key) % size;
    void * item = items[key_index];
}

Upvotes: 18

Views: 83267

Answers (3)

Tim
Tim

Reputation: 1546

Aside from the casting issue (which you don't need as stated before), there is some more intricate things that might go wrong with the code.

if hash() is supposed to return an index to an array, it should return a size_t as well. Since it doesn't, you might get weird effects when key_index is larger than INT_MAX.

I would say that size, hash(), key_index should all be of the same type, probably size_t to be sure, for example:

size_t hash(void * key) {
    //...
}

int main (int argc, char * argv[]) {
    size_t size = 10;
    void * items[size];
    //...
    void * key = ...;

    size_t key_index = hash(key) % size;
    void * item = items[key_index];
}

Upvotes: 3

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215173

All arithmetic types convert implicitly in C. It's very rare that you need a cast - usually only when you want to convert down, reducing modulo 1 plus the max value of the smaller type, or when you need to force arithmetic into unsigned mode to use the properties of unsigned arithmetic.

Personally, I dislike seeing casts because:

  1. They're ugly visual clutter, and
  2. They suggest to me that the person who wrote the code was getting warnings about types, and threw in casts to shut up the compiler without understanding the reason for the warnings.

Of course if you enable some ultra-picky warning levels, your implicit conversions might cause lots of warnings even when they're correct...

Upvotes: 19

Stephen Canon
Stephen Canon

Reputation: 106117

size_t key_index = (size_t)hash(key) % size;

is fine. You actually don't even need the cast:

size_t key_index = hash(key) % size;

does the same thing.

Upvotes: 7

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