Tip 22: all the casting you'll need

14 November 11. [link] PDF version

level: intermediate
purpose: still less obsolete cruft in your life

Part of a series of tips on POSIX and C. Start from the tip intro page, or get 21st Century C, the book based on this series.

There are two (2) reasons to cast a variable from one type to another.

First: when dividing two numbers, an integer divided by an integer will always return an integer, so the following statements will be true:

4/2 == 2
3/2 == 1

That second one is the source of lots of errors. It's easy to fix: if i is an integer, then i + 0.0 is a floating-point number that matches the integer. Don't forget the parentheses, but that solves your problem:

4/(2+0.0) == 2.0
3/(2+0.0) == 1.5

You can also use the casting form:

4/(float)2 == 2.0
3/(float)2 == 1.5

I'm partial to the add-zero form, for æsthetic reasons; you're welcome to prefer the cast-to-float form. But make a habit of one or the other every time you reach for that / key, because this is the source of many, many errors. And not just in C; lots of other languages also like to insist that int / int int. Not that that makes it OK.

Second: array indices have to be integers. It's the law (C standard §, and GCC will complain if you send a floating-point index. So, you may have to cast to an integer, even if you know that in your situation you will always have an integer-valued expression.

4/(float)2 == 2.0 //this is float, not an int.
mylist[4/(float)2]; //So this is an error: floating-point index

mylist[(int)(4/(float)2)]; //This works; take care with the parens

int index=4/(float)2;//This form also works, 
mylist[index];       //and is more legible.

Now that I've covered both of the reasons to cast in C, I can point out the reasons to not bother. Notice that the index variable above was an integer, but the right-hand value was a floating-point number. C auto-casts in this case, truncating down to the right value. If it's valid to assign an item of one type to an item of another type, then C will do it for you without your having to tell it to with an explicit cast; if it's not valid, then you'll have to write a function to do the conversion anyway.

C++ isn't like this: you have to explicitly cast in all cases. Fortunately, you're writing in C, so you can ignore C++ tutorials that tell you to explicitly cast. And as a broad rule that universally works for me: don't bother with anything that uses C/C++ in the title.

In the 1970s and 80s, malloc returned a char* pointer, and had to be cast (unless you were allocating a string), with a form like:

//don't bother with this sort of redundancy:
float* list = (float) malloc(list_length * sizeof(float));

You don't have to do this anymore, because malloc now gives you a void* pointer, which the compiler will comfortably auto-cast to anything.

If you check the examples above, you'll see that I even gave you options to avoid the casting syntax for the two legitimate reasons to cast: adding 0.0 and declaring an integer variable for your array indices. Bear in mind the existence of the casting form var_type2 = (type2) var_type1, because it might come in handy some day, and in a few tips we'll get to declarations that mimic this form. But for the most part, explicit type casting is just redundancy that clutters the page.

[Previous entry: "Tip 21: become a better typist"]
[Next entry: "Tip 23: The limits of sizeof"]