CPSC Notes

Type Casting in C

2 min read

Type casting is the process of converting one data type to another by the programmer, using the casting operator.

Syntax

In C, this would be with brackets like the following:

int x;
double y;
y = (double) x;

Note that in the above, not casting x would not be a compiler error, since casting from an int to a double is not destructive. It is however good practice, and some other cases would require it, most commonly the void pointer type:

void* x;
*x; //You cannot dereference a null pointer, this is a compilation error!
// Instead, cast the pointer to the type it's supposed to be:
int* y;
y = (int*) x;

Extra Info

It’s technically a compiler error only if you pass the compiler flag Werror that makes every C warning into an error, but everyone and their mothers use that flag, especially in an academic setting.

Rules

For numerical values, type casting also follows a specific set of rules:

  • If the original type is unsigned, and it is typecasted to a larger signed type, the value is promoted without sign extension.
  • If the original type is signed, and is typecasted to a larger signed type, the sign is extended. I.e if the leftmost bit is 1, the rest is filled with ones.

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