Why (cast) when we don't have to?

[dissectcode2]

I was just reading in K&R page 44-45 that "conversions take place across assignments; the value of the right side is converted to the type on the left, which is the type of the result......If x is float and i is int, then x = i and i = x both cause conversions; float to int causes truncation"

so if I can type:

int i;
float f;
 
i = f; // and f = i;
 
 

without explicitly casting (float) and (int), then

what's the purpose of casting???

[weaknessforcats]

In your example an explicit cast on your part is unnecessary. The compiler knows both types.

But many C functions have arguments that are addresses of an unknown type. These are the void* arguments. These C functions may also have a second argument (usually an int) that the function can use to determine the type of the void*. Maybe 1 means int* and 2 means Date" where Date is one of your structs. The function can now typecast the void* into the correct type.

In short, you use an explicit cast when only you know what the real type is.

[Banfa]

You might also using casting while doing arithmetic operations in order to ensure you get the correct result, for example

int a = 4;
int b = 5;
double c;
 
c = a / b; /* c has the value 0.0 because the calculation is done 
              in integer arithmetic and then assigned to c */
 
c = (double)a / b; /* c has the value 0.8 because the calculation is done as 
                      floating point arithmetic because a has been cast to a 
                      double and then assign to c*/
 

There can be similar issues resolved with casts when using integers of different bit sizes for the calculation and result.