Need some clarification regarding casting in C

Question

I was just reading about the bad practice of casting the return value of malloc. If I understood correctly, it is absolutely legal to leave the cast as it is do

Answer 1

You can't ask about casting in 'C' w/o understanding that casting covers more than one type of operation. There are essentially two, type conversion and type coercion. In C++, because it has more type info, it's creating 4 types of casts and codified this with an exclusive notation. reinterpret_cast<>, const_cast<>, dynamic_cast<> and static_cast<>. You don't have these in C since all casts have the syntax (ctype) but the reasons for them remain and it helps to understand why casting is required even though your question was about 'C' specifically.

The "need" for static cast is what you show in your example. The compiler will do them for you, even if you don't specify it - however, crank the warning level up high enough, and the compiler will warn you if there is a loss of precision as there is going from double to float (your return bar; statement). Adding a cast tells the compiler the loss of precision was intended.

The second least dangerous cast is a const cast<>. It's used to removed const or volatile from a type. This commonly occurs where structures have internal "caches". So a caller may have a const version of your structure, but an "internal function" needs to update the cache so will have to cast from a pointer to a const struct to a regular struct to update an internal field.

The most dangerous type is a reinterpret cast and why people will go on and on about how bad it is to cast. That's where you're not converting anything, but telling the compiler to reinterpret a value as a totally different type. What is below might have been added by a naive programmer trying to get rid of a compiler error.

    char **ptostr = (char **p) "this is not a good idea"; 

Likely the correct fix was to use an '&' and this is how casting gets a bad reputation. Casts like this can be used for good or evil. I used it in the answer to another question about how to find the smallest power of 2 in order to leverage the power of the FPU in a CPU. A better example of being used for good, is when implementing linked lists. If the links live in the objects themselves, you have to cast from the link pointer back out to the enclosed object (a good use for the offsetof macro if the links can't be at the top of the structure).

A dynamic cast has no language support in C but the circumstance still occurs. If you have a heterogeneous list, then you might verify an object was of a given type using a field in the list's link header. Implemented manually, you would verify the type as being compatible and return NULL if it wasn't. This is special version of the reinterpret cast.

There are many sophisticated programing patterns that require casting so I wouldn't say casting needs to be avoided or indicates something is wrong. The problem with 'C' is how you write the unsafe ones in the same exact way as the safe ones. Keeping it contained and limited is a good practice so you can make sure you have it right (e.g., use library routines, strong typing and asserts if you can).