What's the point of function prototyping?

Question

I'm following a guide to learn curses, and all of the C code within prototypes functions before main(), then defines them afterward. In my C++ learnings, I had heard about function prototyping but never done it, and as far as I know it doesn't make too much of a difference on how the code is compiled. Is it a programmer's personal choice more than anything else? If so, why was it included in C at all?

Answer 1

In C prototyping is needed so that your program knows that you have a function called x() when you have not gotten to defining it, that way y() knows that there is and exists a x(). C does top down compilation, so it needs to be defined before hand is the short answer.

x();
y();
main(){

}

y(){
x();
}

x(){
...
more code ...
maybe even y();
}

Answer 2

It allows you to have a situation in which say you can have an iterator class defined in a separate .h file which includes the parent container class. Since you've included the parent header in the iterator, you can't have a method like say "getIterator()" because the return type would have to be the iterator class and therefore it would require that you include the iterator header inside the parent header creating a cyclic loop of inclusions (one includes the other which includes itself which includes the other again, etc.).

If you put the iterator class prototype inside the parent container, you can have such a method without including the iterator header. It only works because you're simply saying that such an object exists and will be defined.

There are ways of getting around it like having a precompiled header, but in my opinion it's less elegant and comes with a slew of disadvantages. Of couurse this is C++, not C. However, in practice you might have a situation in which you'd like to arrange code in this fashion, classes aside.

Answer 3

I was under the impression that it was so customers could have access to the .h file for libraries and see what functions were available to them, without having to see the implementation (which would be in another file).

Useful to see what the function returns/what parameters.

Answer 4

Function prototyping is a remnant from the olden days of compiler writing. It used to be considered horribly inefficient for a compiler to have to make multiple passes over a source file to compile it.

In C, in certain contexts, referring to a function in one manner is syntactically equivalent to referring to a variable: consider taking a pointer to a function versus taking a pointer to a variable. In the compiler's intermediate representation, the two are semantically distinct, but syntactically, whether an identifier is a variable, a function name, or an invalid identifier cannot be determined from the context.

Since it's not determinable from the context, without function prototypes, the compiler would need to make an extra pass over each one of your source files each time one of them compiles. This would add an extra O(n) factor for any compilation (that is, if compilation were O(m), it would now be O(m*n)), where n is the number of files in your project. In large projects, where compilation is already on the order of hours, having a two-pass compiler is highly undesirable.

Forward declaring all your functions would allow the compiler to build a table of functions as it scanned the file, and be able to determine when it encountered an identifier whether it referred to a function or a variable.

As a result of this, C (and by extension, C++) compilers can be extremely efficient in compilation.

Answer 5

Function prototyping originally wasn't included in C. When you called a function, the compiler just took your word for it that it would exist and took the type of arguments you provided. If you got the argument order, number, or type wrong, too bad – your code would fail, possibly in mysterious ways, at runtime.

Later versions of C added function prototyping in order to address these problems. Your arguments are implicitly converted to the declared types under some circumstances or flagged as incompatible with the prototype, and the compiler could flag as an error the wrong order and number of types. This had the side effect of enabling varargs functions and the special argument handling they require.

Note that, in C (and unlike in C++), a function declared foo_t func() is not the same as a function declared as foo_t func(void). The latter is prototyped to have no arguments. The former declares a function without a prototype.