What is forward declaration of object? When do we need forward declaration?

Subject Object Oriented Programming
NU Year Set: 3.(a) Marks: 5 Year: 2009

The compiler wants to ensure you haven't made spelling mistakes or passed the wrong number of arguments to the function. So, it insists that it first sees a declaration of 'add' (or any other types, classes or functions) before it is used.

This really just allows the compiler to do a better job of validating the code and allows it to tidy up loose ends so it can produce a neat looking object file. If you didn't have to forward declare things, the compiler would produce an object file that would have to contain information about all the possible guesses as to what the function 'add' might be. And the linker would have to contain very clever logic to try and work out which 'add' you actually intended to call when the 'add' function may live in a different object file the linker is joining with the one that uses add to produce a dull or exe. It's possible that the linker may get the wrong add. Say you wanted to use int add(int a, float b), but accidentally forgot to write it, but the linker found an already existing int add(int a, int b) and thought that was the right one and used that instead. Your code would compile, but wouldn't be doing what you expected.

So, just to keep things explicit and avoid the guessing etc, the compiler insists you declare everything before it is used.

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