What are forward declarations in C

Question

At  http   www learncpp com cpp-tutorial 19-header-files   The following is mentioned   add cpp   int add int x  int y        return x   y      main cpp    include  lt iostream gt   int add int x  int y      forward declaration using function prototype  int main         using namespace std      cout  lt  lt   The sum of 3 and 4 is    lt  lt  add 3  4   lt  lt  endl      return 0         We used a forward declaration so that the compiler would know what  add  was when compiling main cpp  As previously mentioned  writing forward declarations for every function you want to use that lives in another file can get tedious quickly    Can you explain  forward declaration  further  What is the problem if we use it in the main   function

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Because C   is parsed from the top down  the compiler needs to know about things before they are used  So  when you reference   int add  int x  int y     in the main function the compiler needs to know it exists  To prove this try moving it to below the main function and you ll get a compiler error   So a  Forward Declaration  is just what it says on the tin  It s declaring something in advance of its use   Generally you would include forward declarations in a header file and then include that header file in the same way that iostream is included

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When the compiler sees add 3  4  it needs to know what that means  With the forward declaration you basically tell the compiler that add is a function that takes two ints and returns an int  This is important information for the compiler becaus it needs to put 4 and 5 in the correct representation onto the stack and needs to know what type the thing returned by add is   At that time  the compiler is not worried about the actual implementation of add  ie where it is  or if there is even one  and if it compiles  That comes into view later  after compiling the source files when the linker is invoked

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int add int x  int y      forward declaration using function prototype      Can you explain  forward declaration    more further  What is the problem if   we use it in the main   function    It s same as  include add h   If you know preprocessor expands the file which you mention in  include  in the  cpp file where you write the  include directive  That means  if you write  include add h   you get the same thing  it is as if you doing  forward declaration    I m assuming that add h has this line   int add int x  int y

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Why forward-declare is necessary in C    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 dll 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   Difference between declaration and definition  As an aside  it s important to know the difference between a declaration and a definition   A declaration just gives enough code to show what something looks like  so for a function  this is the return type  calling convention  method name  arguments and their types  But the code for the method isn t required  For a definition  you need the declaration and then also the code for the function too   How forward-declarations can significantly reduce build times  You can get the declaration of a function into your current  cpp or  h file by  includ ing the header that already contains a declaration of the function  However  this can slow down your compile  especially if you  include a header into a  h instead of  cpp of your program  as everything that  includes the  h you re writing would end up  include ing all the headers you wrote  includes for too  Suddenly  the compiler has  included pages and pages of code that it needs to compile even when you only wanted to use one or two functions  To avoid this  you can use a forward-declaration and just type the declaration of the function yourself at the top of the file  If you re only using a few functions  this can really make your compiles quicker compared to always  including the header   For really large projects  the difference could be an hour or more of compile time bought down to a few minutes   Break cyclic references where two definitions both use each other  Additionally  forward-declarations can help you break cycles  This is where two functions both try to use each other  When this happens  and it is a perfectly valid thing to do   you may  include one header file  but that header file tries to  include the header file you re currently writing     which then  includes the other header  which  includes the one you re writing  You re stuck in a chicken and egg situation with each header file trying to re  include the other  To solve this  you can forward-declare the parts you need in one of the files and leave the  include out of that file   Eg   File Car h   include  Wheel h      Include Wheel s definition so it can be used in Car   include  lt vector gt   class Car       std  vector lt Wheel gt  wheels       File Wheel h  Hmm    the declaration of Car is required here as Wheel has a pointer to a Car  but Car h can t be included here as it would result in a compiler error  If Car h was included  that would then try to include Wheel h which would include Car h which would include Wheel h and this would go on forever  so instead the compiler raises an error  The solution is to forward declare Car instead   class Car         forward declaration  class Wheel       Car  car       If class Wheel had methods which need to call methods of car  those methods could be defined in Wheel cpp and Wheel cpp is now able to include Car h without causing a cycle

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One problem is  that the compiler does not know  which kind of value is delivered by your function  is assumes  that the function returns an int in this case  but this can be as correct as it can be wrong  Another problem is  that the compiler does not know  which kind of arguments your function expects  and cannot warn you  if you are passing values of the wrong kind  There are special  promotion  rules  which apply when passing  say floating point values to an undeclared function  the compiler has to widen them to type double   which is often not  what the function actually expects  leading to hard to find bugs at run-time

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one quick addendum regarding  usually you put those forward references into a header file belonging to the  c pp  file where the function variable etc  is implemented  in your example it would look like this  add h   extern int add int a  int b     the keyword extern states that the function is actually declared in an external file  could also be a library etc    your main c would look like this     include   include  add h   int main

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The term  forward declaration  in C   is mostly only used for class declarations  See  the end of  this answer for why a  forward declaration  of a class really is just a simple class declaration with a fancy name    In other words  the  forward  just adds ballast to the term  as any declaration can be seen as being forward in so far as it declares some identifier before it is used     As to what is a declaration as opposed to a definition  again see What is the difference between a definition and a declaration

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The compiler looks for each symbol being used in the current translation unit is previously declared or not in the current unit  It is just a matter of style providing all method signatures at the beginning of a source file while definitions are provided later  The significant use of it is when you use a pointer to a class as member variable of another class     foo h class bar        This is useful class foo       bar  obj     Pointer or even a reference         foo cpp  include  bar h   include  foo h    So  use forward-declarations in classes when ever possible  If your program just has functions  with ho header files   then providing prototypes at the beginning is just a matter of style  This would be anyhow the case had if the header file was present in a normal program with header that has only functions

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