c++ forward declarations

2/16/13 1.7 Forward declarations Learn C++

www.learncpp.com/cpp-tutorial/17-forward-declarations/ 1/24

1.6 Whitespace and basic formatting 1.8 Programs with multiple files

1.7 Forward declarationsBY A LEX, ON JUNE 2ND, 2007

Take a look at this seemingly innocent sample program called add.cpp:

You would expect this program to produce the result:

The sum of 3 and 4 is: 7

But in fact, it doesnt compile at all! Visual Studio 2005 Express produces the following compile errors:

add.cpp(10) : error C3861: 'add': identifier not found

add.cpp(15) : error C2365: 'add' : redefinition; previous definition was 'formerly unknown identifier'

The reason this program doesnt compile is because the compiler reads files sequentially. When it reaches the

function call to add() inside of main(), it doesnt know what add is, because we havent defined add() until later!

That produces the error on line 10. Then when it gets to the actual declaration of add(), it complains about add

being redefined (which seems slightly misleading, given that it wasnt ever defined in the first place). Often

times, a single error in your code will end up producing multiple warnings.

Rule: When addressing compile errors in your programs, always resolve the first error produced first.

In this case, that means we need to address the fact that the compiler doesnt know what add is. There are

three ways to fix this problem.

The first way is to reorder our function calls so add is defined before main:

12345678910111213

#include int main(){ using namespace std; cout



That way, by the time main() calls add(), it will already know what add is. Because this is such a simple

program, this change is relatively easy to do. However, in a large program, it would be extremely tedious trying

to decipher which functions called which other functions so they could be declared in the correct order.

Furthermore, this option is not always available. Lets say were writing a program that has two functions A and

B. If function A calls function B, and function B calls function A, then theres no way to order the functions in a

way that they will both be happy. If you define A first, the compiler will complain it doesnt know what B is. If

you define B first, the compiler will complain that it doesnt know what A is.

Consequently, a better solution is to use a forward declaration. In a forward declaration, we declare (but do

not define) our function in advance of where we use it, typically at the top of the file. This way, the compiler will

understand what our function looks like when it encounters a call to it later. We do this by writing a declaration

statement known as a function prototype. A function prototype is a declaration of a function that includes the

functions name, parameters, and return type, but does not implement the function.

Heres our original program with a forward declaration:

Now when the compiler reaches add() in main, it will know what add looks like (a function that takes two

integer parameters and returns an integer), and it wont complain.

It is worth noting that function prototypes do not need to specify the names of the parameters. In the above

code, you could also forward declare your function like this:

However, we prefer the method where the parameters are named because its more descriptive to a human

reader.

One question many new programmers have is: what happens if we forward declare a function but do not define

it?

The answer is: it depends. If a forward declaration is made, but the function is never called, the program will

compile and run fine. However, if a forward declaration is made, the function is called, but the program never

defines the function, the program will compile okay, but the linker will complain that it cant resolve the function

call. Consider the following program:

8910111213

int main(){ using namespace std; cout



In this program, we forward declare add(), and we call add(), but we never define add(). When we try and

compile this program, Visual Studio 2005 Express produces the following message:

Compiling...

add.cpp

Linking...

add.obj : error LNK2001: unresolved external symbol "int __cdecl add(int,int)" (?add@@YAHHH@Z)

add.exe : fatal error LNK1120: 1 unresolved externals

As you can see, the program compiled okay, but it failed at the link stage because int add(int, int) was never

defined.

The third solution is to use a header file, which we will discuss shortly.

Quiz

1) Whats the difference between a function prototype and a forward declaration?

2) Write the function prototype for this function:

For each of the following programs, state whether they fail to compile, fail to link, or compile and link. If you are

not sure, try compiling them!

3)

1234567891011

#include "stdafx.h"#include int add(int x, int y); int main(){ using namespace std; cout



4)

5)

6)

Quiz Answers

1) Show Solution

2) Show Solution

3) Show Solution

4) Show Solution

5) Show Solution

6) Show Solution

1234567891011121314

#include int add(int x, int y); int main(){ using namespace std; cout




1.8 Programs with multiple files

Index

1.6 Whitespace and basic formatting

C++ TUTORIAL | PRINT THIS POST

70 comments to 1.7 Forward declarations

Learn C++ - 7.1 -- Function parameters and arguments

September 11, 2007 at 9:09 am Log in to Reply

[...] Forward declarations [...]

beatavenger

November 1, 2011 at 4:42 pm Log in to Reply

xcode doesnt understand add()

Help!

GovZ

December 13, 2007 at 6:44 pm Log in to Reply

Hello guys,

You have a great tutorial here. Worthy of publication, IMHO.

Anyways one question. Does Forward Declaration by function prototyping have an effect on the execution

time or is this only used during the creation of some hash table or something? And that in the resulting

executable, this step is not actually re-read.

I hope I make sense. Thanks for your answers in advance. =)

Alex


GovZ,



Forward declarations are used only to tell the compiler about the existence of a function (or class or

variable) before it is actually implemented.

This information is only used during compile time. Consequently, forward declarations will not make your

executables larger or slower.

josh

January 28, 2008 at 11:34 am Log in to Reply

rying out your second example i wrote:

#include

int add(int x+int y)

{

return x+y;

}

int main()

{

using namespace std;

cout

Alex


Please repost your question with the code embedded inside PRE html tags.

WordPress

June 19, 2011 at 10:45 pm Log in to Reply

If I am right the arguments should not be divided by a positive or plus symbol

but rather by a comma.

Dan

February 1, 2008 at 4:25 pm Log in to Reply

Ahh, ok Im getting a clearer picture of this now (re: my question a few days ago in the

previous functions section

when I asked about the order of functions in Cpp code). This helps explain the reasons why it must be. :)

Thanks Alex!

-Dan

WordPress


Yeah we had this problem back there too! :D



Gary

March 22, 2008 at 7:46 pm Log in to Reply

Hi there. I was wondering how come I have to type #include stdafx.h now. I havent used my

compiler for a while, so I was just wondering. I cant do this program unless I do it, but the

Hello World one works just fine without it.

By the way, in my older programs, the source files say main.cpp but the new one has 2 files, one that

says stdfax.cpp and another one with the title of my project.

Im new to programming, so if my questions sound dumb, bear with me.

Alex


stdafx.cpp is a file that Microsoft compilers use to do precompiled headers (which

makes you program compile faster), if you sent them up correctly. If you dont want to

deal with stdafx.h, you can always turn precompiled headers off (in the project settings).

Learn C++ - 1.8 Programs with multiple files

April 22, 2008 at 5:39 pm Log in to Reply

[...] 2007 Prev/Next Posts 1.7 Forward declarations | Home | 1.9 Header files Saturday, June 2nd,

2007 at 8:26 [...]

Mitul


/* I have writen one another Program like example 5*/

#include

#include

int add(int x, int y, int z);

void main()

{

int x,y,z;

clrscr();

cout > x;

cin >> y;

cin >> z;

cout



Schmeckle


Doesnt main have to be:

int main()

not

void main()

so that main returns a value?

Alex

October 4, 2008 at 9:14 am Log in to Reply

Many (most?) compilers let you get away with using void main() instead of int

main(), and they will implicitly return 0 when you do this. However, its not

technically part of the language, so I avoid it in my examples.

ice

September 7, 2010 at 1:43 pm Log in to Reply

I tried to run your program and came up with and error like this

1> Build started: Project: 1, Configuration: Debug Win32

1> 1.cpp

1>c:\users\marius\documents\visual studio 2010\projects\1\1\1.cpp(2): fatal error C1083: Cannot

open include file: iostream.h: No such file or directory

========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Joyel


This helped explain a lot of things I was previously having problems with.

jeff

December 7, 2008 at 7:23 am Log in to Reply

i am experimenting the code from the quiz number 6 and i try to changed the forward

declaration variables

from

int add(int x, int y, int z);

to



int add(int a, int b, int c);

after that i compiled the code by using microsoft visual studio 2008 and i notice that the code is

successfuly compiled and the program is smoothly running.

why is it that the compiler is not complaining while the forward declaration variable is different from the

implementation?

Alex


Good question! It turns out that the variable names in your forward declarations dont

have to match those in the actual definitions. This is because the variable names in

the forward declarations simply arent used only the types are. You can even omit the variable

names from the forward declaration altogether if you want.

However, I think its generally good practice to put them in. If you see a forward declaration like this:

void doSomething(bool); // what does this bool mean?

Its a lot less meaningful than if you see this:

void doSomething(bool bSilentMode); // now we have an idea

jeff


now its clear and i understand well.

thank you very much!

vkakwani

May 4, 2011 at 10:09 am Log in to Reply

hi alex,

first of all thnks a lot for such nice tutorials.

i know its little too late to ask questions {looking at the postig dates} however i have one

doubt.

forward declaration in case of classes.

can i have forward declaration of a class [class A] in .cpp file. this class definition is present in

other header fil [A.h].

now, in my code i can have a class [class B] containing pointer to this class and forward

declaration [class A;]. this will get compiled and linked.

But if i create an object of class A in class B then i have to do #include A.h also.

Any specific reason for this, my thinking is that compiler would require complete definition of

object getting created inside class B, whereas in case of pointer, well.. its just pointer.



m i thinking correct?

adam


So a function prototype is a kind of forward declaration?

Alex


Its more accurate to say that one use of function prototypes is for forward

declarations. Function prototypes can also be used in header files and in class

declarations (covered in chapter 8). So function prototypes have more uses than just as forward

declarations.

adam


in solution one it says eturn instead of return

[ Fixed! Thanks. -Alex ]

peddi


Can we say the forward declaration in nothing but declaring the function globally, where as

function prototyping is declaring function locally i.e, in the main function? is that only the

difference? If then there is no way we can block the outsider from accessing our function which declared

using forward declaration?

Alex

January 3, 2009 at 6:56 pm Log in to Reply

A forward declaration is a specific type of function prototype that allows you to declare

a function before you actually define it. Unfortunately, theres no way to hide such

functions from outsiders even if you dont declare a function prototype, someone who wanted to

use your function could write a function prototype for it and then use it.

The only way to hide functions is to put them inside classes, which well cover much later in this

tutorial.

Julian


I just found this site today after frustratingly leaving a different learn C++ website, and I must

say, your tutorials are a 100 times better than the other site I was on! :P

It had about one paragraph on functions which was badly worded, and then went on to something about



arrays and loops and binary trees and it had examples but they used functions in a way I didnt even know

was possible and it just confused the crackers outta me.

I like the way these tutorials dont teach everything about functions in the one chapter, they teach the

aspects of functions that would be easy to understand at this point, and then explain the more advanced

features when the reader has a bit more knowledge about C++.

Thankyou! :D

sara

February 24, 2009 at 6:43 am Log in to Reply

i m very weak in programing how can i improve it

Alex


Practice makes perfect! Pick a small project and figure out how to code it. Eg. a

calculator, an address book, a little game, etc You will learn as much from working

on your own code and solving the problems you run into as you will from these tutorials.

csvan

April 23, 2009 at 6:33 am Log in to Reply

I double that. And triple it. I believe practical work is absolutely essential to

building solid programming competence. You can learn all your life, but learning

is in vain if it is only forgotten. Putting learnt knowledge into practice helps solidify it, and, I

believe, can really contribute to it becoming a permanent part of your routine, not easily

forgotten.

Justin

February 27, 2009 at 12:16 pm Log in to Reply

Hi i have a question i wrote my own code (same as yours but didnt copy and paste) and i

got a weird error that i do not under stand here is my code

int add(int x,int y);

{

return x + y;

}

int main()

{


cout



Thankz

Justin

Alex


You have a semicolon after the line declaring add:

int add(int x,int y);

should be:

int add(int x,int y)

Danny


#include stdafx.h

#include

int add(int 1, int 2, int 3, int 4);

int main()

{


cout



should be

#include "stdafx.h"

Also, you need to lose the ; on the end of your forward declaration and your add function

int add(int 1, int 2, int 3, int 4);

and

int add(int 1, int 2, int 3int 4);

should be

int add(int 1, int 2, int 3, int 4)

and

int add(int 1, int , int 3, int 4)

csvan


You almost got it, however you NEED to have a ; at the end of forware

declarations, since they are statements. Thus, it is compleely correct to write:

int add(int a, int b);

but not without the semicolon. C++ statements are ALWAYS concluded with semicolon

(except for some structures, such as while and for loops).

Also, you CANNOT write a parameters name as a number (for example int add(int 1);), that is

not allowed by the compiler.

Keep on coding. Use it for good. :)

csvan


You have 3 errors here:

1. The first line is missing a closing quotation mark () at the end (it should be stdafx.h).

2. The second include statement does not include anything. I think you meant to include iostream,



therefore it should read:

#include

3. The name of a parameter (or any variable as far as I know) can never start with a number, or

simply be a number. Therefore, you will have to rename the parameters both in the function

prototype and definition for add, for example like this:

int add(int a, int b, int c, int 4);

and make the same changes to the body of the function (change 1 to a etc).

Keep coding. Use it for good.

Rinummannobby

May 25, 2009 at 10:56 am Log in to Reply

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=o_O=

August 5, 2009 at 4:07 am Log in to Reply

Well lookie lookie! SPAM!! GTFO!!!!!

Prayrit

July 17, 2009 at 12:06 am Log in to Reply

This is a weird concept to me, because every other language Ive worked with(java, c#,

actionscript) doesnt care where the function is located

StelStav


moved my question at the bottom of the queue

Jon


I dont know if this was mentioned yet, on a brief scan I didnt see it, so forgive me if you

already answered this



You defined a function prototype as:

A function prototype is a declaration of a function that includes the functions name, parameters, and

return type, but does not implement the function. In Question #2 on this page you give us:

1.int DoMath(int first, int second, int third, int fourth)

2.{

3. return first + second * third / fourth;

4.}

Did you mean us to solve for the forward declaration? If not, and you did mean to ask for the Function

Prototype, shouldnt you have given us only the Forward Declaration?

Florian


Hi,

i do have a problem understanding this:

#include

int add(int x, int y); // forward declaration of add() using a function prototype

int main()

{


cout



Florian


Ok,

I was thinking about it for a second, here is my explanation maybe someone can tell me if i am right.

The program does work because c++ is compiled rather than interpreted. Which means as soon as we

have created a prototyp the compiler gets through until the end and the compiled program has overwriten

the first Prototype with the completed declaration (since the later declaration supersedes the first

statement?

Florian

Eugene


Not quite. The prototype does not get overwritten. Rather, the forward declaration

informs the compiler of how a call of that function should look like, and thus the

compiler can do the necessary setup and translate the function call into assembly/machine code.

The function definition defines the code that will be executed when the function is called. In this

case it is in the same source file as the function call, but it could be in a different source file, or

might not even be available at all, e.g., just the object code is available in a shared library.

That said, some functions whose function definitions are available might be inlined, i.e., the function

call is replaced by the code of the function itself.

Michael O.


Hi Alex!

Thanks a lot for these great tutorials, theyve teached me a lot (I began 2 days ago).

I agree with comment #1, these tutorials deserve a publication! I have tried to learn C++ some times before,

but all the books I ended up with were hard to understand, the language was difficult, but these tutorials are

very easy to understand, so again, thanks a lot Alex!

-Michael O.

Jeno


Hi,

Thanks for this wonderful website!!!

I wrote the following code, it compiles, links and executes well, but it seems like the Return Value is

wrong:

// Preprocessor statement



#include

//"Getem" Forward Declaration

int Getem(int one, int two, int three, int four);

//"Main" function declaration

int main()

{

using namespace std; {

std::cout



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Zaman Ali

June 17, 2010 at 7:54 am Log in to Reply

Its very great tutorial , its helpful and very good.

thank you for putting together this wonderful lessons.

C++ Tutorial and Online Ebook


[...] 1.7 Forward declarations [...]

tcp

June 30, 2010 at 6:07 am Log in to Reply

okay so sometimes when i get to

int add(int x, int y);

{

return x + y;

}

the little dropdown-menu-checkbox doesnt appear. why is this? the little alert box to the left of the code is

green, so its not registering as a problem, but it refuses to run the program unless int add is a dropdown

item. (the code is perfect, the only difference between the sites code and my own is the dropdown box).

the error message:

error C2447: { : missing function header (old-style formal list?)

Auge

August 22, 2010 at 8:05 am Log in to Reply

Lets have a look at your code:

int add(int x, int y);

{



return x + y;

}

The semicolon after the function header must be dropped:

int add(int x, int y)

The error message tells you that the compiler doesnt recognize to which function the part within the

curly brackets belongs to.

Thats also because of the semicolon after the function header.

StelStav


Hi all,

I would like to ask what is the reasoning of c++ creators for forward declaration? Why isnt the compiler

designed in such a manner so as to look for a function definition regardless of it being before or after the

actual function calling?

I would imagine that it is enough that the function definition resided in the same scope of the function call.

Would the above (my) logic effect the efficiency of the executable or just the compilation?

Thanks in advance

very nice site btw, good job admins :)

Online Rollenspiele Kostenlos

September 7, 2010 at 11:00 pm Log in to Reply

That was exactly what i needed ! Thank you for your support ! =)

Anders Melen


This tutorial is Perfect! Thank you so much for taking the time to help others learn a new

great programming language! This could surly being published!, but thanks for making it

available to all!

Steven


Love the tutorials,

It says to declare a function you dont need to define it. Then in the definition for function

prototype its a declaration of a function that includes the functions name, parameters, and return type

But when I try to compile

int add(); int main(){... cout



It tells me add() cant take 2 arguments, and forces me to declare

int add(int x, int y)

at the beginning before it will compile/work. Wouldnt that be a declaration WITH a function prototype? or

should

add();

work at the beginning and Im doing something wrong.

donblas

July 16, 2011 at 10:31 pm Log in to Reply

i know its been a while since this was used, but how do you make a program that does more

than add a bunch of numbers?

zingmars


Continue reading the tutorials, and youll understand how :)

sherwood


I did not understand the difference between Forward Declaration and Function Prototyping,

there is a blog that explains this really well. http://blog.onaclovtech.com/2011/07/forward-

declaration-vs-function.html

basically Forward declaration has the function above main and function prototyping just

has the single line prototype above main and the real function(definition) can be elsewhere.

misserwell

August 3, 2011 at 9:29 pm Log in to Reply

Thank you very much, the article is so clear , hope more and more visitor knows the web

Da-Rage44

August 23, 2011 at 7:58 pm Log in to Reply

I have a stupid question, what does it actually mean by the program never defines the

function?

Do we assume we need a return value to define a function?




zingmars


If you only make a forward declaration and never actually make the function the

program never actually makes the function, meaning you will get errors when youre

trying to use the forward declaration.

No you dont need a return value to declare a function (void), it just needs to be declared somewhere

(it doesnt even need to have code in it).

fould12

January 12, 2012 at 2:52 pm Log in to Reply

add.obj : error LNK2001: unresolved external symbol int __cdecl add(int,int) (?

add@@YAHHH@Z)

(?add@@YAHHH@Z) Makes me think of add?YAHHH!!

equasar


On the Quiz question 2, shouldnt be forward declaration instead function prototype?

Daniel Russell Forward declarations

April 6, 2012 at 11:12 pm Log in to Reply

[...] Quiz Answers 1) Show Solution [...]

Amjadb


Can i just make a function call like : int add(int x, int y); before the line of add (3, 4) ?

CSC209Cramming SLog of the most illogical mind

October 20, 2012 at 3:26 pm Log in to Reply

[...] You can find nice example and quiz about it at here, http://www.learncpp.com/cpp-tutorial/17-forward-

declarations/ [...]

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