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2.7 — Forward declarations and definitions

Take a look at this seemingly innocent sample program:

You would expect this program to produce the result:

The sum of 3 and 4 is: 7

But in fact, it doesn’t compile at all! Visual Studio produces the following compile error:

add.cpp(5) : error C3861: 'add': identifier not found

The reason this program doesn’t compile is because the compiler compiles the contents of code files sequentially. When the compiler reaches the function call to add on line 5 of main, it doesn’t know what add is, because we haven’t defined add until line 9! That produces the error, identifier not found.

Older versions of Visual Studio would produce an additional error:

add.cpp(9) : error C2365: 'add' : redefinition; previous definition was 'formerly unknown identifier'

This is somewhat misleading, given that add wasn’t ever defined in the first place. Despite the redundancy of the second error message, it’s useful to note that it is fairly common for a single error to produce (often redundant) multiple compiler errors or warnings.

Best practice

When addressing compile errors in your programs, always resolve the first error produced first and then compile again.

To fix this problem, we need to address the fact that the compiler doesn’t know what add is. There are two common ways to address the issue.

Option 1: Reorder the function calls

One way to address the issue is to reorder the function calls so add is defined before main:

That way, by the time main calls add, the compiler will already know what add is. Because this is such a simple program, this change is relatively easy to do. However, in a larger program, it can be tedious trying to figure out which functions call which other functions (and in what order) so they can be declared sequentially.

Furthermore, this option is not always possible. Let’s say we’re writing a program that has two functions A and B. If function A calls function B, and function B calls function A, then there’s no way to order the functions in a way that will make the compiler happy. If you define A first, the compiler will complain it doesn’t know what B is. If you define B first, the compiler will complain that it doesn’t know what A is.

Option 2: Use a forward declaration

We can also fix this by using a forward declaration.

A forward declaration allows us to tell the compiler about the existence of an identifier before actually defining the identifier.

In the case of functions, this allows us to tell the compiler about the existence of a function before we define the function’s body. This way, when the compiler encounters a call to the function, it’ll understand that we’re making a function call, and can check to ensure we’re calling the function correctly, even if it doesn’t yet know how or where the function is defined.

To write a forward declaration for a function, we use a declaration statement called a function prototype. The function prototype consists of the function’s return type, name, parameters, but no function body (the curly braces and everything in between them), terminated with a semicolon.

Here’s a function prototype for the add function:

Now, here’s our original program that didn’t compile, using a function prototype as a forward declaration for function add:

Now when the compiler reaches the call to add in main, it will know what add looks like (a function that takes two integer parameters and returns an integer), and it won’t complain.

It is worth noting that function prototypes do not need to specify the names of the parameters. In the above code, you can also forward declare your function like this:

However, we prefer to name our parameters (using the same names as the actual function), because it allows you to understand what the function parameters are just by looking at the prototype. Otherwise, you’ll have to locate the function definition.

Best practice

When defining function prototypes, keep the parameter names. You can easily create forward declarations by using copy/paste on your function declaration. Don’t forget the semicolon on the end.

Forgetting the function body

New programmers often wonder what happens if 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 and the function is called, but the program never defines the function, the program will compile okay, but the linker will complain that it can’t resolve the function call.

Consider the following program:

In this program, we forward declare add, and we call add, but we never define add anywhere. When we try and compile this program, Visual Studio 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.

Other types of forward declarations

Forward declarations are most often used with functions. However, forward declarations can also be used with other identifiers in C++, such as variables and user-defined types. Variables and user-defined types have a different syntax for forward declaration, so we’ll cover these in future lessons.

Declarations vs. definitions

In C++, you’ll often hear the words “declaration” and “definition” used, often interchangeably. What do they mean? You now have enough of a framework to understand the difference between the two.

A definition actually implements (for functions or types) or instantiates (for variables) the identifier. Here are some examples of definitions:

A definition is needed to satisfy the linker. If you use an identifier without providing a definition, the linker will error.

The one definition rule (or ODR for short) is a well-known rule in C++. The ODR has three parts:

  1. Within a given file, a function, object, type, or template can only have one definition.
  2. Within a given program, an object or normal function can only have one definition. This distinction is made because programs can have more than one file (we’ll cover this in the next lesson).
  3. Within a given program, types, template functions, and inline functions can have multiple definitions so long as they are identical. We haven’t covered what most of these things are yet, so don’t worry about this for now -- we’ll bring it back up when it’s relevant.

Violating part 1 of the ODR will cause a compile to issue a redefinition error. Violating ODR parts 2 or 3 will cause the linker to issue a redefinition error. Here’s an example of a violation of part 1:

Because the above program violates ODR part 1, this causes the Visual Studio compiler to issue the following compile errors:

project3.cpp(9): error C2084: function 'int add(int,int)' already has a body
project3.cpp(3): note: see previous definition of 'add'
project3.cpp(16): error C2086: 'int x': redefinition
project3.cpp(15): note: see declaration of 'x'

A declaration is a statement that tells the compiler about the existence of an identifier and its type information. Here are some examples of declarations:

A declaration is all that is needed to satisfy the compiler. This is why we can use a forward declaration to tell the compiler about an identifier that isn’t actually defined until later.

In C++, all definitions also serve as declarations. This is why int x appears in our examples for both definitions and declarations. Since int x is a definition, it’s a declaration too. In most cases, a definition serves our purposes, as it satisfies both the compiler and linker. We only need to provide an explicit declaration when we want to use an identifier before it has been defined.

While it is true that all definitions are declarations, the converse is not true: all declarations are not definitions. An example of this is the function prototype -- it satisfies the compiler, but not the linker. These declarations that aren’t definitions are called pure declarations. Other types of pure declarations include forward declarations for variables and type declarations (you will encounter these in future lessons, no need to worry about them now).

The ODR doesn’t apply to pure declarations (it’s the one definition rule, not the one declaration rule), so you can have as many pure declarations for an identifier as you desire (although having more than one is redundant).

Author's note

In common language, the term “declaration” is typically used to mean “a pure declaration”, and “definition” is used to mean “a definition that also serves as a declaration”. Thus, we’d typically call int x; a definition, even though it both a definition and a declaration.

Quiz time

Question #1

What is a function prototype?

Show Solution

Question #2

What is a forward declaration?

Show Solution

Question #3

How do we declare a forward declaration for functions?

Show Solution

Question #4

Write the function prototype for this function (use the preferred form with names):

Show Solution

Question #5

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!

a)

Show Solution

b)

Show Solution

c)

Show Solution

d)

Show Solution


2.8 -- Programs with multiple code files
Index
2.6 -- Whitespace and basic formatting

118 comments to 2.7 — Forward declarations and definitions

  • Colin

    Just to say the site really is very helpful. Keep up the good work.

  • Ashley

    Hi, Alex. Great tutorial. Really enjoying it.

    Just one question about the declarations vs, definitions section above. Why in the first instance(definition) do you say with int x it "instantiates an integer variable named x", ie. memory is allocated for it, but in the second instance(declaration) you say "declares an integer variable named x". Didn't you say at the start of the tutorial that when you write "int x" memory is allocated. So in other words in this case memory should be allocated in both instances and hence both are instantiated. Or did you just add them in both instances to show that int x can be a definition AND a declaration, and hence is initiated(memory allocated to it) in both cases? I read through the comments and actually saw that you replied to Matthew saying int x is both a declaration and definition. I just want to be sure that this was what you wanted to show in your above examples to start with.
    Thanks :)

    • Alex

      Yes, "int x" is both a declaration (telling the compiler that x is an integer variable) AND a definition (allocating memory for integer variable x).

      Later on, when we cover forward declarations of variables, you'll learn it's possible to declare variable x without instantiating it.

  • Iamthatis

    So essentially, a forward declaration behaves like a variable for a function whose definition has not yet sequentially appeared, (similarly to how X can be declared as a variable without being given a value) and the function prototype syntax distinguishes this "variable" as a function?

    • Alex

      Using the word variable is confusing here, because functions aren't variables.

      A forward declaration simply allows us to tell the compiler that an object (a variable, a function, a type, etc...) exists but will be defined later. This allows us to use an object that may be defined later in the file, or perhaps in another file altogether.

      For functions, a forward declaration is done via a function prototype.

  • Unknown

    Can you all please help??
    when i saw the first question i couldn't answer it because the question was :
    What’s the difference between a function prototype and a forward declaration?
    and when i saw the answer i saw "A function prototype can be used to forward declare a function" isn't that mean that prototype is forward declaration or a type of it?
    please replay fast because i cant really continue if i didn't understand something. and by the way thx alex for this awesome guide i really learned alot :).

    • Unknown

      and can you tell me please the Meaning of definition because i don't understand how a function can be defined.

      • Alex

        A declaration tells the compiler that an identifier exists. A definition provides the compiler with enough information to instantiate or execute something.

        In the case of a function, a function declaration is just the prototype. It's enough to tell the compiler that the function exists, but not enough to actually execute it. A function definition includes the prototype and the function body. A function definition is enough for the compiler to translate the function into machine language, so it can be executed.

    • Alex

      A function prototype declares a function's name, return type, and parameters. It does not include the function body.

      A forward declaration tells the compiler that something exists before it is actually defined.

      For functions, a function prototype serves as a forward declaration.

      For other types of objects (e.g. user-defined types) they have a different syntax for doing forward declarations.

      Make sense?

  • Dan

    Hey Alex. Is there a way to tell the difference between a compiler-specific and linker-specific error in Microsoft Visual Studio Express 2013 for Windows Desktop?

  • Okay...compiler executes program sequentially but starts execution from top of main (). If execution starts from top of main(), compiler should know about add() (because it's being called in main), but as written above, it doesn't in this program:

    If add () is defined above main(), the program compiles fine. Does that mean compiler can't jump below main () to find a function's definition.  I know that's stupid, but I proud myself for being stupid.

    • Alex

      Remember that the compiler compiles the program before execution can start. So for purpose of compiling a program, execution doesn't matter.

      When the compiler compiles a file, it does so sequentially, from top to bottom. In your example, when it gets to the call for add(3,4), it checks its internal database to see if it's seen a definition for add(3,4) and says, "nope!". Then it gives you an error.

      There are a few ways to get around this:
      1) Define add() above main()
      2) Use a forward declaration, which tells the compiler "I promise I'll define this later" (passing the buck to the linker)

      So basically, the compiler can't jump below main() to find a function definition while compiling. This is an intentional limitation because the alternative (having the compiler look for a definition) would be both inefficient and complicated. Especially if add() used a definition the compiler hadn't seen yet. In the worst case, that could exponentially increase your compile times.

  • begginner

    ohh yes .thankx i got it

  • begginner

    #include<iostream>
    int add(int x, int y)
    {
        
        
        
    }
    int main(){

    std::cout<<3+4<<add;
    return 0;
    }
    why the output is  71 here?

    • Alex

      Similar answer to the above. 3+4 = 7, so that's where the 7 comes from. Instead of calling function add(), you're printing the memory address that function add lives at. Why this is printing a 1 I have no idea.

      In your code above, "add" should be "add(3,4)" or something.

  • apurva

    what is the difference between
    #include<iostream.h>
    and
    #include"iostream.h"

  • Todd

    I'm loving these tutorials!
    I found a grammatical error - the first I have seen in all these tutorials!

    "A declaration is an statement"

    should be

    "A declaration is a statement"

    Also, "a integer" should be "an integer" in the code:

    Not trying to be annoying - just trying to keep your fantastic tutorials flawless!

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