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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?

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Question #3

How do we declare a forward declaration for functions?

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Question #4

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

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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)

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b)

Show Solution

c)

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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

  • Matthew

    Hi,

    I don't think

    is a definition.

    • Alex

      int x; is a definition (and a declaration). A definition defines how something is implemented -- in the case of variables, it's probably more intuitive to say it instantiates the variable (causes memory to be allocated for it).

      One way to tell it's a definition is to duplicate the line and see if the compiler complains (remember, things can only have one definition.

      If you do this:

      The compiler will complain.

      Although we haven't covered it yet, it is possible to forward declare variables. In that case, the forward declaration is a declaration only (and does not allocate memory -- it just tells the compiler that a variable exists, but it's defined elsewhere).

      I've updated the lesson text to be more clear about what definition means for variables.

  • amin

    you still don't want to make a class for matlab or fortran?:)

  • Bonnev

    "it doesn’t know what add is, because we haven’t defined add() until later! That produces the error on line 10. Then when it gets to the actual declaration of add()"
    which is.. on line 10 as well !! The error should be at line 6.
    "it complains about add being redefined (which seems slightly misleading, given that it wasn’t ever defined in the first place)" which is on line 15, and on which God knows what is written having in mind that the example has only 13 lines

  • Amjadb

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

  • equasar

    On the Quiz question 2, shouldn't be "forward declaration" instead "function prototype"?

  • fould12

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

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

  • Da-Rage44

    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?

    • 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 you're trying to use the forward declaration.
      No you don't need a return value to declare a function (void), it just needs to be declared somewhere (it doesn't even need to have code in it).

    • Alex

      > what does it actually mean by the program never defines the function?

      It means the program doesn't define a function body (the part of the function between the curly braces).

  • misserwell

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

  • 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.

  • donblas

    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?

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