[c++] What are forward declarations in C++?

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 <iostream>

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

int main()
{
    using namespace std;
    cout << "The sum of 3 and 4 is " << add(3, 4) << 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?

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.

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.

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.

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?)

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()
{
.
.
.

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.

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);