What is forward reference in C?

Question

What is forward reference in C with respect to pointers?

Can I get an example?

Answer 1

Forward reference is when you declare a type but do not define it.

It allows you to use the type by pointer (or reference for C++) but you cannot declare a variable.

This is a way to say to the compiler that something exists

Say that you have a Plop structure defined in Plop.h:

struct Plop
{
   int n;
   float f;
};

Now you want to add some utility functions that works with that struct. You create another file PlopUtils.h (let's say you can't change Plop.h):

struct Plop; // Instead of including Plop.h, just use a forward declaration to speed up compile time

void doSomething(Plop* plop);
void doNothing(Plop* plop);

Now when you implement those function, you will need the structure definition, so you need to include the Plop.h file in your PlopUtils.cpp:

#include "PlopUtils.h"
#include "Plop.h" // now we need to include the header in order to work with the type

void doSomething(Plop* plop)
{
   plop->n ...
}

void doNothing(Plop* plop);
{
   plop->f ...
}