Forward Declarations in C++

So far, we've been working under the restrictions that functions must be defined before they are used. For example, we've been unable to create code like shown below:

1int main() {
2  // Add is not defined yet
3  Add(1, 2);
4}
5
6int Add(int x, int y) {
7  return x + y;
8}
9

Because the compiler reads our files top-to-bottom, when it reaches line 3, it encounters a call to a function that has not been defined.

The easy solution we've been using has been to move Add above main. However, we may not want to do this for stylistic reasons. Worse, not all problems of this category can be solved by just moving functions around.

Instead, we can forward declare a function. This is a promise to the compiler that the thing we're trying to use is going to be defined somewhere else. Once all the code is linked together, everything will be resolved.

We'll discuss how our code gets built in a later chapter, and what this "linking" process actually is. For now, lets see how we can forward declare a function, and get our code working.

Function Prototypes in C++

To forward declare a function, we first need to understand what a function prototype is. This is sometimes also called the signature. We can think of prototype as just the heading of the function. In other words, the function without its body. For example, if we have the following Add function:

1int Add(int x, int y) {
2  return x + y;
3}
4

Its prototype would be this:

1int Add(int x, int y);
2

So, the prototype is the function's return type, the function name, and the parameter types.

In this example, the return type is int, the name is Add, and the parameter types are int, int

Within the function prototype, the names of our parameters are optional. They are typically worth including anyway for the benefit of anyone reading our code.

We can infer a lot more about the function just from its prototype if the prototype includes the names of the parameters.

But technically, only the type is relevant for the prototype, so the following is also a valid prototype for our function:

1int Add(int, int);
2

1float Calculate(int x, float y) {
2  return x * y;
3}
4

Using Forward Declaration

Now, we no longer have to define our function before we use it - we just have to declare its prototype before we use it.

Prototypes are typically put near the top of the files that use them, but as long as they're provided before the functions are used, our code will work as expected:

1int Add(int x, int y);
2
3int main() {
4  Add(1, 2);
5}
6
7int Add(int x, int y) {
8  return x + y;
9}
10

1int main() {
2  int x { Calculate() };
3}
4
5int Calculate() {
6  return 10 * 10;
7}
8

1int Calculate();
2
3int main() {
4  int x { Calculate() };
5}
6
7int Calculate() {
8  return 10 * 10;
9}
10

This separation between declaring a function and defining it is likely to feel quite strange, especially at this stage.

However, this is a very important concept. In fact, most C++ code is created this way. This is because the seperation can have some major workflow benefits for larger projects. We'll see why that is very soon.

For now, lets round off this section by introducing loops!