When to Use Forward Declarations in C++

Forward declarations are useful in a few scenarios:

1. To break circular dependencies. If two classes or functions depend on each other, one of them can use a forward declaration to break the cycle.
2. To improve compilation speed. If a header file only uses pointers or references to a type, it can forward declare that type instead of #include-ing its definition. This reduces the amount of code that needs to be parsed when the header is included.

For example, consider these two classes:

1class A {
2  B* b;
3  // ...
4};
5
6class B {
7  A* a;
8  // ...
9};

This code won't compile, because A needs to know about B, and B needs to know about A. We can fix this with a forward declaration:

1class B; // Forward declaration
2
3class A {
4  B* b;
5  // ...
6};
7
8class B {
9  A* a;
10  // ...
11};

Now A knows that B exists (but not its full definition), which is enough for it to have a B* member.