When to Use Forward Declarations in C++

In what situations should I use forward declarations in my C++ code?

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:

class A {
  B* b;
  // ...
};

class B {
  A* a;
  // ...
};

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:

class B; // Forward declaration

class A {
  B* b;
  // ...
};

class B {
  A* a;
  // ...
};

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

Introduction to Functions

Learn the basics of writing and using functions in C++, including syntax, parameters, return types, and scope rules.

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