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:
- 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.
- 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.