Summary of the benefits of Forward declaration in C++:
Reduce compilation dependencies: By declaring in advance, the dependency on header files can be reduced. This reduces compile time because the compiler doesn't need to see the full definition, only the name of the type and the signature of the member functions.
Solve circular dependencies: When two classes refer to each other, circular dependencies can cause compiler errors. By declaring one of the classes ahead of time, you can break the circular dependency, allowing the compiler to compile smoothly.
Improve compilation speed: When only pointers or references to classes need to be used, early declaration can avoid the compiler to generate complete definition overhead, thereby improving compilation speed.
Increase code flexibility: By declaring in advance, the definition details of the class can be hidden in some cases, thereby increasing the flexibility and maintainability of the code.
Reduced compile-time and link-time: In large projects, early declarations can reduce compile-time and link-time overhead, because the full definition does not need to be checked for every type used.
In general, early declaration has many benefits in C++, including reducing compilation dependencies, solving circular dependency problems, improving compilation speed, increasing code flexibility, and reducing compilation and linking time. Where appropriate, forward declarations can be applied to improve the efficiency and maintainability of programs.