Many C++ programmers will be familiar with the tidal wave of recompilations that occur when a seemingly unrelated header file is changed. C++ file dependencies need to be managed otherwise compilation times will grow unchecked. Large build times can cripple the process of creating software and eventually kill a project.
Suppose the class fubar uses the class snafu. Which uses require a #include
// fubar.hpp#include class fubar{public: // parameter type void method1(snafu value_parameter); // 1 void method2(snafu * pointer_parameter); // 2 void method3(snafu & reference_parameter); // 3public: // return type snafu value_return(); // 4 snafu * pointer_return(); // 5 snafu & reference_return(); // 6private: // object state snafu value; // 7 snafu * pointer; // 8 snafu & reference; // 9 private: // class state static snafu shared_value; // 10 static snafu * shared_pointer; // 11 static snafu & shared_reference; // 12};
The only one that requires a #include is number 7. A forward declaration will do for all the rest.
// fubar.hppclass snafu;class fubar{public: // parameter type void method1(snafu value_parameter); // 1 void method2(snafu * pointer_parameter); // 2 void method3(snafu & reference_parameter); // 3public: // return type snafu value_return(); // 4 snafu * pointer_return(); // 5 snafu & reference_return(); // 6private: // object state //snafu value; // 7 snafu * pointer; // 8 snafu & reference; // 9 private: // class state static snafu shared_value; // 10 static snafu * shared_pointer; // 11 static snafu & shared_reference; // 12};
However, be aware that you can’t forward declare a class when:
- The class is a nested class (there is no legal syntax to do it!)
- The class lives in the standard library (prohibited by ISO C++)
- The class is not a class! (e.g. a typedef’d template)
