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CppForwardDeclaration

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(C++) Forward declaration

A technique to speed up the time compiling by saying that a class exists and will get defined later.

class Test; //Forward declaration
class MyClass
{
  public:
  Test * mTest; //Okay: a pointer to this class can be made
};
include "Test.h" //Okay: the class definition comes in later


int main()
{
  MyClass myClass;
  myClass.mTest = new Test; //Okay: use class methods after class definition
  delete myClass.mTest;
}

Using a forward declared class

When a class is forward declared, you

• CAN create a pointer to this class

  Test * myTest;

• CANNOT create an static instance of this CppClass

  Test myTest; //ERROR: 'Class Test unknown or unknown size'

• CANNOT call methods of this CppClass

  Test * myTest = new Test; //ERROR: 'Class Test unknown'

this would call the constructor)

The cause of this speed increase

The increase in speed by forward declaration is due to the fact that header files (.h) are #included multiple times, whereas implementation files .cpp are not. Therefore, keep the header files as simple to compile as possible.

STL forward declarations

The STL also has build-in forward declarations:

include <iosfwd>

After this #include you have a forward declaration of, among others, std::iostream and std::string.

Forward declaration in other namespaces

A forward declaration in other namespaces can also be done using the example below:

//Forward declaration
namespace AnyNamespace //Get in the right namespace
{
  class MyForwardDeclaredClass; //Forward declaration
};

What does NOT work, is:

AnyNamespace::MyForwardDeclaredClass; //DOES NOT WORK!!!

Idioms using forward declarations

The pimpl-idiom speeds up any class's compilation by forward declaring the class's own implementation.
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