在头文件中前向声明另一个命名空间的自定义类型

Question

I am using an external library named custom (not the actual name) which defines many of its data types in its own namespace. Let's assume that the namespace is named as custom as well.

I am trying to make use of a datatype named DataType . DataType is a custom type defined using a typedef, and let's assume that it can be used in a .cpp file by including "custom/datatype.h"

My code has the following setting.

action.h:

class Action
{
    Action() {}
    virtual ~Action() {}

    virtual void foo(custom::DataType*) const = 0;
    ...
}

some_action.h:

#include "action.h"

class SomeAction : public Action
{
    SomeAction() {}

    virtual void foo(custom::DataType*) const override;
    ...
}

some_action.cpp:

#include "some_action.h"
#include "custom/datatype.h"
...
void SomeAction::foo(custom::DataType*) const
{
    ...
}

Is there any proper way to forward-declare DataType in action.h and some_action.h other than including "custom/datatype.h" in action.h ?

Answer 1

Is there any proper way to define DataType in action.h and some_action.h other than including "custom/datatype.h" in action.h ?

You probably don't want to "define" DataType , but to "forward declare" it instead. This allows you to make the compiler aware of a custom::DataType which will be defined later.

You can achieve this as follows. In action.h and some_action.h add the following forward declaration :

namespace custom { class DataType; }

In the .cpp files where you're going to need the definition of DataType , include "custom/datatype.h" .

---

If custom::DataType is a type alias (ie typedef ) , you need to include the header. There isn't a way of forward-declaring type aliases: see this question & answers for related information.

Answer 2

If DataType is a class type or an enum with an underlying integer type, then you can forward declare it in action.h :

namespace custom {
  class DataType1;

  enum DataType2 : int; 
}

The downside is that it may break when you update the library. Well written libraries may have a header full of nothing other than forward declarations, which you may use, so you can look for those (example: iosfwd ).

Otherwise, there is no recourse from including the header you are given by the library writer.