Understanding Incomplete Type in C++
Published: 2023-07-13 13:54:19

In C++, an incomplete type refers to a type that has been declared but not fully defined. It means that the compiler has encountered a declaration of a type, but it doesn't have enough information about the type to determine its size or members. This situation can arise when a type is declared but its definition is not yet available, or when a type is declared with missing or incomplete information.

The concept of incomplete types is important in C++ because the compiler needs complete information about a type to perform various operations, such as allocating memory, determining the size of an object, accessing its members, or calculating offsets. Without complete information, the compiler cannot accurately perform these tasks.

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## Common scenarios leading to incomplete types in C++
Here are some scenarios where incomplete types can occur:

### Forward Declarations
Sometimes, you may need to declare a type before its complete definition is available. This is called a forward declaration. It allows you to declare a class, structure, or enumeration without providing its full definition.

For example:
```cpp
class MyClass;  // Forward declarationclass AnotherClass {
  MyClass* obj; // Pointer to incomplete type
};
```

In this case, MyClass is an incomplete type because its complete definition is not yet available. You can declare pointers or references to incomplete types, but you cannot create objects of that type or access its members until the complete definition is provided.

### Recursive Data Structures
Incomplete types are often used in data structures that have recursive references to themselves. For example:

```cpp
cppCopy code
struct Node {
  int data;
  Node* next; // Pointer to incomplete type
};
```

Here, Node is an incomplete type because it refers to itself through the next pointer. The size of Node cannot be determined until the complete definition of Node is available.

### Class Inheritance
Incomplete types can also occur in the context of class inheritance, particularly when using base classes. 

Consider the following example:
```cpp
class Base; // Incomplete typeclass Derived : public Base { // Inheritance from incomplete type// ...
};
```

Here, Base is an incomplete type because its complete definition is not provided before its usage as a base class in the Derived class.

## Limitations of Incomplete Types
It's important to note that incomplete types have limitations. You cannot create objects of incomplete types, access their members, or perform certain operations that require complete type information.

You can only declare pointers or references to incomplete types. To use the incomplete type, you need to ensure that its complete definition is available before attempting to create objects or access members.

Here are some of the limitations of incomplete types that you should take note:

### Inability to create objects of incomplete types:
When a type is incomplete, the compiler doesn't have enough information about the size or structure of the type. As a result, it's not possible to create objects of incomplete types. This means you cannot directly instantiate an object using the incomplete type.

For example, if you have a class MyClass that is incomplete, you cannot write MyClass myObject; to create an instance of MyClass. This limitation exists because the compiler needs the complete definition of a type to allocate memory and initialize its members properly.

### Restricted access to members of incomplete types:
Incomplete types limit the ability to access the members of the type. Since the compiler doesn't know the complete structure of the type, it cannot determine the existence or accessibility of its members. As a result, you cannot access the members directly using the dot operator or the arrow operator.

For example, if you have an incomplete type MyClass, you cannot write myObject.member to access a member of MyClass. This limitation exists because the compiler needs the complete type information to resolve member access and ensure proper visibility.

### Limitations in operations requiring complete type information:
Incomplete types impose limitations on various operations that require complete type information. These operations include calculating the size of the type, performing pointer arithmetic, determining the alignment requirements, or using certain language features that rely on complete type information.

For example, you cannot use the sizeof operator on an incomplete type because the compiler cannot determine the size without the complete definition. Similarly, you cannot perform pointer arithmetic or use certain language features like virtual functions or inheritance with incomplete types.

## How to Solve the C++ incomplete type is not allowed?
To solve the issue of "C++ incomplete type is not allowed," you need to provide the complete definition of the type that is causing the error. Here are some steps to help you resolve the problem:

1. Identify the incomplete type: Review the error message or compiler warnings to identify the specific type that is incomplete. The error message typically provides information about the line number or the context where the incomplete type is being used.

2. Locate the missing definition: Determine where the missing definition of the incomplete type should be located. This could be in a header file or a source file within your project.

3. Include necessary headers: If the incomplete type is part of a different class or structure defined in another file, make sure you include the appropriate header file that contains the complete definition. Check that the header file is correctly included using the #include directive.

4. Reorder class definitions: If the incomplete type is used as a base class or member within another class, ensure that the class definition containing the incomplete type comes after the definition of the incomplete type itself. This way, the compiler will have the complete definition available when it encounters the class that relies on it.

5. Forward declarations: If you are using a forward declaration of the incomplete type, ensure that the forward declaration is correctly written and precedes the use of the type. In some cases, you may need to modify the forward declaration to include additional information, such as pointers or references to the incomplete type.

6. Resolve circular dependencies: If the incomplete type is part of a circular dependency, where multiple types depend on each other, consider breaking the circular dependency by using techniques like forward declarations, pointers, or references. This way, you can provide the complete definition of the types when needed.

7. Verify type completeness: Double-check that you have provided the complete definition of the type, including all necessary members, methods, and any inheritance relationships. Ensure that the definition is correct and free of syntax errors.

8. Recompile your code: After making the necessary changes to provide the complete definition of the type, recompile your code to check if the error is resolved. If there are no more errors related to incomplete types, it indicates that you have successfully resolved the issue.

By following these steps and ensuring that the compiler has access to the complete definition of the type, you can solve the problem of "C++ incomplete type is not allowed" and proceed with compiling and running your program successfully.

## Forward Declarations and Incomplete Types
In C++, a forward declaration is a declaration of a type without providing its complete definition. It allows you to declare the existence of a type before its definition is available in the current scope. Forward declarations are useful when you need to refer to a type without requiring its complete definition, thus breaking circular dependencies between types or improving compilation times.

Forward declarations enable valid uses of incomplete types in certain scenarios. Some of the common valid uses include:

1. Declaring pointers or references: You can declare pointers or references to an incomplete type. This allows you to work with the type without needing its complete definition. For example, you can declare MyClass* ptr; with a forward declaration of class MyClass;.
2. Function declarations: Forward declarations are commonly used in function prototypes or function parameter lists. You can declare functions that accept or return the incomplete type as a pointer or reference.
3. Resolving circular dependencies: In cases where two or more types depend on each other, forward declarations can be used to break the circular dependency. By forward declaring one or more types, you can satisfy the compiler's requirement for type information while deferring the complete definition to a later point.

While forward declarations can be beneficial, there are certain restrictions and precautions to consider too.

### Restrictions and Precautions for Forward Declaration:

1. Limited usability: Incomplete types have limitations in terms of creating objects or accessing members, as discussed earlier. When using forward declarations, you should be aware that the functionality of the incomplete type will be limited until its complete definition is provided.
2. Incomplete type size: The size of an incomplete type is unknown to the compiler. Therefore, you cannot determine the size of an object or an array of an incomplete type using sizeof. This restriction is important to keep in mind, as it affects memory allocation and usage.
3. Member access limitations: Since the complete structure of an incomplete type is not available, you cannot directly access its members. However, you can still work with pointers or references to the incomplete type. Just ensure that you don't attempt to access members before the complete type definition is available.
4. Order of definitions: When using forward declarations, it's crucial to ensure that the complete definition of the type is provided before any operations that require complete type information. Failure to provide the complete definition in the right order can result in compilation errors.

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Read more: [Understanding Incomplete Type in C++](https://www.lightly-dev.com/blog/cpp-incomplete-type-is-not-allowed/)