Why C++ Containers Don't Allow Incomplete Types?

C++ Standard Template Library (STL) provides various containers such as std::vector, std::list, std::map, and more. These containers are essential for managing collections of objects efficiently. However, we might encounter an issue when trying to use incomplete types with these containers.

In this article, we will learn why C++ containers do not allow incomplete types and discuss the implications and alternatives.

What is an Incomplete Type?

An incomplete type in C++ is a type that is declared but not defined. For instance, declaring a class without providing its full definition:

For Example,

class MyClass; // Incomplete type declaration

Here, MyClass is known to the compiler, but its size and members are not. It becomes a complete type when its full definition is provided.

Why C++ Containers Don't Allow Incomplete Types?

There are several reasons why C++ containers do not allow incomplete types:

1. Memory Management

C++ containers need to know the size of the objects they store to manage memory correctly. For example, std::vector needs to allocate memory for a dynamic array, std::list manages nodes in a doubly linked list, and std::map organizes nodes in a balanced tree structure. Without knowing the size of the objects, the container cannot allocate the correct amount of memory.

2. Element Access and Manipulation

Containers often need to copy, move, or assign elements. This requires knowing the complete type to call the appropriate constructors, destructors, and assignment operators.

3. Type Traits and Metaprogramming

Many C++ containers and algorithms use type traits and template metaprogramming to optimize performance and ensure type safety. Incomplete types cannot be used with type traits, which rely on knowing the complete type's properties.

Therefore, using incomplete types with STL containers results in compilation errors, preventing the program from being built. This limitation ensures that the containers can manage memory, access elements, and use type traits correctly.

Alternative Approaches for Same Functionality

While incomplete types cannot be directly used with STL containers, there are several workarounds to achieve similar functionality:

1. Pointers to Incomplete Types

We can use pointers to incomplete types since pointers have a known size, regardless of the pointed-to type's completeness. This allows the container to manage memory for the pointers, while the actual objects can be defined later.

Example:

// C++ program to use pointers to incomplete types

// Include necessary header files
#include <iostream>
#include <vector>

using namespace std;

// Forward declaration of MyClass (Incomplete type)
class MyClass;

int main()
{
    // Initialize MyClass pointers to nullptr
    MyClass *a = nullptr;
    MyClass *b = nullptr;
    MyClass *c = nullptr;

    // Create a vector of pointers to MyClass
    vector<MyClass *> vec = {a, b, c};

    // Iterate through the vector and print addresses
    for (auto ptr : vec)
    {
        // Print the address stored in each pointer
        cout << ptr << " ";
    }

    // Print a new line character
    cout << endl;

    // Return 0 to indicate successful execution
    return 0;
}

0 0 0 

2. Using std::reference_wrapper

The STL provides std::reference_wrapper, which can be used to store references in STL containers. std::reference_wrapper is a utility that allows references to be stored in standard containers like std::vector.

Example:

// C++ program to use std::reference_wrapper

#include <functional>
#include <iostream>
#include <vector>

using namespace std;

// Definition of MyClass
class MyClass
{
  public:
    int value;
    // Constructor to initialize value
    MyClass(int v) : value(v)
    {
    }
};

int main()
{
    // Create instances of MyClass with different values
    MyClass a(10), b(20), c(30);

    // Create a vector of reference_wrappers
    vector<reference_wrapper<MyClass>> vec = {a, b, c};

    // Iterate through the vector and print values
    for (auto &ref : vec)
    {
        // Print the value of each MyClass instance
        cout << ref.get().value << " ";
    }
    cout << endl;

    return 0;
}

10 20 30 

3. Using Smart Pointers

Smart pointers such as std::shared_ptr and std::unique_ptr can also be used to maintain the lifetime of objects dynamically allocated on the heap. These smart pointers can be stored in containers even if the pointed-to type is incomplete.

Example:

// C++ program to use smart pointers

#include <iostream>
#include <memory>
#include <vector>

using namespace std;

// Definition of MyClass
class MyClass
{
  public:
    int value;
    // Constructor to initialize value
    MyClass(int v) : value(v)
    {
    }
};

int main()
{
    // Create shared pointers to MyClass
    auto a = make_shared<MyClass>(10);
    auto b = make_shared<MyClass>(20);
    auto c = make_shared<MyClass>(30);

    // Create a vector of shared pointers
    vector<shared_ptr<MyClass>> vec = {a, b, c};

    // Iterate through the vector and print values
    for (auto &ptr : vec)
    {
        // Print the value of each MyClass instance
        cout << ptr->value << " ";
    }
    cout << endl;

    return 0;
}

10 20 30 

Conclusion

C++ containers do not allow incomplete types because they need to manage memory, access elements, and use type traits, all of which require knowing the complete type. Understanding this limitation helps in designing C++ programs more effectively, and using workarounds like pointers, smart pointers, and ensuring complete type definitions can help achieve the desired functionality.