How Can I Use `std::unique_ptr` with Incomplete Types in C ?

Using std::unique_ptr with Incomplete Types

In the pimpl idiom, using std::unique_ptr with an incomplete type can lead to compilation errors. Consider the following code:

class window {
  window(const rectangle& rect);

private:
  class window_impl; // defined elsewhere
  std::unique_ptr<window_impl> impl_; // won't compile
};

The error typically arises from attempting to use sizeof on an incomplete type, as seen in line 304.

The Problem

The issue stems from the fact that the compiler needs to know the size of the incomplete type to determine the allocation for the std::unique_ptr. This information is unavailable, resulting in the compilation error.

The Solution

To address this problem, it is essential to implement a destructor for the class holding the unique_ptr. This is because the default destructor generated by the compiler requires a complete declaration of the type. Here's an example:

class foo {
    class impl;
    std::unique_ptr<impl> impl_;

public:
    foo(); // You may need a def. constructor to be defined elsewhere

    ~foo(); // Implement (with {}, or with = default;) where impl is complete
};

By implementing the destructor, the compiler gains the necessary knowledge to handle the destruction of the unique_ptr.

Special Cases

In certain scenarios, using unique_ptr with incomplete types may not be possible:

• Template constructors: If the class has template constructors, the compiler requires the complete type to determine potential exceptions in construction and how to handle destruction.
• Static duration objects: Using unique_ptr at namespace scope may also fail, as the compiler needs to know how to destroy the static duration object. In such cases, a workaround is to define a helper class with a destructor:

class impl;
struct ptr_impl : std::unique_ptr<impl> {
    ~ptr_impl(); // Implement (empty body) elsewhere
} impl_;

By following these guidelines, you can effectively use unique_ptr with incomplete types, ensuring a smooth compilation process.

The above is the detailed content of How Can I Use `std::unique_ptr` with Incomplete Types in C ?. For more information, please follow other related articles on the PHP Chinese website!