### Friendship example: list.cpp

Source: https://joholl.github.io/pitchfork-website

An empty source file that includes the header to ensure it compiles in isolation. This verifies that the header has all necessary includes and forward declarations.

```cpp
// list.cpp
#include "list.hpp"

```

--------------------------------

### PIMPL example: connection.cpp

Source: https://joholl.github.io/pitchfork-website

Implementation file for the connection class. It defines the implementation class and a helper function to initialize the private data, then implements the constructor and destructor.

```cpp
#include "connection.hpp"

namespace acme::detail {

class connection_impl {
    // Whatever we need for the connection...
};

}

namespace {

std::unique_ptr<acme::detail::connection_impl>
init_private_data(const std::string& address) {
    // Initialize and return the connection data...
}

}

acme::connection::~connection() = default;
acme::connection::connection(const std::string& address)
    : _impl{ init_private_data(address) }
{}

```

--------------------------------

### PIMPL example: connection.hpp

Source: https://joholl.github.io/pitchfork-website

Header for a connection class using the PIMPL idiom. It forward-declares the implementation class and holds a unique_ptr to it, hiding the implementation details.

```cpp
// connection.hpp
#include <memory>

namespace acme {

// Forward-declare the private member data type
namespace detail { class connection_ipml; }

// The wrapper class
class connection {
private:
    // The actual PIMPL
    std::unique_ptr<detail::connection_impl> _impl;

public:
    connection(const std::string& address);
    ~connection();
};

}

```

--------------------------------

### Friendship example: list.hpp

Source: https://joholl.github.io/pitchfork-website

Shows a list class template and its iterator as a single logical component using friendship. The iterator has a private constructor that only the list class can call, and the list class declares the iterator as a friend.

```cpp
// list.hpp
namespace acme {

template <typename T>
class list;

namespace detail {

// Class implementing a "list iterator"
template <typename T>
class list_iterator {
public:
    // Public default constructor
    list_iterator() = default;

private:
    // A private constructor used to initialize the iterator to the proper state
    explicit list_iterator(list<T>&);

    // Permit our list class to construct us with the private constructor
    friend class list<T>;
};

}

template <typename T>
class list {
public:
    using iterator = detail::list_iterator<T>;

    // The iterator class is a friend, giving it access to our internals
    friend class detail::list_iterator<T>;
};

}

```

--------------------------------

### Separate header placement directory layout

Source: https://joholl.github.io/pitchfork-website

Shows the resulting directory structure when using separate header placement, with public headers in include/ and compilable sources in src/. The relative path from the source directory to the parent directory of the source file must match that of the header.

```text
<root>/
    include/
        cat/
            sounds/
                meow.hpp
    src/
        cat/
            sounds/
                meow.cpp

```

--------------------------------

### Merged header placement directory layout

Source: https://joholl.github.io/pitchfork-website

Shows the resulting directory structure when using merged header placement, with both header and source files in the same src/ directory. The files of a physical component are siblings in the same directory.

```text
<root>/
    src/
        cat/
            sounds/
                hiss.hpp
                hiss.cpp

```

--------------------------------

### Source tree layout for subdivided circle component

Source: https://joholl.github.io/pitchfork-website

Shows the directory and file layout for the logical component `geo::shapes::circle` when splitting complex member functions `circumference()` and `area()` into separate translation units. The header `circle.hpp` appears in `src/geo/shapes/`, alongside the main implementation and per-function implementations, plus the unit test `circle.test.cpp`.

```plaintext
<root>
    src/geo/shapes/
        circle.hpp
        circle.cpp
        circle.circumference.cpp
        circle.area.cpp
        circle.test.cpp
```

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