PImpl (which stands for Pointer to implementation) is a programming tehnicque to remove implementation details from a class by placing them in a separate class that is accessed through an opaque pointer. Its purpose is to separate interfaces and implementations and minimize compile-time dependencies. In this article, we’ll at how the PImpl idiom is typically implemented in C++ and how C++26 simplifies its implementation.
Implementing with a raw pointer
We can implement the Pimpl idiom using raw pointers and abiding to the Rule of Five. This is a guideline that says that when a class defines a special member function (for resource management) it should define all five of them (destructor, copy constructor, copy assignment operator, move constructor, move assignment operator).
To demonstrate this, we will use a widget that represents a UI element that has a label and can be clicked and on each click a counter is incremented. Therefore, a widget would have a counter and a label but these are implementation details that are hidden behind an implementation class.
This Widget class definition looks as follows:
#pragma once
#include <string>
class Widget
{
public:
Widget(const std::string& name);
~Widget();
Widget(const Widget& other);
Widget& operator=(const Widget& other);
Widget(Widget&& other) noexcept;
Widget& operator=(Widget&& other) noexcept;
void click();
int clickCount() const;
std::string label() const;
private:
struct Impl;
Impl* pimpl_;
};
The Widget::Impl class is an incomplete type here. It’s forward declared and defined in the .cpp file. The Widget class keeps a pointer to an object of this type.
#include "Widget.h"
struct Widget::Impl
{
std::string name;
int clicks = 0;
explicit Impl(std::string n) : name(std::move(n)) {}
};
Widget::Widget(const std::string& name)
: pimpl_(new Impl(name))
{
}
Widget::~Widget()
{
delete pimpl_;
}
Widget::Widget(const Widget& other)
: pimpl_(new Impl(*other.pimpl_))
{
}
Widget& Widget::operator=(const Widget& other)
{
if (this != &other)
{
Impl* tmp = new Impl(*other.pimpl_);
delete pimpl_;
pimpl_ = tmp;
}
return *this;
}
Widget::Widget(Widget&& other) noexcept
: pimpl_(other.pimpl_)
{
other.pimpl_ = nullptr;
}
Widget& Widget::operator=(Widget&& other) noexcept
{
if (this != &other)
{
delete pimpl_;
pimpl_ = other.pimpl_;
other.pimpl_ = nullptr;
}
return *this;
}
void Widget::click() { ++pimpl_->clicks; }
int Widget::clickCount() const { return pimpl_->clicks; }
std::string Widget::label() const { return pimpl_->name; }
The Widget class defines all five special member functions. The move constructor and assignment operator are defined noexcept because they just copy/delete objects and nothing should throw. Moreover, it’s a performance issue because a container such as std::vector<Widget> will only move elements during reallocation if the move constructor is noexcept; otherwise it falls back to copying to preserve its strong exception guarantee.
The Impl object is created during the construction of the Widget and basically stores the state of the widget. The public interface methods of Widget use it to access the state (clicks, name).
A Widget can be used as follows:
#include <iostream>
#include <print>
#include "Widget.h"
int main()
{
Widget a("Button A");
a.click();
a.click();
std::println("clicks {}", a.clickCount()); // prints "clicks 2"
Widget b = a;
b.click();
std::println("clicks {}", b.clickCount()); // prints "clicks 3"
}
A problem that may seem subtle is that constness is not propagated from Widget to Impl. In a const method, such as clickCount() above, we can change the state because even if the Impl pointer is const, the object it points to is not. Therefore, the following would compile and produce unexpected results:
int Widget::clickCount() const { return ++pimpl_->clicks; }
On the other hand, a consequence of the move semantics is that a moved-from Widget object has a null pimpl_ pointer and calling any function that use the pointer is undefined behavior (UB).
Widget a("Button A");
Widget b = std::move(a);
a.click(); // undefined behavior
There are several ways to address this:
- document that moved-from objects cannot be used
- add a check that
pimpl_is not null everywhere before its usage - provide a function that indicates whether the object is in a valid state so clients can query whether they can use the widget or not
Implementing using std::unique_ptr
We can simplify the implementation by using the C++11 std::unique_ptr type instead of a raw pointer. This automatically manages the allocated object so we don’t have to do resource management explicitly.
#pragma once
#include <memory>
#include <string>
class Widget
{
public:
Widget(const std::string& name);
~Widget();
Widget(Widget&&) noexcept;
Widget& operator=(Widget&&) noexcept;
Widget(const Widget& other);
Widget& operator=(const Widget& other);
void click();
int clickCount() const;
std::string label() const;
private:
struct Impl;
std::unique_ptr<Impl> pimpl_;
};
#include "Widget.h"
struct Widget::Impl
{
std::string name;
int clicks = 0;
explicit Impl(std::string n) : name(std::move(n)) {}
};
Widget::Widget(const std::string& name)
: pimpl_(std::make_unique<Impl>(name))
{
}
Widget::~Widget() = default;
Widget::Widget(Widget&&) noexcept = default;
Widget& Widget::operator=(Widget&&) noexcept = default;
Widget::Widget(const Widget& other)
: pimpl_(std::make_unique<Impl>(*other.pimpl_))
{
}
Widget& Widget::operator=(const Widget& other)
{
if (this != &other)
{
*pimpl_ = *other.pimpl_;
}
return *this;
}
void Widget::click() { ++pimpl_->clicks; }
int Widget::clickCount() const { return pimpl_->clicks; }
std::string Widget::label() const { return pimpl_->name; }
In this implementation, the copy constructor and copy assignment are explicitly user-defined. The destructor, move constructor, and move assignment operator are defaulted to the compiler but this must happen in the .cpp file because in the header Widget::Impl is an incomplete type and the unique_ptr‘s deleter needs to know the size of Impl.
Although there is no manual handling of resources anymore, the constness issue remains the same as well as the null pimpl_ object after a move.
Implementing using std::indirect
This is where the std::indirect enters the scene. This is a new vocabulary type from C++26, defined in the <memory> header. It is intended to be used for class members that are dynamically allocated but need to behave like values. It’s designed to be used instead of std::unique_ptr where its semantics (not copyable, does not propagate const, and can be null) are not appropriate. The perfect example for this is Pimpl.
The following snippet shows the Widget class implemented using std::indirect:
#pragma once
#include <memory>
#include <string>
class Widget
{
public:
Widget(const std::string& name);
Widget(const Widget&);
Widget(Widget&&) noexcept;
Widget& operator=(const Widget&);
Widget& operator=(Widget&&) noexcept;
~Widget();
void click();
int clickCount() const;
std::string label() const;
private:
struct Impl;
std::indirect<Impl> pimpl_;
};
#include "widget.h"
struct Widget::Impl
{
std::string name;
int clicks = 0;
explicit Impl(std::string n) : name(std::move(n)) {}
};
Widget::Widget(const std::string& name)
: pimpl_(std::in_place, name)
{}
Widget::Widget(const Widget&) = default;
Widget::Widget(Widget&&) noexcept = default;
Widget& Widget::operator=(const Widget&) = default;
Widget& Widget::operator=(Widget&&) noexcept = default;
Widget::~Widget() = default;
void Widget::click() { ++pimpl_->clicks; }
int Widget::clickCount() const { return pimpl_->clicks; }
std::string Widget::label() const { return pimpl_->name; }
If we compare these implementation, we’ll see that the five special member functions are still declared in the header but they are defaulted (for compiler implementation) in the source file, where the Impl type is complete and known to the compiler (this is the same requirement as for std::unique_ptr).
The std::indirect type owns a single T object allocated on the heap, but behaves like a T member:
- Deep copy: copying the
std::indirect<T>object copies the owned object, so your class’s compiler-generated copy constructor just works. - Const propagation: through a
const std::indirect<T>object, you only get const access to theT. - Never null: it always holds a value, except in the moved-from state (but features a member function called
valueless_after_move()that lets you check the state).
Using std::indirect does not prevent us from having to explicitly declare and default the special member functions but does help us with propagating constness, which means the code is more robust towards accidental changes.
The valueless_after_move() is basically a replacement for null check from unique_ptr. For instance, we can add an assert to all the functions that use the pimpl_ object to ensure they are not invoked after the widget has been moved.
void Widget::click()
{
assert(!pimpl_.valueless_after_move() && "use of moved-from Widget");
++pimpl_->clicks;
}
Another example of using valueless_after_move() is erase-remove from a vector from which we moved out objects. Here is an example:
std::vector<std::indirect<Widget>> widgets = ...;
std::vector<std::indirect<Widget>> selected;
for (auto& w : widgets)
if (select(w))
selected.push_back(std::move(w)); // leaves a valueless widget behind
std::erase_if(widgets, [](const auto& w) {
return w.valueless_after_move(); // remove the dangling object
});
This is possible with std::unique_ptr too, but the check would be a w == nullptr. Therefore, it’s just a more verbose way of checking that an std::indirect object still holds a value or not.
std::indirect comes with a companion, std::polymorphic, but we will look at that in another article.
At the time of writing this, only GCC 16 supports std::indirect.