Prototype Pattern: From a Single Copy Constructor to a clone() That Handles Inheritance

What Problem Are We Actually Solving?

Let's skip the formal definition for a moment. Imagine a concrete scenario: you have an object that is expensive to create. Why is it expensive? Maybe its constructor needs to fetch data from a remote API to populate fields, or it requires a time-consuming calculation, or perhaps it reads a multi-MB configuration template. Now, you need another object that is almost identical to it, differing only in a few fields—for example, 1,000 office location records that differ only by street number, or a swarm of monsters that differ only in health points.

If you stick to the "start from scratch with new and re-run that expensive initialization" approach, that's inefficient—when you have a ready-made, fully initialized object right there. Why not just use it as a template, copy it, and tweak a few fields? This is exactly what the Prototype Pattern solves: use an existing object as a template to create new objects by copying it, rather than constructing from scratch every time.

It sounds deceptively simple—so simple you might ask: isn't this just a copy constructor? Yes, the most primitive form of the Prototype Pattern is just a copy constructor. However, this topic deserves its own article because when that "template object" lives within a type system involving inheritance, polymorphism, and resource ownership, that simple copy constructor starts to fail: it slices objects, loses type information, and can inadvertently share underlying resources. The real question we need to answer is—how do we ensure that "copying an object" faithfully reproduces the "correct derived type" within an inheritance hierarchy and correctly handles resource semantics?

We will walk through this step-by-step: first, looking at the most intuitive approach; then, seeing where it breaks; and finally, deriving a standard, modern C++ solution that is both safe and capable of handling polymorphism.

Step 1: The Most Intuitive Approach—Create a Prototype, Then Copy It

Let's start with the office location example. An Address record holds a street number and accessibility status:

struct Address {
    std::string door_number;     // 门牌号
    bool        accessible {};   // 是否可达
};

The most straightforward way to use a prototype is to first initialize a prototype object, then use it to create copies, and finally modify a few fields.

Address proto;
proto.door_number    = "B-101";
proto.accessible     = true;

// 需要新实例时,从原型拷一份,只改需要变的字段
Address other   = proto;
other.door_number = "A501";

Address another = proto;
another.door_number = "C-77";

You see, Address other = proto; here is a copy construction, representing the most primitive form of the Prototype pattern. For a trivial class like Address, where all fields are value types (std::string, bool), this approach works perfectly—the compiler-generated copy constructor correctly performs a deep copy of the std::string and copies the bool by value, so everything is fine.

However, this "perfectly fine" comes with a premise: the class must have no inheritance and no resource members requiring special handling. The moment someone adds a derived class to Address, this approach immediately exposes its flaws.

Step Two: The Problem Arises — Object Slicing

Let's assume the project evolves over time, and someone extends it by creating ExAddress, which adds an "additional info" field to the base:

struct ExAddress : Address {
    std::string extra;   // 附加信息,比如「靠近咖啡机」
};

Back in the day, to reuse code, we encapsulated the process of "creating an object from a prototype with a different ID" into a function—pay attention to its parameter type:

Address* make_from_proto(const Address* a, const std::string& door_number) {
    Address* t = new Address(*a);   // 用 Address 的拷贝构造
    t->door_number = door_number;
    return t;
}

Now the question arises. What happens if we pass a real ExAddress object from the outside?

ExAddress ex;
ex.door_number = "B-101";
ex.accessible  = true;
ex.extra       = "near-cafeteria";   // 派生类独有字段

Address* p = make_from_proto(&ex, "A501");
// p 指向一个 Address,它的 extra 去哪了?

Inside make_from_proto, the code executes new Address(*a). Since the static type of *a is Address, the copy constructor of Address is invoked. The copy constructor of Address only knows about its own two members; it cannot see, nor does it copy, ExAddress::extra. Consequently, on this line, extra is silently discarded. This is the infamous object slicing in C++: when you use a base class to copy-construct a derived class object, the derived part is sliced off cleanly.

Let's first verify that this actually happens.

Verification: What exactly gets sliced?

Let's write a small program to see if the derived fields still exist after the copy constructor runs:

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#include <iostream>
#include <memory>
#include <string>
#include <typeinfo>

class Address {
public:
    virtual ~Address() = default;
    std::string door_number;
};

class ExAddress : public Address {
public:
    std::string extra;
};

int main() {
    ExAddress ex;
    ex.door_number = "A501";
    ex.extra       = "near-cafeteria";

    // 切片:用 Address 的拷贝构造去拷一个 ExAddress
    Address sliced = ex;

    std::cout << "[sliced]   door_number = " << sliced.door_number << "\n";
    std::cout << "[sliced]   dynamic type = " << typeid(sliced).name() << "\n";

    // 原对象的 extra 还在
    std::cout << "[original] extra = " << ex.extra << "\n";
    return 0;
}

Let's compile and run it:

$ g++ -std=c++23 -O2 -Wall -Wextra prototype_verify.cpp -o prototype_verify
$ ./prototype_verify
[sliced]   door_number = A501
[sliced]   dynamic type = 9Address
[original] extra = near-cafeteria

The result is crystal clear: the dynamic type of the copied sliced has become 9Address (9 is the name length prefix in the mangled name, representing Address). It is no longer an ExAddress—the extra part has been sliced off. This is why a copy constructor hardcoded to the base class cannot support an inheritance hierarchy: it loses the type information right at the first step of information transfer.

Even more critically, we likely cannot change the interface of make_from_proto—because others in the project are also deriving from Address for their own purposes. If you change the parameter type to ExAddress*, all other derived classes will break. We need a way to move the decision of "how to copy" from the caller to the object itself. This is exactly where virtual comes into play.

Step 3: Build Cloning into the Class — Polymorphic clone()

The idea is straightforward: since the problem stems from the caller deciding "which type to copy to," we hand this decision over to the object itself. Let each class know "how I should copy myself," and expose this capability through a virtual function—the caller simply shouts "give me a copy," while the specific type to copy is determined by the object's dynamic type.

We write the base class like this:

class Address {
public:
    virtual ~Address() = default;

    virtual Address* clone() const {        // 基类版本:复制成一个 Address
        return new Address(*this);
    }

    std::string door_number;
    bool        accessible {};
};

Each derived class overrides and calls its own copy constructor:

class ExAddress : public Address {
public:
    std::string extra;

    ExAddress* clone() const override {      // 派生类版本:复制成一个 ExAddress
        return new ExAddress(*this);
    }
};

Note the return types in these two lines: the base class returns Address*, while the derived class returns ExAddress*. This is valid in C++ and is known as a covariant return type. When a derived class overrides a virtual function, the return type can be a pointer or reference to a type derived from the base class's return type. This is a specific language feature designed to support "polymorphic factories" or "polymorphic clones." We will verify that this works correctly in a moment.

Now, let's rewrite that utility function:

std::unique_ptr<Address> make_from_proto(const Address& a,
                                         const std::string& door_number) {
    auto t = a.clone();                      // 走虚派发:动态类型决定复制成谁
    t->door_number = door_number;
    return std::unique_ptr<Address>(t);
}

This line a.clone() is the key to the entire article. Its static invocation is Address::clone, but because clone is a virtual function, the actual version executed depends on the dynamic type of a—if a's true identity is ExAddress, then ExAddress::clone is called here, and the result is a complete ExAddress object, so slicing will never occur again. Let's verify this.

Verification: Does Polymorphic clone Really Preserve the Dynamic Type?

Let's put the above structure into a runnable small program, use a pointer whose static type is the base class but whose dynamic type is the derived class to call clone(), and see exactly who the copied object is:

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#include <iostream>
#include <memory>
#include <string>
#include <typeinfo>

class Address {
public:
    virtual ~Address() = default;
    virtual Address* clone() const { return new Address(*this); }
    virtual void describe() const { std::cout << "Address door=" << door_number << "\n"; }
    std::string door_number;
};

class ExAddress : public Address {
public:
    std::string extra;
    ExAddress* clone() const override { return new ExAddress(*this); }
    void describe() const override {
        std::cout << "ExAddress door=" << door_number << " extra=" << extra << "\n";
    }
};

int main() {
    ExAddress ex;
    ex.door_number = "A501";
    ex.extra       = "near-cafeteria";

    Address* proto = &ex;                     // 静态类型 Address*,动态 ExAddress
    Address* cloned = proto->clone();         // 虚派发 -> ExAddress::clone

    std::cout << "[clone] dynamic type = " << typeid(*cloned).name() << "\n";
    cloned->describe();                        // 走的也是 ExAddress::describe
    delete cloned;
    return 0;
}

Build and Run:

$ g++ -std=c++23 -O2 -Wall -Wextra prototype_verify.cpp -o prototype_verify
$ ./prototype_verify
[clone] dynamic type = 9ExAddress
[proto] dynamic type = 9ExAddress

The dynamic type of the copied object is 9ExAddress—the derived part is fully preserved, and extra came along with it. This is the core value of a polymorphic clone() compared to "direct copy construction": it makes the copy behavior follow the dynamic type, thereby faithfully reproducing the correct derived type without modifying any caller code.

That Return Type: Why I Recommend You Write std::unique_ptr<Base>

Until now, our clone() has returned a raw pointer Address*. While this is the most intuitive approach for teaching purposes, it offloads the burden of "who calls delete" onto the caller—the caller receives an Address* and must remember to delete it themselves. Forgetting to do so results in a memory leak, while deleting it too early leads to a dangling pointer.

A more idiomatic approach in modern C++ is to have clone() directly return a smart pointer that owns the resource:

class Widget {
public:
    virtual ~Widget() = default;
    virtual std::unique_ptr<Widget> clone() const = 0;   // 纯虚,强制子类实现
    virtual void draw() const = 0;
};

Implementation of the subclass:

class Button : public Widget {
public:
    std::string label;

    std::unique_ptr<Widget> clone() const override {
        return std::make_unique<Button>(*this);   // 拷贝构造一个 Button,包进 unique_ptr
    }

    void draw() const override {
        std::cout << "Button: " << label << "\n";
    }
};

Here is a detail worth noting: the subclass clone() returns a std::unique_ptr<Widget>, but the function body uses std::make_unique<Button>(...) which creates a std::unique_ptr<Button>. There is an implicit conversion happening here. This works because Button is a derived class of Widget, and std::unique_ptr allows for the conversion from a derived class pointer to a base class pointer—it has a non-throwing implicit constructor. We will verify this together in a moment.

You might ask: can we let the subclass return std::unique_ptr<Button> and use covariant return types? There is a historical limitation in C++ here: std::unique_ptr<Derived> is not a derived type of std::unique_ptr<Base>. They are independent, parallel types instantiated from different templates, so smart pointers do not support covariant return types. In other words, for a virtual function returning a unique_ptr, the base class and the derived class must have exactly the same return type (here, std::unique_ptr<Widget>), relying on the implicit conversion within the function body to "recover" the specific type. This is a small price to pay for the safety of ownership semantics, and it is worth it.

Let's verify both the "unique_ptr version of clone" and that implicit conversion together:

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#include <iostream>
#include <memory>
#include <typeinfo>

class Base {
public:
    virtual ~Base() = default;
    virtual std::unique_ptr<Base> clone() const = 0;
};

class Derived : public Base {
public:
    std::unique_ptr<Base> clone() const override {
        return std::make_unique<Derived>(*this);   // unique_ptr<Derived> -> unique_ptr<Base>
    }
};

int main() {
    std::unique_ptr<Base> proto = std::make_unique<Derived>();   // 同样的隐式转换
    auto cloned = proto->clone();
    std::cout << "[unique_clone] dynamic type = " << typeid(*cloned).name() << "\n";
    return 0;
}

$ g++ -std=c++23 -O2 -Wall -Wextra prototype_verify.cpp -o prototype_verify
$ ./prototype_verify
[unique_clone] dynamic type = 7Derived

The actual type is 7Derived, which shows that the implicit conversion did not lose type information, and virtual dispatch works as expected. The conclusion is: implement clone() to return a std::unique_ptr<Base>, let ownership follow the return value, and the caller doesn't have to write a single delete.

The Real Pitfall Lies Ahead: clone Is Not Just Mindless new Derived(*this)

At this point, the framework for a polymorphic clone() is clear. But there is a pitfall that many introductory materials gloss over, yet it bites back hardest in practice—does clone() perform a shallow copy or a deep copy?

new Derived(*this) invokes the copy constructor of Derived. The default copy constructor synthesized by the compiler performs a "member-wise copy" for each member: value-type members (like std::string, std::vector) invoke their own copy constructors (usually deep copies), but pointer members only copy the address value, not the data they point to. This means that if your class contains raw pointers, or members like std::shared_ptr where "copying implies sharing," the two objects produced by the default copy constructor will share underlying resources—you think clone() produced an independent copy, but they are actually holding hands behind the scenes.

Let's first verify how the default copy constructor behaves with shared members:

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#include <iostream>
#include <memory>

class SharedBuffer {
public:
    explicit SharedBuffer(int v) : data_(std::make_shared<int>(v)) {}
    // 默认合成拷贝构造:shared_ptr 拷贝 -> 引用计数 +1,两个对象指向同一块
    int  get() const { return *data_; }
    void set(int v)  { *data_ = v; }
private:
    std::shared_ptr<int> data_;
};

int main() {
    SharedBuffer a(10);
    SharedBuffer b = a;          // 默认浅拷(共享底层 int)
    a.set(999);
    std::cout << "[shared shallow] b.get() = " << b.get()
              << " (follows a's mutation)\n";
    return 0;
}

$ g++ -std=c++23 -O2 -Wall -Wextra prototype_verify.cpp -o prototype_verify
$ ./prototype_verify
[shared shallow] b.get() = 999 (follows a's mutation)

The result is straightforward: we only modified a, but b changed as well—because they both point to the same underlying int. If your prototype's intent is to "provide an independent copy where modifying the original does not affect the copy," this default behavior is a bug.

Even more dangerous are raw pointers. If a member is int* data_ instead of shared_ptr<int>, the default copy constructor will cause two objects to point to the same heap memory. When they destruct, each will call delete, resulting in a double free and immediate undefined behavior (UB).

Therefore, before writing clone(), you must clarify the copy semantics for every member in the class: which are value semantics (follow the default), which are shared semantics (e.g., large read-only buffers intentionally shared, so use shared_ptr and accept sharing), and which are ownership semantics (exclusive resources, so copies must be deep copies, otherwise copying should be prohibited). The implementation of clone() must clearly enforce these semantics, rather than just mindlessly writing new Derived(*this).

This pitfall is more common than you think

Many tutorial clone() example classes only contain a few std::string fields—so-called "clean" classes where the default copy constructor happens to be correct. This creates the illusion that "clone is just a one-liner." In real-world scenarios, once your class holds a std::unique_ptr (non-copyable, the implicitly generated copy constructor is deleted, so your clone() won't compile), or holds raw pointers / shared_ptr, you must hand-write the copy constructor and the deep copy logic for clone(), or explicitly adopt shared semantics. Don't just copy examples; first ask yourself: after copying this class, should the two objects share underlying resources?

Managing Prototypes: The Prototype Registry

At this point, we have objects that can clone correctly. However, there is still an engineering challenge: prototypes themselves are often "expensive to create" objects—you don't want to recreate a prototype every time you need to use it. A natural approach is to store commonly used prototypes centrally and retrieve them by name or ID when needed, then clone() a copy. This is the Prototype Registry.

It looks very similar to the Factory pattern, but the difference is: a factory "creates a new object based on parameters," while a registry "copies from a pre-stored template." The registry stores prototypes internally and exposes a "clone by name" interface:

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#include <iostream>
#include <memory>
#include <string>
#include <unordered_map>

class Widget {
public:
    virtual ~Widget() = default;
    virtual std::unique_ptr<Widget> clone() const = 0;
    virtual void draw() const = 0;
};

class Button : public Widget {
public:
    std::string label;
    explicit Button(std::string l) : label(std::move(l)) {}
    std::unique_ptr<Widget> clone() const override {
        return std::make_unique<Button>(*this);
    }
    void draw() const override { std::cout << "Button: " << label << "\n"; }
};

class TextField : public Widget {
public:
    std::string placeholder;
    explicit TextField(std::string p) : placeholder(std::move(p)) {}
    std::unique_ptr<Widget> clone() const override {
        return std::make_unique<TextField>(*this);
    }
    void draw() const override { std::cout << "TextField: " << placeholder << "\n"; }
};

class WidgetRegistry {
public:
    void register_proto(const std::string& name, std::unique_ptr<Widget> proto) {
        protos_[name] = std::move(proto);
    }

    std::unique_ptr<Widget> create(const std::string& name) const {
        auto it = protos_.find(name);
        if (it == protos_.end()) return nullptr;     // 没注册 -> 返回空,调用方自己处理
        return it->second->clone();                  // 从模板克隆一份
    }

private:
    std::unordered_map<std::string, std::unique_ptr<Widget>> protos_;
};

Here is how we use it: first, we register a few "carefully tuned" prototypes, and afterwards, we can retrieve them by name at any time, always getting an independent copy:

int main() {
    WidgetRegistry registry;

    // 注册原型:这些原型可以预先做昂贵的初始化
    registry.register_proto("ok_button",
        std::make_unique<Button>("OK"));
    registry.register_proto("cancel_button",
        std::make_unique<Button>("Cancel"));
    registry.register_proto("name_field",
        std::make_unique<TextField>("enter your name"));

    // 按名字克隆,改了克隆体不影响原型,也不影响其他克隆体
    auto b1 = registry.create("ok_button");
    auto b2 = registry.create("ok_button");
    auto f1 = registry.create("name_field");

    if (b1) b1->draw();    // Button: OK
    if (b2) b2->draw();    // Button: OK  (独立副本,改 b1 不影响 b2)
    if (f1) f1->draw();    // TextField: enter your name
}

This structure represents the prototype of a configuration-based creation system—prototypes in the registry can be loaded from configuration files or scripts, and the entire set of objects is assembled at runtime by name. Monster spawning in games, UI theme switching, and "copy-paste" in document editors often rely on this mechanism behind the scenes. The cost is that you must invest effort in managing the registration and lifecycle of the prototypes; if the prototypes themselves are mutable and the registry is accessed across threads, you will need additional locking to ensure consistency. However, for scenarios where "objects need to be created in batches based on templates," this maintenance cost is worthwhile.

Summary

Let's review the entire evolutionary path:

Stage	Approach	Why it falls short
Direct prototype copy	Address other = proto;	Works for trivial classes, but causes slicing with inheritance
Utility function for fixed base copy	new Address(*a)	Slicing: loses derived parts and dynamic type
Polymorphic clone()	Base declares virtual clone(), derived classes override	Supports the inheritance hierarchy (basically sufficient)
clone() returns unique_ptr	Returns std::unique_ptr<Base>	Solves ownership, caller is free from delete
Prototype Registry	Registry clone() by name	Configuration-based batch creation, but requires managing registration/lifecycle

Keep these key conclusions in mind:

• The essence of the Prototype pattern is "using copy instead of construction". Its simplest form is just a copy constructor line; you only need to upgrade to a polymorphic clone() when dealing with inheritance hierarchies or resource management.
• Slicing is the primary killer of copying in polymorphic scenarios—new Base(*derived_ptr) silently discards the derived portion. The cure is to make cloning a virtual function, letting the dynamic type decide what to copy.
• clone() returning std::unique_ptr<Base> is the standard approach in modern C++. Covariant return types only work for raw pointers or references; smart pointers do not support covariance, so the base and derived classes must write the same return type, relying on implicit conversion to recover the specific type.
• clone() is not a mindless new Derived(*this)—first clarify the copy semantics of every member (value / shared / ownership). The default copy constructor performs a shallow copy for pointers, and classes containing unique_ptr are not copyable by default.
• When you need to create objects in batches based on templates, layering a Prototype Registry to index prototypes by name and clone() on demand is a common skeleton for configuration-based creation.

Accompanying Compilable Project

The examples in this section have a complete compilable project (.h + main + CMakeLists.txt) in the repository at code/volumn_codes/vol4/design-patterns/Prototype/. Run cmake -S . -B build && cmake --build build to reproduce the outputs shown above.

References

• cppreference: Virtual functions (Covariant return type) (Since C++98, virtual functions return covariant pointers/references)
• cppreference: Copy constructors (Default copy constructor member-wise copy semantics)
• cppreference: std::unique_ptr (Implicit conversion from derived to base, why covariance isn't possible)
• GoF, Design Patterns: Elements of Reusable Object-Oriented Software — Chapter on Prototype
• Dmitri Nesteruk, Hands-On Design Patterns with C++ and .NET Core — Prototype pattern chapter (Polymorphic clone and registry practices)