std::unique_ptr: Exclusive Ownership

Learn how std::unique_ptr provides zero-overhead exclusive ownership, how to use make_unique, custom deleters for C resources, and factory function patterns.

What Is unique_ptr?

std::unique_ptr is a smart pointer that owns an object exclusively — there is exactly one unique_ptr pointing to any given object. When the unique_ptr is destroyed (goes out of scope), it automatically deletes the owned object.

Key properties:
- Non-copyable — copying would create two owners, violating exclusive ownership.
- Moveable — ownership can be transferred via std::move.
- Zero overhead — sizeof(unique_ptr) == sizeof(T*) (with default deleter). The generated code is identical to raw pointer usage with manual delete.
- This is the default smart pointer. Reach for unique_ptr first; only use shared_ptr when you genuinely need shared ownership.

make_unique: The Only Correct Way

std::make_unique(args...) (C++14) is the only correct way to create a unique_ptr. It is exception-safe, concise, and avoids writing new.

unique_ptr_basics.cpp

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

struct Connection {
    std::string host;
    int port;
    Connection(std::string h, int p) : host(std::move(h)), port(p) {
        std::cout << "Connected to " << host << ":" << port << '\n';
    }
    ~Connection() {
        std::cout << "Disconnected from " << host << '\n';
    }
    void query(const std::string& sql) {
        std::cout << "Executing: " << sql << '\n';
    }
};

int main() {
    // CORRECT: use make_unique
    auto conn = std::make_unique<Connection>("localhost", 5432);
    conn->query("SELECT * FROM users");

    // Access the raw pointer (non-owning)
    Connection* raw = conn.get();  // does NOT transfer ownership

    // Transfer ownership
    auto conn2 = std::move(conn);
    // conn is now nullptr
    if (!conn) std::cout << "conn is null after move\n";

    // conn2 destroyed here — destructor called automatically
}  // Output: "Disconnected from localhost"

Custom Deleters for C Resources

Many C libraries return raw pointers that must be freed with a specific function (not delete). unique_ptr supports custom deleters to handle these. The deleter becomes part of the type, so it has zero overhead when using a function pointer or stateless lambda.

custom_deleter.cpp

#include <cstdio>
#include <memory>
#include <iostream>

// Custom deleter for FILE*
struct FileDeleter {
    void operator()(FILE* f) const {
        if (f) {
            std::fclose(f);
            std::cout << "File closed\n";
        }
    }
};

// Type alias for clarity
using UniqueFile = std::unique_ptr<FILE, FileDeleter>;

// Factory function
UniqueFile open_file(const char* path, const char* mode) {
    FILE* f = std::fopen(path, mode);
    if (!f) throw std::runtime_error("Cannot open file");
    return UniqueFile(f);
}

int main() {
    try {
        auto file = open_file("/tmp/test.txt", "w");
        std::fputs("Hello, RAII!\n", file.get());
        // file automatically closed when scope exits
    } catch (const std::exception& e) {
        std::cerr << e.what() << '\n';
    }

    // Lambda deleter (less common, but useful for one-offs)
    auto deleter = [](int* p) { std::cout << "custom free\n"; delete p; };
    std::unique_ptr<int, decltype(deleter)> p(new int(42), deleter);
}

unique_ptr with Arrays

std::unique_ptr manages a dynamically allocated array and calls delete[] automatically. It provides operator[] instead of operator* and operator->.

``cpp auto arr = std::make_unique(100); // 100 zero-initialized ints arr[0] = 42;``

However, std::vector is almost always preferable — it knows its size, supports iteration, and is just as efficient. Use unique_ptr only when interfacing with C APIs or when you need a non-resizable, non-copyable buffer.

Factory Functions Returning unique_ptr

Returning unique_ptr from factory functions is the idiomatic C++ ownership pattern. The caller receives exclusive ownership. If the caller wants shared ownership, they can convert: std::shared_ptr sp = factory();.

factory_unique_ptr.cpp

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

class Shape {
public:
    virtual ~Shape() = default;
    virtual double area() const = 0;
    virtual std::string name() const = 0;
};

class Circle : public Shape {
    double radius_;
public:
    explicit Circle(double r) : radius_(r) {}
    double area() const override { return 3.14159 * radius_ * radius_; }
    std::string name() const override { return "Circle"; }
};

class Rectangle : public Shape {
    double w_, h_;
public:
    Rectangle(double w, double h) : w_(w), h_(h) {}
    double area() const override { return w_ * h_; }
    std::string name() const override { return "Rectangle"; }
};

// Factory — caller owns the result
std::unique_ptr<Shape> make_shape(const std::string& type, double a, double b = 0) {
    if (type == "circle")    return std::make_unique<Circle>(a);
    if (type == "rectangle") return std::make_unique<Rectangle>(a, b);
    return nullptr;  // unknown shape
}

int main() {
    auto s = make_shape("circle", 5.0);
    if (s) {
        std::cout << s->name() << " area: " << s->area() << '\n';
    }
    // s automatically deleted here
}

Pitfall

Never pass unique_ptr by const reference to a function that only observes the object — pass a raw pointer or reference instead. unique_ptr parameters signal ownership transfer.

- void use(Widget* w) — observes, no ownership
- void sink(std::unique_ptr w) — takes ownership (call with std::move)
- void reseat(std::unique_ptr& w) — may replace the owned object

Never construct two unique_ptr from the same raw pointer — both will call delete, causing double-free. This is why make_unique is preferred: there is no raw pointer to accidentally reuse.

Best Practice

1. Default to unique_ptr — it should be your first choice for heap allocation.
2. Always use make_unique — never write unique_ptr(new T(...)).
3. Return unique_ptr from factories — it implicitly converts to shared_ptr if the caller needs it.
4. Pass by value to transfer ownership — void sink(unique_ptr p). The caller must use std::move.
5. Pass raw pointer/reference to observe — void use(const T& obj) or void use(T* obj).
6. Mark move constructor/assignment noexcept — critical for types stored in containers.

Key Takeaways

unique_ptr provides exclusive ownership with zero overhead vs raw pointers
Always create via std::make_unique(args...) — never use raw new
Non-copyable but moveable — ownership transfers via std::move
Custom deleters enable RAII for C resources (FILE*, sockets, etc.)
Return unique_ptr from factory functions — it converts to shared_ptr if needed
Pass raw pointer or reference when a function only observes, not owns

Quiz — Test Your Knowledge

(15 XP)

1. What is the overhead of `std::unique_ptr<T>` compared to a raw `T*` (with default deleter)?

2. How should you pass a `unique_ptr` to a function that needs to take ownership?

3. Why is `std::make_unique` preferred over `unique_ptr<T>(new T(...))`?

Dynamic Memory: new, delete & Their Dangers

shared_ptr, weak_ptr & Ownership Graphs