Impara Using std::move | Implementing Move Operations

The utility std::move is an essential tool in modern C++ that enables move semantics. It is a standard library function that casts an lvalue (an object with a name and address) to an rvalue reference. By doing so, it allows move constructors or move assignment operators to be invoked, rather than their copying counterparts. However, it's important to understand that std::move does not move anything by itself—it simply makes it possible for move operations to occur.

main.cpp


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#include <iostream>
#include <utility>
#include <string>

class Buffer {
public:
    Buffer(size_t size) : size_(size), data_(new char[size]) {
        std::cout << "Constructed Buffer of size " << size_ << "\n";
    }

    // Move constructor
    Buffer(Buffer&& other) noexcept
        : size_(other.size_), data_(other.data_) {
        other.data_ = nullptr;
        other.size_ = 0;
        std::cout << "Moved Buffer\n";
    }

    // Move assignment operator
    Buffer& operator=(Buffer&& other) noexcept {
        if (this != &other) {
            delete[] data_;
            data_ = other.data_;
            size_ = other.size_;
            other.data_ = nullptr;
            other.size_ = 0;
            std::cout << "Move-assigned Buffer\n";
        }
        return *this;
    }

    // Destructor
    ~Buffer() {
        delete[] data_;
        std::cout << "Destroyed Buffer\n";
    }

    // Deleted copy constructor and assignment
    Buffer(const Buffer&) = delete;
    Buffer& operator=(const Buffer&) = delete;

    void print() const
    {
        std::cout << "Buffer at " << static_cast<void*>(data_)
                  << " size " << size_ << "\n";
    }

private:
    size_t size_;
    char* data_;
};

int main()
{
    Buffer a(10);
    Buffer b(20);

    std::cout << "Before move:\n";
    a.print();
    b.print();

    // Move assignment: b takes resources from a
    b = std::move(a);

    std::cout << "After move:\n";
    a.print();
    b.print();
}

Use std::move when you no longer need the original object and want to transfer its resources. Do not use it on objects you intend to use afterward.

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Sezione 2. Capitolo 3

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main.cpp


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#include <iostream>
#include <utility>
#include <string>

class Buffer {
public:
    Buffer(size_t size) : size_(size), data_(new char[size]) {
        std::cout << "Constructed Buffer of size " << size_ << "\n";
    }

    // Move constructor
    Buffer(Buffer&& other) noexcept
        : size_(other.size_), data_(other.data_) {
        other.data_ = nullptr;
        other.size_ = 0;
        std::cout << "Moved Buffer\n";
    }

    // Move assignment operator
    Buffer& operator=(Buffer&& other) noexcept {
        if (this != &other) {
            delete[] data_;
            data_ = other.data_;
            size_ = other.size_;
            other.data_ = nullptr;
            other.size_ = 0;
            std::cout << "Move-assigned Buffer\n";
        }
        return *this;
    }

    // Destructor
    ~Buffer() {
        delete[] data_;
        std::cout << "Destroyed Buffer\n";
    }

    // Deleted copy constructor and assignment
    Buffer(const Buffer&) = delete;
    Buffer& operator=(const Buffer&) = delete;

    void print() const
    {
        std::cout << "Buffer at " << static_cast<void*>(data_)
                  << " size " << size_ << "\n";
    }

private:
    size_t size_;
    char* data_;
};

int main()
{
    Buffer a(10);
    Buffer b(20);

    std::cout << "Before move:\n";
    a.print();
    b.print();

    // Move assignment: b takes resources from a
    b = std::move(a);

    std::cout << "After move:\n";
    a.print();
    b.print();
}

Use std::move when you no longer need the original object and want to transfer its resources. Do not use it on objects you intend to use afterward.

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Grazie per i tuoi commenti!

Sezione 2. Capitolo 3