Impara Move Assignment Operators | Implementing Move Operations

The move assignment operator transfers resources from an rvalue to an existing object, releasing any resources the target already owns. You use it to efficiently "move" ownership of dynamically allocated memory or other resources from a temporary or expiring object to another, instead of copying. The operator must handle self-assignment and ensure no resource leaks. This is especially important when working with classes that manage their own resources, such as dynamic arrays or file handles.

main.cpp


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#include <iostream>
#include <cstring>

class String {
    char* data;
public:
    // Constructor
    String(const char* str = "") {
        data = new char[std::strlen(str) + 1];
        std::strcpy(data, str);
        std::cout << "Constructed: " << data << "\n";
    }
    // Destructor
    ~String() {
        delete[] data;
        std::cout << "Destroyed\n";
    }
    // Move constructor
    String(String&& other) noexcept : data(other.data) {
        other.data = nullptr;
        std::cout << "Move constructed\n";
    }
    // Move assignment operator
    String& operator=(String&& other) noexcept {
        if (this != &other) { // Self-assignment check
            delete[] data; // Release current resource
            data = other.data; // Transfer ownership
            other.data = nullptr; // Reset source
            std::cout << "Move assigned\n";
        }
        return *this;
    }
    // Print method
    void print() const
    {
        if (data)
            std::cout << "String: " << data << "\n";
        else
            std::cout << "String: (null)\n";
    }
};

int main()
{
    String a("Hello");
    String b("World");
    a = std::move(b); // Move assignment
    a.print();
    b.print();
    a = std::move(a); // Self-assignment test
    a.print();
}

The move assignment operator above first checks for self-assignment, then releases any owned resources before taking ownership from the source object. After moving, the source object's pointer is set to nullptr to avoid double deletion. This ensures that resources are not leaked or deleted twice, and that the target object now owns the resource previously held by the source.

Why check for self-assignment?

Self-assignment can occur in rare cases (such as a = std::move(a)). The check prevents deleting the resource before moving it, which would result in undefined behavior or a crash.

What should you do with the source object?

After moving, set the source's pointers to nullptr or reset its state to avoid double deletion. This ensures safe cleanup and prevents resource management errors.

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Sezione 2. Capitolo 2

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