Lære Why Copying Isn’t Always Efficient | Introduction to Move Semantics

Copying objects in C++ can be expensive, especially when dealing with large resources such as dynamically allocated memory or containers. Each copy operation may involve allocating new memory and duplicating the contents, which can significantly impact performance.

main.cpp


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#include <iostream>
#include <cstring>

class LargeBuffer {
public:
    LargeBuffer(size_t size)
        : size_(size), data_(new char[size]) {
        std::cout << "Constructed LargeBuffer of size " << size_ << std::endl;
        std::memset(data_, 0, size_);
    }

    // Expensive copy constructor
    LargeBuffer(const LargeBuffer& other)
        : size_(other.size_), data_(new char[other.size_]) {
        std::cout << "Copied LargeBuffer of size " << size_ << std::endl;
        std::memcpy(data_, other.data_, size_);
    }

    // Destructor
    ~LargeBuffer() {
        std::cout << "Destroyed LargeBuffer of size " << size_ << std::endl;
        delete[] data_;
    }

    size_t size() const { return size_; }

private:
    size_t size_;
    char* data_;
};

void processBuffer(LargeBuffer buf)
{
    std::cout << "Processing buffer of size " << buf.size() << std::endl;
}

int main()
{
    LargeBuffer buf1(1024 * 1024); // 1 MB buffer
    processBuffer(buf1); // Copies buf1 into the function
}

In the example above, notice how copying the object leads to additional memory allocations and data duplication. This inefficiency becomes more pronounced as the size of the resource grows.

Copying is necessary when you need two independent objects with the same state. For example, when you want to preserve the original while modifying the copy.

Move semantics allow you to transfer resources instead of duplicating them, which is especially useful for temporary objects or when ownership can be transferred.

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