Copy vs Move in C++: Key Differences and When to Use Each

In C++, copy duplicates an object's data creating a new independent object, while move transfers ownership of resources from one object to another without copying. Move operations are faster because they avoid expensive data duplication by reusing existing resources.

Quick Comparison

This table summarizes the main differences between copy and move operations in C++.

Aspect	Copy	Move
Operation	Duplicates all data to create a new object	Transfers ownership of resources without duplication
Performance	Slower due to deep copying	Faster by reusing resources
Source Object State	Source remains unchanged	Source is left in a valid but unspecified state
Use Case	When you need two independent objects	When you want to avoid expensive copies
Syntax	Copy constructor or copy assignment operator	Move constructor or move assignment operator
Resource Handling	Allocates new resources	Steals existing resources

Key Differences

Copy operations create a new object by duplicating the data from an existing object. This means all the resources like memory or file handles are separately allocated for the new object, making it independent from the original. Because of this duplication, copying can be expensive in terms of performance, especially for large or complex objects.

Move operations, introduced in C++11, allow the program to transfer ownership of resources from one object to another without copying the underlying data. Instead of duplicating, the move operation 'steals' the resource pointers from the source object and leaves it in a valid but unspecified state. This makes moves much faster than copies, especially for objects managing dynamic memory or system resources.

In code, copy constructors and copy assignment operators handle copying, while move constructors and move assignment operators handle moving. Using move semantics can greatly improve performance by avoiding unnecessary deep copies.

Code Comparison

Here is an example showing how copying works with a simple class managing a dynamic array.

cpp

#include <iostream>
class Buffer {
public:
    int* data;
    int size;

    Buffer(int s) : size(s), data(new int[s]) {}

    // Copy constructor
    Buffer(const Buffer& other) : size(other.size), data(new int[other.size]) {
        for (int i = 0; i < size; ++i) {
            data[i] = other.data[i];
        }
    }

    ~Buffer() { delete[] data; }
};

int main() {
    Buffer buf1(3);
    buf1.data[0] = 1; buf1.data[1] = 2; buf1.data[2] = 3;

    Buffer buf2 = buf1; // Copy

    std::cout << "buf2 data: ";
    for (int i = 0; i < buf2.size; ++i) {
        std::cout << buf2.data[i] << " ";
    }
    std::cout << std::endl;
    return 0;
}

Output

buf2 data: 1 2 3 

Move Equivalent

Now the same class with move constructor to transfer ownership instead of copying.

cpp

#include <iostream>
class Buffer {
public:
    int* data;
    int size;

    Buffer(int s) : size(s), data(new int[s]) {}

    // Move constructor
    Buffer(Buffer&& other) noexcept : size(other.size), data(other.data) {
        other.size = 0;
        other.data = nullptr;
    }

    ~Buffer() { delete[] data; }
};

int main() {
    Buffer buf1(3);
    buf1.data[0] = 1; buf1.data[1] = 2; buf1.data[2] = 3;

    Buffer buf2 = std::move(buf1); // Move

    std::cout << "buf2 data: ";
    for (int i = 0; i < buf2.size; ++i) {
        std::cout << buf2.data[i] << " ";
    }
    std::cout << std::endl;
    return 0;
}

Output

buf2 data: 1 2 3 

When to Use Which

Choose copy when you need a completely independent object with its own resources, such as when you want to keep the original object unchanged and use the duplicate separately. Copying is safer but can be slower for large or complex objects.

Choose move when you want to transfer ownership of resources efficiently, especially for temporary objects or when the original object is no longer needed. Moving avoids expensive copying and improves performance by reusing existing resources.

In modern C++, prefer move semantics whenever possible to optimize resource management and speed, but ensure the source object is not used after being moved from except to assign or destroy it.

Key Takeaways

Copy creates a full duplicate of an object’s data, while move transfers ownership without copying.

Move operations are faster and more efficient, especially for objects managing dynamic resources.

Use copy when you need independent objects; use move to optimize performance with temporary or expendable objects.

Move leaves the source object in a valid but unspecified state, so avoid using it afterward except to assign or destroy.

Modern C++ encourages move semantics to improve speed and resource management.