C++ Move Semantics

lvalues and rvalues

An lvalue has a name and a persistent memory address. An rvalue is a temporary value with no address.

int x = 42;      // x is lvalue; 42 is rvalue
string s = "hi"; // s is lvalue; "hi" is rvalue
string t = s;    // copy: expensive for large strings

std::move and Move Constructor

std::move transfers ownership of resources instead of copying them — no memory allocation, much faster for large objects:

#include <iostream>
#include <vector>
#include <string>
using namespace std;

class Buffer {
  public:
    vector<int> data;
    Buffer(int size) : data(size, 0) {
        cout << "Constructed, size=" << data.size() << endl;
    }
    // Move constructor
    Buffer(Buffer&& other) noexcept : data(move(other.data)) {
        cout << "Moved!" << endl;
    }
};

int main() {
    Buffer b1(1000000);
    Buffer b2(move(b1));   // move, not copy
    cout << "b1 size: " << b1.data.size() << endl;   // 0 (stolen)
    cout << "b2 size: " << b2.data.size() << endl;   // 1000000
    return 0;
}

Output:

Constructed, size=1000000
Moved!
b1 size: 0
b2 size: 1000000

Rvalue References

void process(string&& s) {   // rvalue reference parameter
    cout << "Moving: " << s << endl;
}

process("temporary");       // OK — rvalue
string name = "Alice";
process(move(name));        // cast lvalue to rvalue

#include <iostream>
#include <vector>
#include <string>
using namespace std;

int main() {
    vector<string> words = {"alpha","beta","gamma","delta","epsilon"};
    string moved = move(words[2]);
    cout << "Moved: "    << moved    << endl;   // gamma
    cout << "Original: " << words[2] << endl;   // (empty)
    return 0;
}

Summary

• Move semantics transfer resource ownership instead of copying.
• T&& is an rvalue reference; std::move() casts to one.
• Move constructors and move assignment operators avoid expensive deep copies.
• Follow the Rule of Five when managing resources.