Value Categories and References

Move Reference Transfer

std::move turns a named object into an rvalue expression so ownership can be transferred.

move expression `std::move` does not move by itself; it marks an object as movable so a move constructor or move assignment can use it.
moved-from owner A moved-from `std::unique_ptr` becomes empty after its ownership is transferred.

Move Reference Transfer

amount
move_reference_transfer.cpp
Replay: real traced execution (multi-file project)
#include <iostream>
#include <memory>

int main() {
    int amount = 12;

    std::unique_ptr<int> source = std::make_unique<int>(amount);
    std::unique_ptr<int> target = std::move(source);

    std::cout << "sourceEmpty=" << (source == nullptr) << std::endl;
    std::cout << "targetValue=" << *target << std::endl;
    return 0;
}

#include <iostream>
#include <memory>

int main() {
    int amount = 5;

    std::unique_ptr<int> source = std::make_unique<int>(amount);
    std::unique_ptr<int> target = std::move(source);

    std::cout << "sourceEmpty=" << (source == nullptr) << std::endl;
    std::cout << "targetValue=" << *target << std::endl;
    return 0;
}

#include <iostream>
#include <memory>

int main() {
    int amount = 30;

    std::unique_ptr<int> source = std::make_unique<int>(amount);
    std::unique_ptr<int> target = std::move(source);

    std::cout << "sourceEmpty=" << (source == nullptr) << std::endl;
    std::cout << "targetValue=" << *target << std::endl;
    return 0;
}

  1. amount ← 12, source ← (empty), target ← (empty)

    4int main() {5    int amount→ 12 = 12; //@amount=5, 3067    std::unique_ptr<int> source→ (empty) = std::make_unique<int>(amount12);8    std::unique_ptr<int> target→ (empty) = std::move(source(empty));910    std::cout << "sourceEmpty=" << (source(empty) == nullptr) << std::endl;11    std::cout << "targetValue=" << *target(empty) << std::endl;12    return 0;13}
    outputsourceEmpty=1
targetValue=12

  1. amount ← 5, source ← (empty), target ← (empty)

    4int main() {5    int amount→ 5 = 5;67    std::unique_ptr<int> source→ (empty) = std::make_unique<int>(amount5);8    std::unique_ptr<int> target→ (empty) = std::move(source(empty));910    std::cout << "sourceEmpty=" << (source(empty) == nullptr) << std::endl;11    std::cout << "targetValue=" << *target(empty) << std::endl;12    return 0;13}
    outputsourceEmpty=1
targetValue=5

  1. amount ← 30, source ← (empty), target ← (empty)

    4int main() {5    int amount→ 30 = 30;67    std::unique_ptr<int> source→ (empty) = std::make_unique<int>(amount30);8    std::unique_ptr<int> target→ (empty) = std::move(source(empty));910    std::cout << "sourceEmpty=" << (source(empty) == nullptr) << std::endl;11    std::cout << "targetValue=" << *target(empty) << std::endl;12    return 0;13}
    outputsourceEmpty=1
targetValue=30