Distinguish Copying from Transfer
Distinguish Copying from Transfer
Copying a standard container creates its own elements.
Moving enables transfer of resources, but the moved-from object’s exact value is generally not promised unless its type specifies it.
Before this lesson, review Share and Observe Ownership when needed.
What you’ll learn
- Use copy and move to distinguish Copying from Transfer.
- Explain how the complete example produces 70 → 90.
- Make the practice change and explain why the result changes or stays the same.
Follow a complete C++ example
Moving enables transfer of resources, but the moved-from object’s exact value is generally not promised unless its type specifies it.
Read the declarations above main before tracing its statements. Each declared name supplies the value, type or operation used later. The std:: prefix selects a name from the standard library, and cout << sends a value to the output stream.
C++17 · main.cpp · complete program
#include <iostream>
#include <vector>
#include <utility>
int main() {
std::vector<int> marks {
70, 80
}
;
auto copy = marks;
copy.at(0) = 90;
auto moved = std::move(copy);
std::cout << marks.at(0) << '\n' << moved.at(0) << '\n';
return 0;
}
Expected output
70
90Connect the code to its result
C++17 · source used in this explanation
#include <iostream>
#include <vector>
#include <utility>
int main() {
std::vector<int> marks {
70, 80
}
;
auto copy = marks;
copy.at(0) = 90;
auto moved = std::move(copy);
std::cout << marks.at(0) << '\n' << moved.at(0) << '\n';
return 0;
}
Expected output
70
90Copying a standard container creates its own elements. Moving enables transfer of resources, but the moved-from object’s exact value is generally not promised unless its type specifies it.
Trace the result: Read the complete program below from its starting values to the output operations. Each std::cout statement displays the value supplied at that point; \n begins the next line. The resulting lines, in order, are 70 → 90. In this example, distinguish what the program calculates or changes from what it actually displays.
Distinguish Copying from Transfer: cause and effect
Operation
Copying a standard container creates its own elements.
Effect on school data
Moving enables transfer of resources, but the moved-from object’s exact value is generally not promised unless its type specifies it.
Observed result
The program displays these lines after the shown operations.
70 90
Important mistakes and boundaries
Do not assume every moved-from vector must be empty. It remains valid for permitted operations such as destruction or reassignment.
Change the school example
C++17 · edit this complete example
#include <iostream>
#include <vector>
#include <utility>
int main() {
std::vector<int> marks {
70, 80
}
;
auto copy = marks;
copy.at(0) = 90;
auto moved = std::move(copy);
std::cout << marks.at(0) << '\n' << moved.at(0) << '\n';
return 0;
}
Your task: Change the copied first mark to 95. Before running your edited program, predict the new output and explain which stored value, condition or object changes. Keep the rest of the example intact.
Use the complete source repeated here; this task does not depend on remembering code from another section.
Worked answer and reasoning
Worked variation: Change the copied first mark to 95. The original stays 70 and moved.at(0) is 95.
Copying a standard container creates its own elements. Moving enables transfer of resources, but the moved-from object’s exact value is generally not promised unless its type specifies it.
Do not assume every moved-from vector must be empty. It remains valid for permitted operations such as destruction or reassignment.
For a baseline run, save this source as main.cpp and use clang++ -std=c++17 -Wall -Wextra -pedantic main.cpp -o school, or equivalent g++ flags. Recompile after editing so that you run the updated executable.
The source and output below belong to the edited program. Compare them with the original example to see the consequence of this single change.
C++17 · worked variation · complete program
#include <iostream>
#include <vector>
#include <utility>
int main() {
std::vector<int> marks {
70, 80
}
;
auto copy = marks;
copy.at(0) = 95;
auto moved = std::move(copy);
std::cout << marks.at(0) << '\n' << moved.at(0) << '\n';
return 0;
}
Expected output
70
95Check your understanding
C++17 · original program for this question
#include <iostream>
#include <vector>
#include <utility>
int main() {
std::vector<int> marks {
70, 80
}
;
auto copy = marks;
copy.at(0) = 90;
auto moved = std::move(copy);
std::cout << marks.at(0) << '\n' << moved.at(0) << '\n';
return 0;
}
Check a common mistake
What to remember
Copying a standard container creates its own elements. Do not assume every moved-from vector must be empty. It remains valid for permitted operations such as destruction or reassignment.
Useful connections: Share and Observe Ownership, Give a Task a Name.
Language and design guidance: C++ Core Guidelines and C++ working draft. The draft describes an evolving language; these examples deliberately target C++17.
Reaching the recap and completing the quizzes are separate milestones.