Run a Task in Another Thread
Run a Task in Another Thread
A thread provides another execution path.
Join the school worker before reading the mark it writes, so the report observes a completed task rather than racing with it.
Before this lesson, review Build a Complete Student Report when needed.
What you’ll learn
- Use threads to run a Task in Another Thread.
- Explain how the complete example produces 72.
- Make the practice change and explain why the result changes or stays the same.
Follow a complete C++ example
Join the school worker before reading the mark it writes, so the report observes a completed task rather than racing with 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 <thread>
int main() {
int marks = 0;
std::thread worker([&marks] {
marks = 72;
}
);
worker.join();
std::cout << marks << '\n';
return 0;
}
Expected output
72Connect the code to its result
C++17 · source used in this explanation
#include <iostream>
#include <thread>
int main() {
int marks = 0;
std::thread worker([&marks] {
marks = 72;
}
);
worker.join();
std::cout << marks << '\n';
return 0;
}
Expected output
72A thread provides another execution path. Join the school worker before reading the mark it writes, so the report observes a completed task rather than racing with 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 72. In this example, distinguish what the program calculates or changes from what it actually displays.
Important mistakes and boundaries
Do not destroy a joinable std::thread. This example reads only after join; unsynchronized reads during the worker’s write would create a data race.
Change the school example
C++17 · edit this complete example
#include <iostream>
#include <thread>
int main() {
int marks = 0;
std::thread worker([&marks] {
marks = 72;
}
);
worker.join();
std::cout << marks << '\n';
return 0;
}
Your task: Have the worker store 80. 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: Have the worker store 80. After joining, the report prints 80.
A thread provides another execution path. Join the school worker before reading the mark it writes, so the report observes a completed task rather than racing with it.
Do not destroy a joinable std::thread. This example reads only after join; unsynchronized reads during the worker’s write would create a data race.
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 <thread>
int main() {
int marks = 0;
std::thread worker([&marks] {
marks = 80;
}
);
worker.join();
std::cout << marks << '\n';
return 0;
}
Expected output
80Check your understanding
C++17 · original program for this question
#include <iostream>
#include <thread>
int main() {
int marks = 0;
std::thread worker([&marks] {
marks = 72;
}
);
worker.join();
std::cout << marks << '\n';
return 0;
}
What to remember
A thread provides another execution path. Do not destroy a joinable std::thread. This example reads only after join; unsynchronized reads during the worker’s write would create a data race.
Useful connections: Build a Complete Student Report, 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.