Receive a Background Result
Receive a Background Result
A future represents a result that may become available later.
Std::async with an explicit async launch policy runs a computation and get retrieves its value or propagates its exception.
Before this lesson, review Protect Shared Changes when needed.
What you’ll learn
- Use async and futures to receive a Background Result.
- Explain how the complete example produces 75.
- Make the practice change and explain why the result changes or stays the same.
Follow a complete C++ example
Std::async with an explicit async launch policy runs a computation and get retrieves its value or propagates its exception.
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 <future>
int main() {
auto result = std::async(std::launch::async, [] {
return 35 + 40;
}
);
std::cout << result.get() << '\n';
return 0;
}
Expected output
75Connect the code to its result
C++17 · source used in this explanation
#include <iostream>
#include <future>
int main() {
auto result = std::async(std::launch::async, [] {
return 35 + 40;
}
);
std::cout << result.get() << '\n';
return 0;
}
Expected output
75A future represents a result that may become available later. Std::async with an explicit async launch policy runs a computation and get retrieves its value or propagates its exception.
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 75. In this example, distinguish what the program calculates or changes from what it actually displays.
Important mistakes and boundaries
Scheduling order is not guaranteed. Future get consumes its result for an ordinary future; shared_future has different access behavior.
Change the school example
C++17 · edit this complete example
#include <iostream>
#include <future>
int main() {
auto result = std::async(std::launch::async, [] {
return 35 + 40;
}
);
std::cout << result.get() << '\n';
return 0;
}
Your task: Return 40 + 45 from the background calculation. 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: Return 40 + 45 from the background calculation. The retrieved total is 85.
A future represents a result that may become available later. Std::async with an explicit async launch policy runs a computation and get retrieves its value or propagates its exception.
Scheduling order is not guaranteed. Future get consumes its result for an ordinary future; shared_future has different access behavior.
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 <future>
int main() {
auto result = std::async(std::launch::async, [] {
return 40 + 45;
}
);
std::cout << result.get() << '\n';
return 0;
}
Expected output
85Check your understanding
C++17 · original program for this question
#include <iostream>
#include <future>
int main() {
auto result = std::async(std::launch::async, [] {
return 35 + 40;
}
);
std::cout << result.get() << '\n';
return 0;
}
What to remember
A future represents a result that may become available later. Scheduling order is not guaranteed. Future get consumes its result for an ordinary future; shared_future has different access behavior.
Useful connections: Protect Shared Changes, 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.