C++ condition_variable::wait() Function

The std::condition_variable::wait() function in C++, is used to block the thread until a specified condition is met. It is used in conjunction with the std::mutex and a std::unique_lock, ensuring the thread-safe execution. When the condition becomes true, the function reacquires the lock and resumes the execution.

Syntax

Following is the syntax for std::condition_variable::wait() function.

void notify_all() noexcept;void wait (unique_lock<mutex>& lck);
or
void wait (unique_lock<mutex>& lck, Predicate pred);

Parameters

• lck − It indicates a unique_lock object whose mutex object is currently locked by this thread.
• pred − It indicates a callable object or function that takes no arguments and returns a value that can be evaluated as a bool.

Return value

This function does not return anything.

Example 1

Let's look at the following example, where we are going to coordinates two threads.

#include <iostream>
#include <thread>
#include <condition_variable>
#include <mutex>
std::mutex a;
std::condition_variable b;
bool turn = false;
void x1() {
   std::unique_lock < std::mutex > lock(a);
   std::cout << "x1 waiting for its turn....\n";
   b.wait(lock, [] {
      return turn;
   });
   std::cout << "x1 proceeding...\n";
}
void x2() {
   std::this_thread::sleep_for(std::chrono::seconds(1));
   {
      std::lock_guard < std::mutex > lock(a);
      turn = true;
      std::cout << "x2 signals Thread 1's turn.\n";
   }
   b.notify_one();
}
int main() {
   std::thread a1(x1);
   std::thread a2(x2);
   a1.join();
   a2.join();
   return 0;
}

Output

Output of the above code is as follows −

x1 waiting for its turn....
x2 signals Thread 1's turn.
x1 proceeding...