Wt

std::shared<SqlConnection> connection = ...; // setup sql connection
auto pool = std::make_shared<Wt::Dbo::FixedSqlConnectionPool>(std::move(connection), 2);

auto con1 = pool->getConnection();
auto con2 = pool->getConnection();

// all connections of pool are now used.

auto thread1 = std::thread([pool](){
  std::cout << "[thread1] Requesting connection ..." << std::endl;
  pool->getConnection();
  std::cout << "[thread1] Connection obtained" << std::endl;
});

auto thread2 = std::thread([pool](){
  std::cout << "[thread2] Requesting connection ..." << std::endl;
  pool->getConnection();
  std::cout << "[thread2] Connection obtained" << std::endl;
});

// wait for 1 second to ensure both threads are waiting for a connection to become available
std::this_thread::sleep_for(std::chrono::seconds(1));

// release two connections immediately after each other
std::cout << "Returning con1" << std::endl;
pool->returnConnection(std::move(con1)); // will wake a single thread (e.g. thread1)
std::cout << "Returning con2" << std::endl;
pool->returnConnection(std::move(con2)); // will probably not wake another thread (e.g. thread2) as `impl_->freeList.size()` is probably 2 due to thread1 not having taken yet the freed connection.

std::cout << "Waiting for threads to finish" << std::endl;

thread1.join(); 
thread2.join();

std::cout << "OK. No deadlock occured." << std::endl; // probably never reached.

[thread1] Requesting connection ...
[thread2] Requesting connection ...
Dbo.FixedSqlConnectionPool: no free connections, waiting for connection
Dbo.FixedSqlConnectionPool: no free connections, waiting for connection
Returning con1
Returning con2
Waiting for threads to finish
[thread1] Connection obtained