C ++ 11无法将std :: condition_variable :: wait从'void'转换为'bool'

Question

I'm trying out the examples in 'The C++ Programming Language' 4th edition and in particular, there's a description of how a condition_variable is used. The code snippet is as follows:

class Message { // object to be communicated
    // ...
};
queue<Message> mqueue;    // the queue of messages
condition_variable mcond; // the variable communicating events
mutex mmutex;             // the locking mechanism


void consumer()
{
    while(true) {
        unique_lock<mutex> lck{mmutex};           // acquire mmutex
        while (mcond.wait(lck)) /* do nothing */; // release lck and wait;
                                                  // re-acquire lck upon wakeup
        auto m = mqueue.front();                  // get the message
        mqueue.pop();
        lck.unlock();                             // release lck
        // ... process m ...
    }
}

However compilation fails on the line containing mcond.wait(lck) with:

error: could not convert 'cond.std::condition_variable::wait((* & lck))' from 'void' to 'bool' .

The documentation for wait lists it with a void return type. Is this an error in the book (at least I couldn't find it in the errata)? Has the standard been updated since the book came out (about two years ago)? If so, how should I use wait correctly in this case?

I'm using Lubuntu 14.04 64bit, my gcc version is 4.9.2, I'm compiling it in NetBeans with:

g++ -m64 -pthread -Wextra -c -g -Wall -std=c++11 -MMD -MP -MF "build/Debug/GNU-Linux-x86/main.od" -o build/Debug/GNU-Linux-x86/main.o main.cpp

EDIT: I've just realised that this has been already spotted in the errata for the previous editions of the book (which I neglected to check prior to posting this question, consulting only the 4th edition one). Still I hope it is of use to anyone who comes across it.

Answer 1

If you wait with a timeout (relative using wait_for() or absolute using wait_until() ), then the return type will be bool . Otherwise, it looks like a simple error in the book.

Answer 2

You shoud delete while condition:

mcond.wait(lock);

Errata for 2nd and 3rd printings of The C++ Programming Language