Question

data_cond.wait(lk, [this]{return !data_queue.empty();});

invalid argument after compile with

g++ -std=c++0x -Wall -pthread threadpool.cc -o hello 

originally from book is

data_cond.wait(lk, []{return !data_queue.empty();});

Answer 1

That looks rather like code from my book: C++ Concurrency in Action, especially with the lack of [this] capture (a typo which will be fixed in the final printing).

Unfortunately, there is a bug in g++ 4.5 and 4.6, where the compiler cannot handle lambdas with [this] captures in class templates. This even applies to simple templates. You can see the problem with the simple classes below:

struct X
{
    int i;
    void foo() {
        [this] { ++i; };
    }
};

template<typename T>
struct Y
{
    T i;
    void foo() {
        [this] { ++i; };
    }
};

Both g++ 4.5 and g++ 4.6 will give “invalid type argument” errors on the lambda in Y::foo, but happily accept the same code in X::foo.

Hopefully this will be fixed in a future version of g++. In the mean time, I would suggest using an explicit around the wait call:

while(data_queue.empty())
{
    data_cond.wait(lk);
}