Error while excuting a simple boost thread program

Question

Could you tell mw what is the problem with the below boost::thread program

#include<iostream>
#include<boost/thread/thread.hpp>

boost::mutex mutex;

class A
{
public:
A() : a(0) {}

void operator()()
{
          boost::mutex::scoped_lock lock(mutex);

}
private:
int a;

};

int main()
{
    boost::thread thr1(A());
    boost::thread thr2(A());
    thr1.join();
    thr2.join();

}

I get the error message: error: request for member 'join' in 'thr1', which is of non-class type 'boost::thread()(A ()())' BoostThread2.cpp:30: error: request for member 'join' in 'thr2', which is of non-class type 'boost::thread ()(A ()())'

Answer 1

You have stumbled on something wonderfully known as the most vexing parse. The quickest way to fix that is to add an extra set of parentheses:

boost::thread thr1((A()));

You can also introduce a temporary:

A tmp1;
boost::thread thr1(tmp1);

In the most vexing parse, what you think is generating a temporary is parsed as if it's a function taking no parameters. It then treats thr1 as a prototype to a function that takes a single parameter (which is the function mentioned previously) and returning a boost::thread.

Answer 2

This is a classic C++ trap. thr1 is not what you think it is (a thread object). It's the declaration of a function that takes an instance of A as a parameter. Wrap it in parentheses to force the intended interpretation:

boost::thread thr1((A()));
boost::thread thr2((A()));

The detailed explanation

Syntactically, the original syntax is equivalent to:

boost::thread thr1(A);

Why is the compiler allowed to ignore the empty parentheses? Frankly, I'm not sure — I'm not a C++ language expert — but I think it has to with the following train of thought: A *a = A (*a), therefore A * = A (*); likewise, A a = A (a), therefore A = A ().

Look to the future!

The upcoming C++ standard will fix this as a byproduct of its new initialisation syntax:

boost::thread thr1{A()};
boost::thread thr2{A()};