error: no matching function for call to 'boost::shared_lock<boost::shared_mutex>::shared_lock(const Lock&)'

Question

I have implemented a ReadLock like following:

In my myClass.h

#include <boost/thread/locks.hpp>
#include <boost/thread/shared_mutex.hpp>

typedef boost::shared_mutex Lock;
typedef boost::shared_lock< Lock > ReadLock;

Lock myLock;

In myClass.cpp:

void ReadFunction() const
{
    ReadLock r_lock(myLock); // Error!
    //Do reader stuff
}

The code works in VS2010 but failed with GCC4.0. The compiler is throwing error at ReadLock saying there is no matching function. I suspect is the "const" correctness problem with the variable myLock. When I removed the const in the function declaration, the error disappeared. Can anybody explain this to me? Why this works under windows but not with gcc?

Any suggestion here? Thanks.

Answer 1

You should either remove the const qualifier from ReadFunction(), because qualifying a non-member function with cv or ref qualifiers is illegal and doesn't even makes sense; or you encapsulate whatever you are trying to do in a class.

---

void ReadFunction() const
{
    ReadLock r_lock(myLock); // Error!
    //Do reader stuff
}

const can only be applied to member functions. The above code isn't a member function, if it was, it would be, (for example, a class named MyClass):

void MyClass::ReadFunction() const
{
    ReadLock r_lock(myLock);
    //Do reader stuff
}

And in that case, you would typically need to make lock a mutable member. by declaring it like this:

class MyClass{
    ....
    mutable Lock myLock;
};