How to have multiple objects of a class with an std::mutex? - c++

Question

I have the following error:
filesystem.hpp:11:7: error: use of deleted function ‘std::mutex& std::mutex::operator=(const std::mutex&)’
In file included from /usr/include/c++/6.1.1/mutex:44:0,
from includes.hpp:11,
from htmlparser.hpp:4,
from htmlparser.cpp:1:
/usr/include/c++/6.1.1/bits/std_mutex.h:98:12: note: declared here
mutex& operator=(const mutex&) = delete;
^~~~~~~~
...about which there are already some questions asked (like this one and that one). Based on these I tried the following class code:
class Filesystem {
private:
std::string dir = getCurrentPath();
mutable std::mutex fsMut;
public:
Filesystem() {}
~Filesystem() {}
Filesystem(const Filesystem&) : fsMut() { }
//Few more functions
};
...which sadly enough doesn't work (or even change the errors).
Now how does my code differ from the previously mentioned questions: In two classes I have in the private section the declaration Filesystem fs;. Which in my opinion would be totally fine, however, while for one class it passes for the other class this error is returned (along with the errors how that class and Filesystem are deleted implicitly).
So I am not copying or moving the class afaik, but what does go wrong then? And how can I alter my code to make it work?
EDIT:
Full error:
htmlparser.cpp: In member function ‘strSet Htmlparser::parseLinks(std::__cxx11::string, std::__cxx11::string, std::__cxx11::string)’:
htmlparser.cpp:10:39: error: use of deleted function ‘Robotsparser& Robotsparser::operator=(const Robotsparser&)’
rsmap[origUrl] = Robotsparser(origUrl);
^
In file included from htmlparser.hpp:5:0,
from htmlparser.cpp:1:
robotsparser.hpp:7:7: note: ‘Robotsparser& Robotsparser::operator=(const Robotsparser&)’ is implicitly deleted because the default definition would be ill-formed:
class Robotsparser {
^~~~~~~~~~~~
robotsparser.hpp:7:7: error: use of deleted function ‘Filesystem& Filesystem::operator=(const Filesystem&)’
In file included from robotsparser.hpp:5:0,
from htmlparser.hpp:5,
from htmlparser.cpp:1:
filesystem.hpp:11:7: note: ‘Filesystem& Filesystem::operator=(const Filesystem&)’ is implicitly deleted because the default definition would be ill-formed:
class Filesystem {
^~~~~~~~~~
filesystem.hpp:11:7: error: use of deleted function ‘std::mutex& std::mutex::operator=(const std::mutex&)’
In file included from /usr/include/c++/6.1.1/mutex:44:0,
from includes.hpp:11,
from htmlparser.hpp:4,
from htmlparser.cpp:1:
/usr/include/c++/6.1.1/bits/std_mutex.h:98:12: note: declared here
mutex& operator=(const mutex&) = delete;
^~~~~~~~
And the other classes:
class Robotsparser {
private:
std::string url;
Filesystem fs;
public:
Robotsparser(std::string u) : url(u) {}
~Robotsparser() {}
};
class A {
private:
std::mutex crawlMut;
Filesystem fs;
public:
A(std::string);
};
Class A is compiled earlier in the Makefile, which might explain why it gives the error at class Robotsparser.

Answer 1

Your error means that there is an assignment operation trying to happen somewhere...
In your Filesystem class, the compiler didn't complain of the copy constructor:
Filesystem(const Filesystem&) : fsMut() {} //because there is no copying of fsMut here
But, the compiler generates a copy assignment operator for you since you didn't define one. And in the compiler generated one, it calls the copy assignment operator of each member.
For what I think your intent are: You should define all your Copy/Move Assignment operators (and Constructors), and make sure you do not try to copy/move the mutex owned by any of the instances.
Filesystem(const Filesystem& f2)
{
std::lock_guard<std::mutex> lk2(f2.fsMut);
/*do your stuff but do not copy f2.fsMut*/
}
Filesystem(Filesystem&& f2)
{
std::lock_guard<std::mutex> lk2(f2.fsMut);
/*do your stuff but do not move f2.fsMut*/
}
Filesystem& operator = (const Filesystem&)
{
std::lock(this->fsMut, f2.fsMut);
std::lock_guard<std::mutex> lk1(this->fsMut, std::adopt_lock);
std::lock_guard<std::mutex> lk2(f2.fsMut, std::adopt_lock);
//do your stuff but do not copy fsMut
return *this;
}
Filesystem& operator = (Filesystem&& f2)
{
std::lock(this->fsMut, f2.fsMut);
std::lock_guard<std::mutex> lk1(this->fsMut, std::adopt_lock);
std::lock_guard<std::mutex> lk2(f2.fsMut, std::adopt_lock);
//do your stuff but do not move fsMut
return *this;
}
Full illustration here: http://coliru.stacked-crooked.com/a/75d03fd564f8b570
Also, consider using, lock_guard's on both mutexes and std::lock to lock both mutexes in the copy/move assignment operators.
Though I still have my reservations on your intent, I saw a member declaration like:
mutable std::mutex fsMut;
The use of mutable is to modify members from a const member function; here its typically for being able to lock/unlock the mutex from a const member function.

Answer 2

What does the first bit of the error say?
htmlparser.cpp: In member function ‘strSet Htmlparser::parseLinks(std::__cxx11::string, std::__cxx11::string, std::__cxx11::string)’:
htmlparser.cpp:10:39: error: use of deleted function ‘Robotsparser& Robotsparser::operator=(const Robotsparser&)’
rsmap[origUrl] = Robotsparser(origUrl);
It says that in Htmlparser::parseLinks, on this line
rsmap[origUrl] = Robotsparser(origUrl);
you're using a copy assignment operator that doesn't exist.
The reason it doesn't exist is that the compiler didn't generate it for you, because it can't, because the mutex member of your class isn't copyable.
However, your real question is why the compiler is trying to use this in the first place:
So I am not copying or moving the class afaik
but you can see, on the line the compiler quoted, an assignment. You haven't shown what rsmap is, but looking at operator[] for std::map shows that it default constructs an element, returns a reference to the new value, and then your code copy-assigns to it. The same is true for std::unordered_map.
If your class isn't copyable or assignable, you can't do this - you need to construct the object in place. The emplace method does this, giving code something like:
rsmap.emplace(origUrl, origUrl);
Alternatively, you can keep your existing code and write the copy/move constructors and assignment operators.