Copy Class with std::mutex

Question

I have a class with a std::mutex as a member. I am trying to create an array of such class

class C {  int x;  std::mutex m; };  int main() {   C c[10];   //later trying to create a temp C   C temp = c[0]; } 

Clearly the above is not possible as mutex object is not copyable. The way to solve it is through a copy constructor.

However, I am having problem in creating a copy constructor. I have tried

C (const C &c) {    x = c.x;     //1. m    //2. m()    //3. m = c.m } 

I am not sure what is the right syntax out of the 3 choices. Please help.

Answer 1

You shouldn't write any of these lines. Your implementation of copy constructor is equivalent to:

C (const C &c) : x(), m() {    x = c.x; } 

So new instance of mutex m is default initialized which means that one of the default constructors will be called. It may be safely used.

However, there are several conserns about this code. I.e. if m protects x, you should explicitly lock it before accessing value:

C (const C &c) {     std::lock_guard<std::mutex> (c.m);     x = c.x; } 

which would require to declare m as mutable (because c is const reference in copy ctor).

mutable std::mutex m; 

In the end, you can see that copying objects with mutexes inside is confusing, and if C is public class, it'll confuse its users, so think twice before implementing copying of it.