C++ capture move initialized is const?

Question

I am trying to move a local variable into the capture of lambda.

#include <thread>
#include <iostream>

// Moveable but not copyable object.
class WorkUnit
{
    public:
        WorkUnit(int)                               {}
        WorkUnit(WorkUnit&&)            noexcept    {}
        WorkUnit& operator=(WorkUnit&&) noexcept    {return *this;}
        WorkUnit(WorkUnit const&)                   = delete;
        WorkUnit& operator=(WorkUnit const&)        = delete;

        // Non const function.
        void doWork()
        {
            std::cerr << "Work\n";
        }
};

int main()
{
    WorkUnit    data(4);

    // Use C++14 generalized lambda capture.
    std::thread test([data{std::move(data)}]()
        {
            // here it is complaining the `data` is a const value.
            // Is there a way to capture this as a non const?
            data.doWork();
        }
    );
    test.join();
}

When I compile I get this.

> g++ -std=c++14 WU.cpp
Test.cpp:26:13: error: member function 'doWork' not viable: 'this' argument has type 'const WorkUnit',
      but function is not marked const
            data.doWork();
            ^~~~

I was expecting the captured value to be none const.

Answer 1

You could use mutable:

mutable - allows body to modify the parameters captured by copy, and to call their non-const member functions

Unless the keyword mutable was used in the lambda-expression, the function-call operator is const-qualified and the objects that were captured by copy are non-modifiable from inside this operator().

std::thread test([data{std::move(data)}]() mutable
    {
        // the function-call operator is not const-qualified;
        // then data is modifiable now
        data.doWork();
    }
);

It's worth noting that this allows to modify on the object captured by copy, which has nothing to do with the original object.