Using std::thread to call an overriden method from a derived class

Question

I am trying to get something like this to work:

int main()
{
    class Base
    {
    public:
        Base() = default;
        virtual void print() = 0;
        void CallThread()
        {
            std::thread(&Base::print, *this);
        };
    };

    class derived1 : public Base
    {
    public:
        derived1() = default;
        void print() { printf("this is derived1\n"); }

    };
    class derived2 : public Base
    {
    public:
        derived2() = default;
        void print() { printf("this is derived2\n"); }

    };
    Base* ptr;
    ptr = new derived1();
    ptr->CallThread();

    return 0;
}

The result I want to happen is :

this is derived1.

The problem is it won't even compile. I am using VisualStudio 2013.

The error is :

Error 2 error C3640: 'main::Base::[thunk]: __thiscall main'::2'::Base::`vcall'{0,{flat}}' }'' : a referenced or virtual member function of a local class must be defined

Any ideas how to make this work?

edit: just to be clear, what I'm trying to do in the thread is more complicated, this is just an example of the structure of my program (so lambda isn't a good idea for me)

Not sure on the error, but shouldn't std::thread(&Base::print, *this); be std::thread(&Base::print, this); so as to not create a copy of Base? Or was the copy intended? — Niall
– Niall, Commented Aug 29, 2014 at 13:06
@Niall You've identified the problem: you cannot create instances of abstract classes. std::thread(&Base::print, *this) tries to create a copy of Base for the thread. Either std::ref or a pointer is required. There's a lifetime problem once you use a pointer or reference, though. Also, the thread must be detached or joined, otherwise an exception will be thrown. — dyp
– dyp, Commented Aug 29, 2014 at 13:08

Mike Seymour · Accepted Answer · 2014-08-29 14:38:31Z

11

Your code is fine, except for two things:

std::thread(&Base::print, *this);

should be:

std::thread(&Base::print, this).join(); // join & no asterisk

edited Aug 29, 2014 at 14:38

Mike Seymour

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answered Aug 29, 2014 at 13:12

Piotr Skotnicki

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5 Comments

Niall Over a year ago

Locally defined classes need to have the member methods defined inside the class body (even if empty), so virtual void print() {} (for gcc and msvc). That said, I've learnt a whole lot of new things about local classes, here.

Piotr Skotnicki Over a year ago

@Niall: you have mixed something, it's fine to have pure-virtual in local class, what you can't do is to have class A{void print();}; void A::print() {} locally

Niall Over a year ago

This seems to be a variation in implemementation between gcc and msvc. Local class must have the functions declared in the body, but you cannot have a pure specifier =0 and a definition together, so =0 {} would not be allowed. I did not know this... Msvc does accept the code with both the specifier and definition together thought, gcc doesn't.

Dor Ioushua Over a year ago

This almost did the trick for me - but it still didn't work, I had to remove the pure specifier ( = 0 ) and just leave empty brackets on the overloaded print() function of Base class - and that worked.

Piotr Skotnicki Over a year ago

@DorIoushua: §9.8.2: "Member functions of a local class shall be defined within their class definition, if they are defined at all."

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