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Possible Duplicate:
Start thread with member function

I'm VERY new to C++. My experience has mostly been with javascript and java.

I'm using Xcode on Lion. The following code gives me a compilation error "Reference to non-static member function must be called; did you mean to call it with no arguments?"

class MyClass {
private:
    void handler() {
    }

public:
    void handleThings() {
        std::thread myThread(handler);
    }
};

I also tried this->handler, &handler, and other variations, but none of them worked. This code compiles though and accomplishes what I want it to:

class MyClass {
private:
    void handler() {
    }

public:
    void handleThings() {
        std::thread myThread([this]() {
            handler();
        });
    }
};

Why can't I pass a reference to a member function? Is my work-around the best solution?

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3
  • 2
    that is not a "workaround"... that is the right way to do your task (one of). the second one, is to use std::bind instead of lambda. to call a method of a class you have to provide an instance of that class (this pointer as first implicit parameter of any non static member-function). passing just an address of the method you want to call (w/o the instance) to std::thread ctor is not enough obviously. Commented Jan 22, 2013 at 7:14
  • depending on your tasks consider to use std::async instead of std::thread... in case if running thread just calc smth and exits (i.e. not an even loop handler) Commented Jan 22, 2013 at 7:28
  • I get it now. Your first comment zaufi as well as this answer stackoverflow.com/a/12765270/711902 taught me something. Commented Jan 22, 2013 at 7:28

3 Answers 3

Reset to default
17 
std::thread myThread(&MyClass::handler, this);
myThread.join();
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Comments

6

You can use std::mem_fun, if you dont want to use a lamda.

Are you able to do this?

std::thread myThread(std::mem_fun(&MyClass::handler),this);

std::thread accepts arguments to the function (which is the first argument) and this is passed as an argument to the mem_fun object, which then calls the handler function on this.

You could also quite simply do the below, courtesy - Start thread with member function

std::thread myThread(&MyClass::handler,this);
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1 Comment

you don't need mem_fun here.
5

Use lambda to access class members within a member function, you need to capture this. [this] is mandatory in below code: 

void handleThings() 
{
    std::thread myThread([this]() {
        handler();
    });
}

You could capture this by reference but it's not as efficient as capturing it by value. because going through the reference requires double indirection (modulo compiler optimizations)

void handleThings() {
        std::thread myThread([&]() {
            handler();
        });
    }

Lambdas are usually a better choice than bind.

  • Easier for readers to understand.
  • More efficient.
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5 Comments

this is just a pointer, so passing/capturing by value is quite ok... depending on completeness of C++11 support in compiler this->handler() may be required to resolve ambiguity.
How can I learn more about lambda vs. bind performance? Does this answer stackoverflow.com/a/8884445/711902 seem to indicate that performance is usually the same, at least with GCC 4.7? Or is the answerer talking about something else?
I haven't test it yet, but I read from C++11 books, like Overview of the New C++ (C++0x) mentions that.
What about std::thread myThread(&MyClass::handler, this) like bamboon suggested in stackoverflow.com/questions/10673585/…? Is that doing the same thing as std::bind? That seems like the cleanest solution.
@TrevorDixon, yes that works perfectly. It's very similar to bind and does the same thing in this case, but is not always exactly the same because the way thread handles its arguments doesn't support everything that bind supports, see stackoverflow.com/a/21066704/981959 for a bit more detail

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