How can I call a lambda function from another lambda function

Question

Consider the contrived class below with a method A() that contains two lambda functions, a1() and a2(). I'd like to be able to invoke a2 from inside a1. However, when I do so, (the second line inside a1), I get the error

Error: variable “cannot be implicitly captured because no default capture mode has been specified”

I don't understand this error message. What am I supposed to be capturing here? I understand that using [this] in the lambda definition gives me access to methods in class foo but I'm unclear as to how to do what I want.

Thanks in advance for setting me straight on this.

class foo
{
   void A()
   {
      auto a2 = [this]() -> int
      {
         return 1;
      };

      auto  a1 = [this]() -> int
         {
            int result;
            result = a2();
            return result;
         };

      int i = a1();
      int j = a2();
   }
};

Brian Bi · Accepted Answer · 2017-11-21 00:11:03Z

22

You need to capture a2 in order to odr-use a2 within the body of a1. Simply capturing this does not capture a2; capturing this only allows you to use the non-static members of the enclosing class. If you expect a2 to be captured by default, you need to specify either = or & as the capture-default.

[this, &a2]  // capture a2 by reference
[this, &]    // capture all odr-used automatic local variables by reference, including a2

answered Nov 21, 2017 at 0:11

Brian Bi

123k13 gold badges197 silver badges353 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

David Over a year ago

Thanks so much for the quick response but I'm still unclear. While [this, &a2] does work, [this, &] does not.

zzxyz Over a year ago

@David - It may be useful to specify a compiler and version. In VS 2017, for instance, it has to be [&, this] although the error message is quite clear.

David Over a year ago

@zzxyz . I'm using XCode 9.x and you're absolutely right, you have to put the & before 'this'. Now everything works --- thanks so much to both you and Brian

Solomon Slow Over a year ago

More specifically, The reason why capturing this does not also capture a2 is because a2 is a local variable. Capturing this to get access to a2 doesn't make any more sense than writing this->a2 in the main body of the foo::A() function would make.

Ying Xiong Over a year ago

@Brian, quick question, does a2 have to be captured by reference? Would [this, a2] work in this case?

|

Collectives™ on Stack Overflow

How can I call a lambda function from another lambda function

1 Answer 1

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related