How to move non-copyable local variable out of lambda in C++?

Question

I want to implement a simple runtime check macro, so it works like:

CHECK(expr) << "Some problem!";

I wrote a simplified logging class to do so:

class Log {
 public:
  Log() = default;
  Log(const Log&) = delete;

  ~Log() {
    cout << this << " dtor" << endl;
    cout << stream_.str() << endl;
  }

  template <class T>
  Log& operator<<(const T& info) {
    cout << this << " <<" << endl;
    stream_ << info;
    return *this;
  }

 private:
  stringstream stream_;
};

And let the macro be:

#define CHECK(expr) \
  if (!(expr)) [] { /* See attempts below */ }()

Now let's try to implement the lambda.

Attempt #1

The simplest way should be:

[] {
  Log log;
  log << "A";
  return log;
}

But no luck - the copy constructor is deleted:

error: use of deleted function 'Log::Log(const Log&)'

Attempt #2

Ok, let's move the local variable explicitly:

[] {
  Log log;
  log << "A";
  return move(log);
}

Hmm, still no luck.

Attempt #3

A desperate attempt that certainly shouldn't work:

[]() -> Log&& {
  Log log;
  log << "A";
  return move(log);
}

It compiles and even runs, but the operator << is called after destructor:

0xbfe84064 dtor
A
0xbfe84064 <<

Help me to figure out what am I doing wrong with returning a variable from lambda?

I'd really avoid return local variable from function (including lambda expression). In your case - just `#define CHECK(e) if (!(e)) Log{} << "A"`. Even better for some other reason `#define CHECK(e) if (e) {} else Log{} << "A"` — PiotrNycz, Jan 24 '16 at 18:32
@PiotrNycz It's not a good variant for me, since I want to print a stack trace along with a user-provided message - that's why I use `Log` inside lambda to do some stuff. — abyss.7, Jan 24 '16 at 18:40
Not a problem at all. You can create object that prints stack track - like `Log{} << stackTracePrinter`. Or you can do everything you can imagine in some Log specialization constructor... — PiotrNycz, Jan 24 '16 at 19:57

score 5 · Accepted Answer · edited May 23 '17 at 11:52

Make the move constructor default, i.e.

Log(Log&&) = default;

because otherwise the existence of a user-provided copy ctor (even if deleted) disables the move ctor. You should also return log; instead of return move(log);, as the default move ctor will be invoked (since the copy ctor is deleted). See e.g. this for more details on why return move should in general be avoided.

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How to move non-copyable local variable out of lambda in C++?

Attempt #1

Attempt #2

Attempt #3

1 Answers1