c++11 - How to move non-copyable local variable out of lambda in C++? -

i want implement simple runtime check macro, works like:

check(expr) << "some problem!"; 

i wrote simplified logging class so:

class log {  public:   log() = default;   log(const log&) = delete;    ~log() {     cout << << " dtor" << endl;     cout << stream_.str() << endl;   }    template <class t>   log& operator<<(const t& info) {     cout << << " <<" << endl;     stream_ << info;     return *this;   }   private:   stringstream stream_; }; 

and let macro be:

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

now let's try implement lambda.

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attempt #1

the simplest way should be:

[] {   log log;   log << "a";   return log; } 

but no luck - copy constructor deleted:

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

attempt #2

ok, let's move local variable explicitly:

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

hmm, still no luck.

attempt #3

a desperate attempt shouldn't work:

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

it compiles , runs, operator << called after destructor:

0xbfe84064 dtor 0xbfe84064 << 

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help me figure out doing wrong returning variable lambda?

make move constructor default, i.e.

log(log&&) = default; 

because otherwise existence of user-provided copy ctor (even if deleted) disables move ctor. should return log; instead of return move(log);, default move ctor invoked (since copy ctor deleted). see e.g. this more details on why return move should in general avoided.

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