CODEWITHSUNDEEP
c++c++11gccclosuresdestructor
vollitwr
vollitwr

Reputation: 439

C++: "illogical" destructor behavior

I want just to print a vector with for_each.

using namespace std;
struct Print {
   int z = 0;
   void operator()(int i) {
      if (z) cout << ' ';
      cout << i;
      z = 1;
   }
   ~Print() {
      if (z) cout << endl;
   }
};
main() {
   vector<int> v = {1, 2, 3, 4, 5};
   for_each(begin(v), end(v), Print());
}

It works wrong, it calls destructor two times and prints two newlines instead of one. Why? Can anybody explain the logic of this odd behavior? BTW it works fine with the ugly global variable.

int z;
struct Print {
   void operator()(int i) {
   . . .
};

I am using GCC.

Upvotes: 1

Views: 66

Answers (3)

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385295

It's not "wrong". You forgot that the predicate is copied, at least once. In fact, it is legal for the predicate to be copied many times. Your class should take that into account; and not by making its members static (which breaks subsequent invocations). std::for_each is not really the right tool for this job.

Upvotes: 3

vollitwr
vollitwr

Reputation: 439

I found much better solution. Both constructors should be explicitly included into Print class.

Print() {}
Print(const Print&) {}

Thanks to Bo Persson and Gene who explained the logic.

Upvotes: 0

Bo Persson
Bo Persson

Reputation: 92341

Check the signature of for_each

template< class InputIt, class UnaryFunction >
UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f );

It returns a copy of the function object.

Upvotes: 1

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