C++ destructor called after calling constructor

Question

By looking at the example:

#include <iostream>
int wow=0;
class Foo{
   int cow = 0;
public:
  Foo(){
    std::cout << "Foo +\n";
    cow = 0;
    ++wow;
  }
  Foo(int n){
    std::cout << "Foo has " << n << "\n";
    cow = n;
    ++wow;
  }
  ~Foo(){
     std::cout << cow << " ~ Foo -\n";
   }
  void print(){
    std::cout << cow << " is the foo#\n";
  }
};

int main(){
  void * bar = ::operator new(sizeof(Foo));
  Foo * a = new(bar) Foo;
  *a = Foo(10);
  std::cout << wow << std::endl;
  a->~Foo();
  ::operator delete(bar);
  return 0;
}

and compiling and running it, the console shows:

Foo+
Foo has 10
10 ~ Foo -
2
10 ~ Foo -

My question is, why is the destructor called upon calling the constructor?

Should the first destructor call be 0 ~ Foo - ? Since that is the first Foo that is overwritten by Foo(10)?

how do you compile this? I get some errors https://godbolt.org/z/qM1cj9W3s — 463035818_is_not_an_ai, Apr 26 '21 at 08:47
Fixed the `void bar` to `void * bar`, please check @largest_prime_is_463035818 — itchyfeet, Apr 26 '21 at 09:14

score 2 · Accepted Answer · edited Apr 26 '21 at 08:56

2

In this assignment statement

 *a = Foo(10);

there is created a temporary object of the type Foo that is assigned to the object specified by the expression *a using the default copy assignment operator (neither copy or move constructor is called here). After the assignment the temporary object is deleted. The undeclared variable cow (it seems it is a data member of the class Foo) of the object pointed to by the pointer a now contains the same value 10. And in the end of the program the object pointed to by the pointer a is also deleted.

As a result you will get two messages

10 ~ Foo -
10 ~ Foo -

The first one is generated by the destructor of the temporary object and the second one is generated by the object pointed to by the pointer a.

edited Apr 26 '21 at 08:56

463035818_is_not_an_ai

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answered Apr 26 '21 at 08:47

Vlad from Moscow

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About "using the default copy assignment operator": shouldn't the default _move_ assignment operator be called here since the argument is a prvalue? – chi Apr 26 '21 at 09:12
@Vlad from Moscow Yes cow is a member of Object Foo. Sorry about that. I already fixed the code. – itchyfeet Apr 26 '21 at 09:31
Wait I am still confused, how did the temporary `Foo` get the `int cow` to be `10`? When in fact it is the constructor that I call to be `10`? – itchyfeet Apr 26 '21 at 09:33
Can you make another example please? – itchyfeet Apr 26 '21 at 09:34
1

@chi According at least to the C++ 14 Standard if there is user-defined destructor then the move assignment operator is not implicitly declared. – Vlad from Moscow Apr 26 '21 at 14:50
@itchyfeet You are explicitly call the constructor like Foo(10). – Vlad from Moscow Apr 26 '21 at 14:52

C++ destructor called after calling constructor

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