Beginner question about destructors!

[baxy]

Hi,

I am wondering why does the below code not behave as i would expect:

#include <iostream>
using namespace std;

struct error{};

class Trace{
  public:
    Trace(const char *string_pt = "done") ;
    ~Trace();
  private:
   const char *string;
};

// declare constructor

Trace::Trace (const char *string_pt) 
	: string(string_pt)
{
  cout << "+ " << string << endl;
}

// declare destructor
Trace::~Trace(){
  cout << "- " << string << endl;
}

int main(){
  Trace string("A1");
  Trace string1("A2");
  
  for(int i = 0; i<3;i++){
    Trace tp("inLoop");
  }
  Trace *tpp =0;
  {
    Trace string("B1");
    Trace *string1 = new Trace("B2");
    tpp = new Trace("Here I am!");
  }
  
  delete tpp;
  return 0;

}

During destruction it destructs everything except string1. This i claim based upon the output:

+ A1
+ A2
+ inLoop
- inLoop
+ inLoop
- inLoop
+ inLoop
- inLoop
+ B1
+ B2
+ Here I am!
- B1
- Here I am!
- A2
- A1

What happened to B2 ???

Thank you

baxy

[manasij7479]

C++ isn't a garbage collected language, so make sure to 'delete' your garbage, and not cause memory leaks.

    Trace string("B1");
 //this is created on the stack and destructed on scope exit.

    Trace *string1 = new Trace("B2");
 //this is allocated on the heap and exists until you explicitly call delete.

Btw, write the same code using a smart pointer (as oppossed to the normal pointer here) in the inner loop, and you'll see it destructed when not required anymore.

[Elysia]

#include <memory>
auto string = std::make_shared<Trace>("B2");

[Elkvis]

I don't see std::make_shared in the standard library documentation. can you point to a reference?

[manasij7479]

std::make_shared - cppreference.com

[grumpy]

make_shared was not in the 1998 C++ standard. IIRC (I haven't checked relevant documentation) it was introduced as part of boost, and is now a feature of C++-11.