Question

I have a C++ code mentioned below:

#include<iostream>
#include<stdexcept>

const long MAX = 10240000;

class Widget{
      public:
             Widget(){
                      ok = new int [1024];
                      prob = new int [100*MAX];
             }
             ~Widget(){
                       std::cout<<"\nDtoR of Widget is called\n";
                       delete ok; ok = NULL;
                       delete prob; prob = NULL;
             }
             //keeping below public: Intentionally
              int* ok;
              int* prob;
};


void func(){
    Widget* pw = NULL;  // <<<<--------------- Issue in Here
    try{
        pw = new Widget;
    }
    catch(...){
               delete pw;
               std::cout<<"\nIn catch BLOCK\n";
               if(pw->ok == NULL){
                      std::cout<<"\n OK is NULL\n";
               }
               throw;
    }
    delete pw;
}

int main(){
    try{
          func();
    }
    catch(std::bad_alloc&){
                     std::cout<<"\nError allocating memory."<<std::endl;
    }

    std::cout<<std::endl;
    system("pause");
    return 0;
}

Now in function func(), i am seeing two different behavior depending on if i do not set pointer ‘pw’ to NULL and if i do set ‘pw’ pointer to NULL (like code above). I was under the impression that it is ‘good’ practise to first set pointer to NULL and then initialize it. But when i initilize it to NULL then the output just shows “In catch BLOCK” and later app crashes. but if i do not set pw pointer to NULL, then i see destructor of pw is called and following output is shown w/o any app crash.

DtoR of Widget is called

In catch BLOCK

OK is NULL

Error allocating memory.

Press any key to continue . . .

My question is why such difference in one case when we are not initializing ‘pw’ pointer to NULL and in other case we are setting it to NULL. Why destructor is called in one case and why it was not called in another.

Note: The intent of this code is to throw bad_alloc exeption.

Answer 1

If you do not set pw to NULL then you will leave it uninitialized. Then, when new operator inside a “try” block throws an exception, it never returns, and you get into catch block. Since new never returned, pw will still be not initialized, and you will pass a random address to delete. That gives you an undefined behavior.