Question

I got this code from the c++ primer book, which was meant to explain the delete operator. However, what I don’t understand is how the program calls the two functions and how they interact.

// delete.cpp -- using the delete operator
#include <iostream>
#include <cstring> // or string.h
using namespace std;
char * getname(void); // function prototype

int main()
{
    char * name; // create pointer but no storage

    name = getname(); // assign address of string to name
    cout << name << " at " << (int *) name << "\n";
    delete [] name; // memory freed

    name = getname(); // reuse freed memory
    cout << name << " at " << (int *) name << "\n";
    delete [] name; // memory freed again

    return 0;
}

char * getname() // return pointer to new string
{
    char temp[80]; // temporary storage
    cout << "Enter last name: ";
    cin >> temp;

    char * pn = new char[strlen(temp) + 1];
    strcpy(pn, temp); // copy string into smaller space

    return pn; // temp lost when function ends
}

The book provided the following sample run:

Enter last name: Fredeldumpkin
Fredeldumpkin at 0x004326b8
Enter last name: Pook
Pook at 0x004301c8

What I don’t understand is how and why “Enter last name: ” was executed twice, why the char * getname() function was executed before int main(), and how the two functions interact with each other.

Answer 1

“Enter last name” was printed twice because it gets printed in getname() and getname() is called twice.

getname() is not executed before int main(), it is declared. It must be declared so that when the compiler is compiling main() (which uses getname()), the compiler knows what is to be done.

main() is the first piece of executable code (that a developer normally has influence over, but there are exceptions). Everything that happens in your program is because either main() does it, or something main() calls (directly or indirectly) does it. In your sample, main() will: do the following:

1. call getname()
2. print something based on the return of getname()
3. release resources that were allocated inside getname()
4. call getname()
5. print something based on the return of getname()
6. release resources that were allocated inside getname()