Question

What is the output of the following C++ code? string *str; string fName, lName; str = &fName *str = "Miller"; str = &lName *str = "Tommy"; cout << fName << " " << lName << endl;

Short answer

The output is: Miller Tommy

Step-by-step solution

Analyze Pointer Initialization

Begin by noticing that the variable `str` is declared as a pointer to a string (`string *str;`). This means `str` can store the address of any string variable.

Assign Address to Pointer

The pointer `str` is assigned the address of `fName` with the statement `str = &fName`. At this point, `str` points to `fName`. Any dereference of `str` will now affect `fName`.

Modify the Value through Pointer

The line `*str = "Miller";` assigns the value "Miller" to what `str` is pointing to, which in this case is `fName`. Thus, `fName` is now "Miller".

Reassign Pointer to New Address

Next, `str` is reassigned to point to `lName` with `str = &lName`. Now, `str` points to `lName` and not `fName`.

Modify the Value of lName

The line `*str = "Tommy";` sets the content of `lName` to "Tommy" since `str` currently points to `lName`.

Output the Result

Finally, `cout << fName << " " << lName << endl;` prints out the values of `fName` and `lName`. Since `fName` was set to "Miller" and `lName` to "Tommy", the output will be `Miller Tommy`.

Key concepts

Pointer Initialization

In programming with C++, pointers are variables that hold memory addresses instead of actual data values. A pointer initialization is the process where you set up a pointer variable to store an address. For example, in the code provided, the pointer `str` is declared with `string *str;`.
Here, the asterisk (*) indicates that `str` is a pointer to a string. However, at this point, `str` has not yet been initialized to point to any specific memory address. It is simply declared as a pointer capable of pointing to a `string` type variable.
Proper pointer initialization is crucial because using an uninitialized pointer can lead to undefined behavior or crashes. It’s like having a pointer without a map; you won't know what it points to unless initialized with a memory address utilizing an assignment.

Dereferencing Pointers

Dereferencing a pointer in C++ involves accessing or modifying the value at the memory address to which the pointer points. This is achieved using the asterisk (*) operator.
In this context, after the initialization `str = &fName;`, the pointer `str` points to the memory location of the `fName` variable. When the code `*str = "Miller";` is executed, the `*str` refers to the content that `str` points to—`fName`.
By dereferencing `str`, you directly access and modify `fName`. This step is key since it allows you to change `fName` to "Miller" by utilizing the pointer rather than accessing the variable directly by its name. Understanding dereferencing is essential for effectively using pointers in various programming scenarios.

Pointer Assignment

Pointer assignment refers to setting a pointer to point to a specific memory address where a variable resides. This is typically done using the address-of operator (&).
In the example provided, `str = &fName;` assigns the address of `fName` to the pointer `str`. Subsequently, performing an assignment like `str = &lName;` changes the memory address stored in `str` to that of `lName`.
Through pointer assignment, `str` no longer points to its original memory location of `fName`, but rather to `lName`. Understanding pointer assignment is essential in C++ as it also allows dynamic data management by changing pointer targets during program execution, which adds flexibility and efficiency in your code.

String Manipulation

String manipulation using pointers in C++ involves changing the content of a string by pointing the pointer to its memory location and altering its value. In this exercise, we manage strings without directly operating on `fName` or `lName` by name in every step.
Instead, `str` is used to modify these variables. After dereferencing `str` when `str` is assigned first to `fName` and later to `lName`, we see the direct manipulation of these strings.
The code uses string pointers to replace the content of `fName` and `lName` to "Miller" and "Tommy" respectively. This method illustrates the power of pointers in efficient data manipulation by simply changing the focus of the pointer `str`. This is a fundamental technique in C++, enhancing both the control and versatility in handling string variables.