Error with Dynamic Arrays

[Catalyst8487]

I am writing a program that asks the user for their first, middle, and last name, then concatenates the 3 fixed size arrays into one array that is dynamically created based on the length of the full name.

When executing the program, I get this error:

Unhandled exception at 0x1026f8e0 (msvcr90d.dll) in Lab5.exe: 0xC0000005: Access violation reading location 0x00000000.

It occurs in the DisplayName function. I noticed when I return to Main after calling my Allocate function that the ptrName shows <bad pointer> in the debug menu.

Here is the code for my 3 files:

Lab5header.h

// Function Prototype
void Allocate(char [], char [], char [], char *);
void DisplayName(char *);

Main.cpp

// Pre-defined headers
#include <iostream>
#include <cstring>

// User-defined headers
#include "Lab5Header.h"

using namespace std;

const int ARRAY_SIZE = 12;

int main()
{
	// Declare fixed-sized array
	char firstName[ARRAY_SIZE] = "";
	char middleName[ARRAY_SIZE] = "";
	char lastName[ARRAY_SIZE] = "";

	char *ptrName = NULL;

	// Variable to check whether user wants to continue program or not.
	char answer = 'y';

	do
	{
		cout << "WARNING: First, middle, and last name can only be up to 12 characters long.";
		cout << endl << endl;

		// Get users last, first, then middle name.
		cout << "Please give me your last name. > ";
		cin >> lastName;

		cout << endl << "Please give me your first name. > ";
		cin >> firstName;

		cout << endl << "Please give me your middle name. > ";
		cin >> middleName;

		// Find the total length of the users full name.  Then, dynamically allocate a new
		// array for the users full name.
		Allocate(lastName, firstName, middleName, ptrName);

		// Call function to display the full name in its correct order
		DisplayName(ptrName);
	}
	while (answer == 'y' || answer == 'Y');

	// Delete the array we created
	delete [] ptrName;
	ptrName = NULL;

	system("pause");
	return 0;
}

Implementation.cpp

// Pre-defined headers
#include <iostream>
#include <cstring>

// User-defined headers
#include "Lab5Header.h"

using namespace std;


void Allocate(char lastName[], char firstName[], char middleName[], char *ptrName)
{
	// Determine the length of the array	
	int arrayLength = strlen(firstName) + strlen(middleName) + strlen(lastName) + 3;

	// Create new Array
	ptrName = new char [arrayLength];

	// Copy and append First, Middle, and Last names into the new array
	strcpy(ptrName, firstName); 
	strcat(ptrName, " ");
	strcat(ptrName, middleName);
	strcat(ptrName, " ");
	strcat(ptrName, lastName);

	cout << endl << ptrName << endl;

	return;
}

void DisplayName(char *ptrName)
{
	cout << endl << endl << ptrName;
	return;
}

I know that I need to pass the pointer by reference but all my attempts at this end up giving me more errors.

[bling]

ptrName gets initialized inside Allocate, but this information doesn't go back to the calling function, so when you call DisplayName, ptrName is still NULL. you need to return the pointer value.

[Catalyst8487]

That is the problem bling. In Allocate, the last cout line where I print ptrName... it print what I expect it to. Back in main, when I call DisplayName, ptrName is no longer what I am expecting it to be.

In my previous usage with pointers I would create the pointer then assign it the address of the variable to it. But since I have to create this new array dynamically, I don't think I can do the same that I've done in the past.

I guess what it's coming down to is how to I pass ptrName that isn't currently pointing to anything?

[bling]

when you're passing pointers, it's really just passing a number.

imagine Allocate having the following signature:
void Allocate(char last[], char first[], char mid[], int ptrName);

that's what's really happening...

[Daved]

You can either return the pointer as bling suggested or pass the pointer in by reference.

[Elysia]

https://apps.sourceforge.net/mediawi...arameter_names

[C_ntua]

That link asks for a password (dont know why).
In any case, if you wanted to work with pointers you would one of these:

void Allocate(char lastName[], char firstName[], char middleName[], char** ptrName){...}
char* Allocate(char lastName[], char firstName[], char middleName[], char *ptrName){...}
void Allocate(char lastName[], char firstName[], char middleName[], char*& ptrName){...}

The best way for me is the third, because you won't have to change anything from your code.
Or alternative, you use the second, but better it would be like this (emitting one unecessary parameter):

char* Allocate(char lastName[], char firstName[], char middleName[])
{
    ...
    char* ptrName = new char[ArrayLength]
    ...
    return ptrName;
}

//in main
char* ptrName = Allocate(lastName, firstName, middleName);

[Elysia]

The 3rd is the best C++-way.
The first is the C way.