Difference between char pointer and char array

Question

I have defined 2 variables, one a pointer and one an array

char* ptr;
char* array;

ptr = "12345";
array = new int[5];

for(int i = 0; i < 5; i++)
    array[i] = i;

while(*ptr != 0)
        cout << *ptr++ << endl;

//Get garbage values
for(int i = 0; i < 5; i++)
    cout << ptr[i];

I was wondering what are the major differences between the variables. And why I get garbage values when I try to print the values in "ptr[]" the array way, but it's perfectly fine when iterate through the vales. I can't seem to understand how my variable "ptr" can point to 5 characters, since it should only be able to point to one.

Hint: ptr++ changes the value of ptr. You never reset it. — GManNickG
– GManNickG, Commented Dec 5, 2012 at 2:35

imreal · Accepted Answer · 2012-12-05 02:44:18Z

7

There are many differences between pointers and arrays, but the reason you are getting garbage, is that by the time you use ptr with an index, it already points to the null terminator of "12345".

Everytime you do this:

*ptr++;

ptr points to the next element it used to point (i.e. 1, 2, 3, ...). When the loop ends, it points to the null terminator \0 and then when you try to index it with i, it points to unknown memory.

I suggest you use a temp pointer to iterate through the elements instead:

const char* ptr; // declaring ptr constant in this case is a good idea as well

...

ptr = "12345";
char *tmp = ptr;

...

while(*tmp != 0)
    cout << *tmp++ << endl;

edited Dec 5, 2012 at 2:44

answered Dec 5, 2012 at 2:35

imreal

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5 Comments

Josh Over a year ago

Is there a way to reset the variable rather than going into a loop and decrementing it?

GManNickG Over a year ago

@SidharthMudgal: Huh? This code is well-defined: int main() { const char* ptr = "12345"; }.

imreal Over a year ago

@SidharthMudgal How can it cause a buffer overflow? Granted it should not be modified which is why I suggested using a constant pointer.

Jeremy Friesner Over a year ago

Assigning a string pointer is not like doing a strcpy(), it's only setting the pointer to point to a different location (in this case, to a location in the executable where the string "12345" has been stored statically by the compiler).

GManNickG Over a year ago

@SidharthMudgal: That's fine too (though that needs to be const char* in C++11). Why don't you expand on how that causes overflows?

Keith · Accepted Answer · 2012-12-05 02:56:58Z

2

You really have three concepts in play here: pointers to characters, dynamic arrays and string literals:

    // Initialise a character pointer to point to a dynamic array of characters
    char* array = new char[5];

    // Assign values pointed to.
    for(int i = 0; i < 5; i++)
        array[i] = i;
    // Read back the values.
    for(int i = 0; i < 5; i++)
         std::cout << array[i];

    // Initialise a character pointer to point to a string literal.
    const char* const ptr = "12345";

    const char* charIterator = ptr;
    while(*charIterator != 0)
        std::cout << *charIterator << std::endl;

    //Another way to read values.
    for(int i = 0; i < 5; i++)
        std::cout << ptr[i];

Note the double use of const in the expression const char* const ptr. The first means that the character values are constant, the second the pointer itself is constant. It is the latter that prevents writing ptr++, and losing your only handle to the address of the start of the string literal "12345".

answered Dec 5, 2012 at 2:56

Keith

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2 Comments

Josh Over a year ago

So const char* ptr = ... causes the "12345" to never change. Why should I add the const after char*? What is the advantage?

Keith Over a year ago

As noted above, it gives an error if you write ptr++, which was the reason for you getting the garbage values,

NPatel · Accepted Answer · 2012-12-05 03:46:15Z

0

If you use pointer ptr directly,the address which ptr stored that point to the array's value would be change.In this loop:

while(*ptr != 0)

   cout << *ptr++ << endl;

now, ptr points to the end of the array(\0).

Maybe you should execute the last loop before the loop above.

Usually,do as Nick's way is more appropriate.

edited Dec 5, 2012 at 3:46

NPatel

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answered Dec 5, 2012 at 3:24

Damon Pan

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1 Comment

Josh Over a year ago

When ptr reaches the end or \0 and I print out that value, will it show an empty space?

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