run-time break error with pointer to a char array

Question

C++ - This program gives a run-time break error at line 2.

char * ptr = "hello";
(*ptr)++;            // should increment 'h' to 'i'
cout<<ptr<<endl;     // should display 'iello'

Unhandled exception at 0x004114b0 in test.exe: 0xC0000005: Access violation writing location 0x00417830.

Any idea why it is giving this error? Whereas if I run the following code, it works absolutely fine.

char arr[] = "hello";
char * ptr = arr;
(*ptr)++;           // increments 'h' to 'i'
cout<<ptr<<endl;    // displays 'iello'

cnicutar · Accepted Answer · 2011-08-16 08:10:27Z

3

Because you are trying to change read-only memory. There is a C FAQ which is adequate, even if this is a C++ question.

Basically when saying char *ptr = "hello" the compiler is free to place "hello" in read-only memory so it's not safe to try to write to it.

Another C FAQ might be useful:

What is the difference between these initializations?
char a[] = "string literal";
char *p  = "string literal";

answered Aug 16, 2011 at 8:10

cnicutar

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4 Comments

Saad Over a year ago

They way I am changing it in the second portion of code, isn't that also read-only memory?

cnicutar Over a year ago

@Saad Read the link I provided, it explains that too :-)

Saad Over a year ago

thanks! much appreciated. But why does compiler treats *ptr++ and ptr++ the same way? Its only when I place brackets around (*ptr)++ that it behaves differently.

cnicutar Over a year ago

@Saad Look for "operator precedence" in your book.

sameer karjatkar · Accepted Answer · 2011-08-16 08:12:58Z

-1

When you declare char * ptr = "hello"; it means ptr is pointing to a constant string

when you say ptr++ , You are trying to change the base address which is not correct

answered Aug 16, 2011 at 8:12

sameer karjatkar

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3 Comments

Saad Over a year ago

ptr++ or *ptr++ doesnt give any error. But when I place quotes, (*ptr)++ then it gives an error. I am not trying to change the base address, rather modifying the contents located on that address.

Chowlett Over a year ago

While it's true that ptr is pointing to a constant string, ptr++ would be perfectly valid (moving the pointer on). However, the question involves (*ptr)++ - incrementing the value currently pointed to.

sameer karjatkar Over a year ago

as @cnicutar pointed out char * ptr = "hello"; instructs the compiler that ptr is pointing to a constant string . whereas in case of char arr[] = "hello"; char * ptr = arr; ptr is a non const pointer

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