Swap 2 char of a C++ array - c++

Question

I have a problem with memory access. When i = 0, Visual Studio throws an exception with reference to a line as labelled in the following code.
Can't access at 0x00AD8B3B and 0x00AD8B3B equals scr+np-i
How can I fix the for-loop body so as to fix this issue?
int o_strrev(char* scr)
{
int np = strlen(scr) - 1;
char tmp;
if (!scr) return -1;
if (!*scr) return -1;
for (int i = 0; i < np / 2; i++)
{
tmp = scr[np-i];
scr[np-i] = scr[i]; # this line
scr[i] = tmp;
}
return 0;
}

Answer 1

As pointed out by #Revolver_Ocelot, you are probably passing a const char* from a string literal. Since these are, by definition, constant, you can't modify them in the way you're trying to. You need some way to convert const char* into a non constant char*. Something along these lines will work for you:
string str = "string";
char* cstr = new char[str.length() + 1];
strcpy(cstr, str.c_str());
I've compiled your code in g++ using non constant char* and it works fine. Just remember to deallocate your char* when you're done with it. We don't want memory leaks ;)

Answer 2

I see you are trying to reverse a string.
Might I suggest a simpler way
void reverse(char *string_var)
{
int length, c;
char *begin, *end, temp;
length = strlen(string_var);
begin = string_var;
end = string_var;
for (c = 0; c < length - 1; c++)
end++;
for (c = 0; c < length/2; c++)
{
temp = *end;
*end = *begin;
*begin = temp;
begin++;
end--;
}
}
Make sure you pass character array of the form
char word[64]
Or maybe by recursion
void reverse(char *x, int begin, int end)
{
char c;
if (begin >= end)
return;
c = *(x+begin);
*(x+begin) = *(x+end);
*(x+end) = c;
reverse(x, ++begin, --end);
}

Answer 3

You're most likely calling o_strrev with a string literal, some thing like:
o_strrev("This ain't gonna work!");
This will compile with most compilers and at most generate a warning. Try cranking up your warning level.
Instead, you want to make a dynamic copy of your string on the heap and work with a char* pointer to that, something like:
string str = "This is gonna work!!"
size_t len = str.size();
char* my_string = new char[len + 1];
strncpy(my_string, str.data(), len)
my_string[len] = '\0';
o_strrev(my_string);
// ...
delete[] my_string;