Double pointer as argument to function - c++

Question

I want to pass a double pointer as argument to a function, but I cant see what I am doing wrong.
#include <iostream>
#include <string>
using namespace std;
void changeString(string ***strPtr) {
strPtr = new string**[1];
*strPtr = new string*[1];
**strPtr = new string("hello");
//cout << strPtr[0][0][0];
}
int main()
{
string **strPtr;
changeString(&strPtr);
//cout << strPtr[0][0];
return 0;
}
The cout in changeString works fine, but the cout in main throws the exception read access violation. strPtr was 0xCCCCCCCC.

Answer 1

Your example is basically equivalent to this:
void changeString(string **strPtr) {
strPtr = new string*[1];
*strPtr = new string("hello");
//cout << strPtr[0][0];
}
int main()
{
string *strPtr;
changeString(&strPtr);
//cout << strPtr[0];
return 0;
}
And that is basically equivalent to this:
void changeString(string *strPtr)
{
strPtr = new string("hello"); // Changes the pointer, not the string!
//cout << strPtr[0];
}
int main()
{
string str;
changeString(&str);
//cout << str;
return 0;
}
At this point it should start to become obvious that you are assigning a new value to the pointer, not the pointed-to object. Consider this:
void change(SomeType t)
{
t = somethingElse;
}
int main()
{
SomeType t;
change(t); // Does not change t.
}
In your case, SomeType happens to be string* (or string***) - you are just overwriting a local variable.
To fix your code, just skip the first line in changeString:
http://coliru.stacked-crooked.com/a/88874ee3601ef853
void changeString(string ***strPtr)
{
*strPtr = new string*[1];
**strPtr = new string("hello");
cout << strPtr[0][0][0];
}
int main()
{
string **strPtr;
changeString(&strPtr);
cout << strPtr[0][0];
return 0;
}