C++ check if pointer is valid? - c++

Question

I'm having problems with valid pointers in C++. I'm using one object in different threads, so I can't just set the pointer to NULL and return. Here's what I'm trying:
int main()
{
char *ptr = new char[1024]; //assume PTR = 0x12345678
changePtr(ptr); //after calling this,
//ptr is not NULL here.
return 0;
}
void changePtr(char *ptr)
{
delete [] ptr; //ptr = 0x12345678
ptr = NULL; //ptr = NULL
}
How can I change ptr to NULL for both functions?

Answer 1

change the signature of changePtr to:
void changePtr(char **ptr)
{
delete [] *ptr; //ptr = 0x12345678
*ptr = NULL; //ptr = NULL
}
And call it using:
changePtr(&ptr);

Answer 2

In C++, use reference parameter:
void changePtr(char *&ptr) {
delete [] ptr; //ptr = 0x12345678
ptr = NULL; //ptr = NULL
}
In C, you need to pass pointer to pointer, which is basically same thing with less pretty syntax.
You do not need to change the calling code. But you must give a modifiable variable as argument when calling, can't give for example NULL or nullptr, same as you can't do &NULL.

Answer 3

If you really want to manage memory in such a complex, error-prone way, then pass a reference to, rather than a copy of, the caller's pointer:
void changePtr(char *&ptr)
// ^
Much better would be to use a smart pointer; they are designed so that it's very difficult to leave them dangling when the target is deleted:
int main()
{
std::unique_ptr<char[]> ptr(new char[1024]); //assume PTR = 0x12345678
changePtr(ptr); //after calling this,
//ptr is empty here.
return 0;
}
void changePtr(std::unique_ptr<char[]> & ptr)
{
ptr.reset();
}
although if I wanted a dynamic array, I'd avoid new altogether and use std::vector.

Answer 4

I have seen a great many questions about how to check a pointer for validity. A large number of these questions have been about Windows. There may not be a general way to check in C++, but for a Windows specific solution the following seems to work on my system:
#include <windows.h>
#include <stdio.h>
int main(int argc, char **argv)
{
MEMORY_BASIC_INFORMATION lpBuffer;
int cActualBytes;
cActualBytes = VirtualQuery(&main, &lpBuffer, sizeof(lpBuffer)); // Can we get info about main?
if (!cActualBytes)
{
printf("Nope, you can't do that \n");
return 2;
}
if (cActualBytes != sizeof(lpBuffer))
{
printf("Surprise! Expected %d bytes, got %d\n", sizeof(lpBuffer), cActualBytes);
}
printf("Information for main\n");
printf("---------------------------\n");
printf("C reports pointer %p, Virtual Alloc sees it as %p\n",&main,lpBuffer.BaseAddress);
return 0;
}