C / C++ / MFC

void GetMemory(char* p)
	{
		if(p != NULL)
		{
			delete []p;
			p = NULL;
		}
		if(p == NULL)
		{
			p =  new char[1000];
			*(p+10) = 'a';
		}
	}
int main()
{
char* str = NULL;
str = new char[100];
GetMemory(str);
/* */
}

sharion wrote:

Above code snippet indicates that before and after GetMemory(str);point str is being revised.I have no idea why such happens.

int main()
{
   char* str = NULL;
   str = new char[100];
   std::cout << (int)str <<std::endl;
   GetMemory(str);
   std::cout << (int)str <<std::endl;
/* */
}

Stuart Dootson wrote:

Of course, the memory that str points at is no longer allocated, so you don't want to be using str after the GetMemory call!

sharion wrote:

The value of point str is changed when second it is allocated memory. Do you agree?

sharion wrote:

Actually, str is allocated successfully after GetMemory.When you count the memory size of str between memory allocated two times you can draw this conclusion.

int main()
{
char* str = NULL; // str is assigned the value NULL
str = new char[100]; // str is assigned a value that points to allocated memory
GetMemory(str); // The value of str is COPIED and we call GetMemory
        void GetMemory(char* p) // The parameter p is now a COPY of str
	{
		if(p != NULL) // p is not NULL, so we execute the if statement's body
		{
			delete []p; // delete p. This does not alter the VALUE of p, but it deallocates the memory pointed to by both p AND str.
			p = NULL; // Assign NULL to p. This DOES NOT ALTER THE VALUE OF str.
		}
		if(p == NULL) // p is now NULL, so we execute the if statement's body
		{
			p =  new char[1000]; // Allocate memory and assign its address to p. WE DO NOT ASSIGN THE VALUE TO str. str DOES NOT CHANGE.
			*(p+10) = 'a'; // Set a value within the allocated block of memory
		}
	} // Return to main, DISCARDING p

// We're now back in main. The value of str HAS NOT CHANGED. It now points to an unallocated area in memory
} // End

str = new char[100]; 
int n1= (int)strlen((const char*)str);
GetMemory(str);
int n2= (int)strlen((const char*)str);

sharion wrote:

The first fact,you will find the value of *(str+10) is equal to 'a'.

#include <iostream>

void GetMemory(char* p)
	{
		if(p != NULL)
		{
			delete []p;
			p = NULL;
		}
		if(p == NULL)
		{
			p =  new char[1000];
			*(p+10) = 'a';
		}
	}
int main()
{
char* str = NULL;
str = new char[100];
char* str2 = new char[100];
GetMemory(str);
/* */
}

mass85 wrote:

Mac is not yet defined, because "Mac.h" was not included in global.h, although I wrote #include "mac.h".

I can't uinderstand why it happens. Why do directives #ifndef, #define stop compiler from including this header. When I delete these directives (#pragma once too) I get multiple inclusion error.

ilostmyid2 wrote:

i like to upload it like himself, but i don't know how.

ilostmyid2 wrote:

i like to upload it (the code), but i still don't know how!