CODEWITHSUNDEEP
c++
Peter
Peter

Reputation: 311

Pointer address in c++

I apologize in advance for the silliness of this question. But I couldn't find the way of understanding this outcome. I'm trying to figure out what the pointer really is in C++. Following is just a simple code printing an address in different ways. 

int a = 15000;
char *b = reinterpret_cast<char*>(a); //equivalent of char *b = (char *)a
int *m = &a;

//(1)
printf("&a:%p\n",&a);           //0x7fff5fbff878
printf("&*m:%p\n",&*m);         //0x7fff5fbff878
printf("m:%p\n",m);             //0x7fff5fbff878

//(2)
printf("a:%p\n",(int*)a);       //0x3a98
printf("&*b:%p\n",&*b);         //0x3a98
printf("b:%p\n",b);             //0x3a98
printf("*m:%p\n",(int*)*m);     //0x3a98

printf("&b:%p\n",&b);           //0x7fff5fbff870

printf("&m:%p\n",&m);           //0x7fff5fbff868

//(3)
std::cout << "b:" << b << std::endl; //error: Segmentation fault: 11

So the question is

Upvotes: 2

Views: 165

Answers (1)

Jts
Jts

Reputation: 3527

why the address of (1) and (2) are different

Because in the first example (1), you use the & address of operator, which gives you the address where the "a" variable is stored, example 0x50302040.

But the second example (2), you are using the value that address (0x50302040) points to, which returns 15000 which equals to 0x3a98 in hexadecimal.

what happened when an int is cast to a char*

If int has a value of 15000, now char* points to the address 15000, which is clearly wrong.

why the error (3) has occurred while 'printf("b:%p\n",b)' is working.

Because your pointer is a char* pointer, therefore the character sequence overload (ostream& operator<< (ostream& os, const char* s); is used, and since your pointer doesn't point to a character sequence (in fact, it doesn't point to anything since you assigned an invalid value to it), your program crashes.

Theres a ostream& operator<< (void* val); overload that will just print the address of the pointer, but you need to explicitly cast your char* pointer to a void* pointer to use it:

std::cout << "b:" << (void*)b << std::endl; // Prints 0x3a98 (15000)

Upvotes: 3

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