C++ - Null Pointers

Question

I'm learning about pointers in C++. I wrote this simple program to show what I had a problem with:

#include <iostream>

using namespace std;

int main() {
    cout << "test1";
    char *ptr = 0;
    cout << ptr;
    cout << "test2";
}

When I run the program, it doesn't output "test2" at the end, instead only "test1". This should mean that it crashed when I tried to print out the value of ptr? I tried stepping through it in Eclipse debugger and it looks like every line gets executed but doesn't it throw an error or something?

For starters, try this: cout << "test2" << endl; And see if it prints out test2. The previous line should print out the address of ptr. — kiss-o-matic, Oct 10 '14 at 01:38
You are right -- sorry for my bad semantics. I'm not sure if it's implementation-defined though. I use this regularly with GCC on OSX & Linux. — kiss-o-matic, Oct 10 '14 at 01:43

Keith Thompson · Accepted Answer · 2014-10-11T06:47:30.207

3

char *ptr = 0;
cout << ptr;

There's an overload of the << operator that takes a char* operand, which it assumes is a pointer to a C-style string.

For pointer types other than char*, the << operator would print the value of the pointer (which is an address), but treating a null char* pointer as if it pointed to a C-style string causes undefined behavior. In any case, it's not going to print the pointer value.

To print the pointer value, you can convert it to void*:

cout << "test1\n";
char *ptr = 0;
cout << static_cast<void*>(ptr) << "\n";
cout << "test2" << "\n";;

edited Oct 11 '14 at 06:47

answered Oct 10 '14 at 01:45

Keith Thompson

254,901
44
429
631

I believe static_cast is best here. – Neil Kirk Oct 10 '14 at 01:47
I can say that at least in C++11, *Requires: s shall not be a null pointer.* ergo no check. C++03 is the same way. – chris Oct 10 '14 at 02:13

score 2 · Answer 2 · answered Oct 10 '14 at 01:45

Normally you can output a pointer to cout and it will print the address contained. However, when you output a char * it is interpreted as a C-style null-terminated string. In this case, it's a null pointer and does not point to a string.

Try casting it to a void * before outputting it.

C++ - Null Pointers

2 Answers2