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NULL Pointer for C++
Extending a language for what it’s lacking
Time: 06/17/09 3:33:03 am
Tags: Null, GCC [1], Assembly [5], C [8]
Most relevent 4 of 11 posts with shared tags:

I’ve recently been frustrated by the fact that NULL in C++ is actually evaluated as integral 0 instead of a pointer with the value of 0. This problem can be seen in the following example:

class String
{
	String(int i)  { /* ... */ } //Convert a number to a string
	String(char* i){ /* ... */ } //Copy a char* string directly into the class
};

String Foo(NULL); //This would give the string "Foo" the value "0" instead of a char* to (void*)0

The solution I came up with, which my good friend Will Erickson (aka Sarev0k) helped me revise, is as follows:

#undef NULL //If NULL is already defined, get rid of it
struct NULL_STRUCT { template <typename T> operator T*() { return (T*)0; } }; //NULL_STRUCT will return 0 to any pointer
static NULL_STRUCT NULL; //NULL is of type NULL_STRUCT and static (local to the current file)

After coming up with this way of doing it, I found out this concept is already a part of the new C++0x standard as nullptr, but since it is not really out yet, I still need a solution for the current C++ standard.


After getting this to work how I wanted it, I tested it out to make sure it is optimized correctly in compilers. When the compiler knows a value will be 0, it can apply lots of special assembly tricks.

Microsoft Visual C++ got it right by seeing that NULL was just 0 and applying appropriate optimizations, but GCC missed an optimization step and didn’t detect that it was 0 down the whole pipe. GCC, to my knowledge, however, isn’t exactly known for its optimization.


Example code:
BYTE* a=...; //Set a to an arbitrary value (best if brought in via an external method [i.e. stdin] so the compiler doesn’t make assumptions about the variable)
bool b=(a==NULL); //Set to b if a is 0 (NULL)
What MSVC6 outputs (and what it should be after optimization):
test eax,eax	//logical and a against itself to determine if it is 0 or not
sete al		//Set the lowest byte of eax to 1 if a is 0
What GCC gives
xor edx,edx	//Temporarily store 0 in edx for later comparison. This is a 0 trick, but 1 step higher than it could be used at.
cmp edx,eax	//Compare a against edx (0)
sete al		//Set the lowest byte of eax to 1 if a equals the value in edx

On a side note, it has been quite painful going from using assembly in Microsoft Visual C++ to GCC for 2 reasons:
  • I hate AT&T (as opposed to Intel) assembly syntax. It is rather clunky to use, and every program I’ve ever used is in Intel syntax (including all the Intel reference documentation). I tried turning on Intel syntax through a flag when compiling through GCC, but it broke GCC. :-\
  • Having to list which assembly registers are modified/used in the extended assembly syntax. This interface is also very clunky and, I have found, prone to bugs and problems.

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