simple question about pointers and casting

steve1_rm · 03-26-2008, 09:40 PM

Hello,

I am just experimenting with some code. And I am not sure why I need to cast to an unsigned char*.

I get this error: cannot convert from 'unsigned char' to 'unsigned char *'
I can understand that the pointer is an unsigned char. But if I am assigning the address of the myFloat which I am casting a unsigned char. Why do I need to cast it as a pointer as well?

Code:

float myFloat = 10000;
unsigned char *ptrChar;
ptrChar = (unsigned char *) &myFloat;

//Why can't I just do this?
ptrChar = (unsigned char) &myFloat;

The following example using a int:

Code:

int myInt = 1000;
int *ptrInt = &myInt;

As the pointer is an int and I am assigning the address of myInt this works perfectly ok.

However, with the example above I have converted to a unsigned char.

Many thanks for clearing this up.

Steve

dwks · 03-26-2008, 09:51 PM

It's because "unsigned char" is a different type from "unsigned char *". Observe.

Code:

char c;
char *p = &c;
unsigned char uc = (unsigned char)c;
unsigned char *up = (unsigned char *)p;

An unsigned char is (usually) one byte. It usually stores a value from 0 to 255.
A unsigned char * is usually four or eight bytes. It stores the address of another variable, the address of an unsigned char somewhere else in memory.

So unsigned char and unsigned char * are completely different types. You have to tell the compiler which of them you want to use. Both can be used, but both give you very different values.

Code:

unsigned char c = 'a';
unsigned char *anotherp = (unsigned char *)c;
unsigned char anotherc = (unsigned char)c;

The value of anotherp is dubious, but it can be done if you really want to.

Rationale: when specifying types, use the name of the type (like "unsigned char"), as well as any levels of indirection (with asterisks).

Incidentally, casting a float * to an unsigned char * isn't usually much use unless you are trying to examine each byte of the float individually or something.

cpjust · 03-27-2008, 06:46 AM

Also, if you're using C++ you shouldn't use C style casts. C++ has 4 safer casts:
static_cast
dynamic_cast
const_cast
reinterpret_cast

dwks · 03-28-2008, 02:25 PM

In this case you'd probably use static_cast<>. Assuming you're using C++, of course.

03-26-2008, 09:40 PM	#1
steve1_rm UK2 Join Date: Sep 2003 Posts: 110	simple question about pointers and casting Hello, I am just experimenting with some code. And I am not sure why I need to cast to an unsigned char. I get this error: cannot convert from 'unsigned char' to 'unsigned char ' I can understand that the pointer is an unsigned char. But if I am assigning the address of the myFloat which I am casting a unsigned char. Why do I need to cast it as a pointer as well? Code: float myFloat = 10000; unsigned char ptrChar; ptrChar = (unsigned char ) &myFloat; //Why can't I just do this? ptrChar = (unsigned char) &myFloat; The following example using a int: Code: int myInt = 1000; int *ptrInt = &myInt; As the pointer is an int and I am assigning the address of myInt this works perfectly ok. However, with the example above I have converted to a unsigned char. Many thanks for clearing this up. Steve
steve1_rm is offline

03-26-2008, 09:51 PM	#2
dwks Frequently Quite Prolix Join Date: Apr 2005 Location: Canada Posts: 7,698	It's because "unsigned char" is a different type from "unsigned char ". Observe. Code: char c; char p = &c; unsigned char uc = (unsigned char)c; unsigned char up = (unsigned char )p; An unsigned char is (usually) one byte. It usually stores a value from 0 to 255. A unsigned char * is usually four or eight bytes. It stores the address of another variable, the address of an unsigned char somewhere else in memory. So unsigned char and unsigned char * are completely different types. You have to tell the compiler which of them you want to use. Both can be used, but both give you very different values. Code: unsigned char c = 'a'; unsigned char anotherp = (unsigned char )c; unsigned char anotherc = (unsigned char)c; The value of anotherp is dubious, but it can be done if you really want to. Rationale: when specifying types, use the name of the type (like "unsigned char"), as well as any levels of indirection (with asterisks). Incidentally, casting a float * to an unsigned char * isn't usually much use unless you are trying to examine each byte of the float individually or something. __________________ dwk Seek and ye shall find. quaere et invenies. "Simplicity does not precede complexity, but follows it." -- Alan Perlis "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra "The only real mistake is the one from which we learn nothing." -- John Powell Other boards: DaniWeb, TPS Unofficial Wiki FAQ: cpwiki.sf.net My website: http://dwks.theprogrammingsite.com/ Projects: codeform, xuni, atlantis, nort, etc.
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03-27-2008, 06:46 AM	#3
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,285	Also, if you're using C++ you shouldn't use C style casts. C++ has 4 safer casts: static_cast dynamic_cast const_cast reinterpret_cast
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03-28-2008, 02:25 PM	#4
dwks Frequently Quite Prolix Join Date: Apr 2005 Location: Canada Posts: 7,698	In this case you'd probably use static_cast<>. Assuming you're using C++, of course. __________________ dwk Seek and ye shall find. quaere et invenies. "Simplicity does not precede complexity, but follows it." -- Alan Perlis "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra "The only real mistake is the one from which we learn nothing." -- John Powell Other boards: DaniWeb, TPS Unofficial Wiki FAQ: cpwiki.sf.net My website: http://dwks.theprogrammingsite.com/ Projects: codeform, xuni, atlantis, nort, etc.
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