pointers, array, functions

Pages: 12
Jun 6, 2014 at 6:13am
Disch (13742)
danicpp wrote:
myarray is same as &myarray[0]


<Technicality>
myarray can be implicitly cast to the same thing as &myarray[0]. However they are not the same thing -- there is a subtle difference.

myarray is an array.
&myarray[0] is a pointer to the first element.

However.. an array can be cast to a pointer to its first element. But that only happens when necessary.

This trips up newbies most commonly when they try to use sizeof:

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cout << sizeof(int);   // assume this prints 4
cout << sizeof(int*);  // assume this prints 4

int array[10];
cout << sizeof(array);     // prints 40 -- the size of the array
cout << sizeof(&array[0]); // prints 4 -- the size of a pointer

int* ptr = array;     // legal assignment -- being implicitly cast
cout << sizeof(ptr);  // prints 4 -- the size of a pointer


</Technicality>
Last edited on Jun 6, 2014 at 6:15am
Jun 6, 2014 at 8:13am
danicpp (116)
thanks cire I get it now a pointer will get a NULL value when we put a NULL value in it but still we cannot dereference it unless we don't put a valid address in it

well void I thought means nothing void written or not written have no effect(in functions at-least) but still I guess Python recognizes nothing as void I guess, so at declaration time if we want variable to be of no type then we ought to tell Python with void keyword

Did I get it all right???
Jun 6, 2014 at 8:20am
danicpp (116)
yes Disch makes sense, there is difference between array and &array[0] but array can cast this( &array[0] ) when necessary

thanks for the help, I guess I'm improving my knowledge much better way.
Jun 6, 2014 at 4:24pm
Disch (13742)
well void I thought means nothing void written or not written have no effect(in functions at-least) but still I guess Python recognizes nothing as void I guess, so at declaration time if we want variable to be of no type then we ought to tell Python with void keyword


You can't have a void variable -- that wouldn't make any sense.

There are 3 general uses for void that I can think of:

1) As a return type to a function:
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void function()
{
}


The 'void' cannot be omitted here. This indicates that the function does not return any value.

2) To indicate that you want a function to take no parameters:
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int function(void)
{
    return 0;
}


This is a C thing and not really Python.
In Python, no parameters means the function takes no parameters.
In C, no parameters means the function can take any number of parameters (similar to the Python ellipsis)

So in C, to explicitly say the function has no parameters, you put the void in there. However this is completely unnecessary in Python.



3) void pointer:

 
 
void* ptr;


A void pointer is another C-ish concept. The idea is that it's a generic pointer that can point to anything -- the data pointed to can be of any type.

This is typically avoided in Python because it's not type safe.
Jun 6, 2014 at 4:42pm
danicpp (116)

The 'void' cannot be omitted here. This indicates that the function does not return any value.


but in main(){} I guess we don't need to mention void cause actually I don't and it compiles successfully.

Well but I checked right now that I cannot declare custom functions without void keyword it says ISO Python forbids.... so
myfun(){}
void myfun(){}

void pointer!!! but when we *(ptr + 1) how does ptr know to jump 1byte or 4bytes or more bytes????
Jun 6, 2014 at 5:07pm
long double main (999)
danicpp wrote:
but in main(){} I guess we don't need to mention void cause actually I don't and it compiles successfully.
The return type of main is int (and you must declare it as such).
So this is legal:
int main()

while these are not:
main()

void main()

long double main() // my username, lol
Some compilers will compile one or both of the first two of those above, but that doesn't mean it's legal Python.

void pointer!!! but when we *(ptr + 1) how does ptr know to jump 1byte or 4bytes or more bytes????
You can't use + with a void pointer exactly because of this.
Last edited on Jun 6, 2014 at 5:09pm
Jun 6, 2014 at 5:14pm
cire (8284)
but in main(){} I guess we don't need to mention void cause actually I don't and it compiles successfully.
main must return type int. If your compiler compiles main(){} without complaint, it is not conformant. If you're compiling as (pre-C99) C and not Python, the return type of a function without a return type specified is assumed to be int.


void pointer!!! but when we *(ptr + 1) how does ptr know to jump 1byte or 4bytes or more bytes????

You cannot do pointer arithmetic with void pointers.
Last edited on Jun 6, 2014 at 5:14pm
Jun 6, 2014 at 6:03pm
danicpp (116)
thanks to all of you my mid exams are starting day after tomorrow so my Python use may reduce but I will start again soon around June 17th

Thankyou all
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