C++Talk.NET

shaun · Guest

Dear all,
I realized an error in a previous post, I reproduce it here because I'm
still not sure how to solve it:

I want to make a templated function which points to one-past-the-end of
a simple array, to pass to a range constructor for a const vector.
Here is some demonstration code:

#include <iostream>
using namespace std;
const double a[]={
2.3,4.5,6.6,8.0,10.0
};

template<class T> T * endof( T parray[]){
int elementSize = sizeof(parray[0]);
int arraySize = sizeof(parray);
cout<<"Inside the function:"< cout<<"Arraysize: "< cout<<"Pointer to array start "< int numel= arraySize/elementSize;
return (parray+numel);
};

int main (int argc, char * const argv[]) {
const double * pEnd=endof(a);
cout<<"Returned pointer: "< cout<<"Outside the function: "< cout<<"Array size: "< cout<<" Element size: "< cout<<"Pointer to array start: "< int numElements=sizeof(a)/sizeof(a[0]);
const double * pEnd2= a+numElements;
cout <<"Pointer to one-past-end: "< cout<<"Pointer to a[5]"<<&(a[5])< return 0;
}

...but my templated function doesn't work at all, here is the output:

Inside the function:
Arraysize: 4 Element size: 8
Pointer to array start 0xe9ef0
Returned pointer: 0xe9ef0

Outside the function:
Array size: 40 Element size: 8
Pointer to array start: 0xe9ef0
Pointer to one-past-end: 0xe9f18

Pointer to a[5]0xe9f18

.....
I can see that inside the function, the argument parray[] has somehow
lost its 'arrayness', so the sizeof(parray) returns the size of the
pointer. Outside the function, this works fine...am i missing some
detail of syntax or is my whole philosophy crooked?

cheers

shaun

Rolf Magnus · Guest

shaun wrote:

Ben Radford · Guest

shaun wrote:

Kai-Uwe Bux · Guest

shaun wrote:

shaun · Guest

OK, thanks all...after some experimenting, I came up with:

#include <iostream>
using namespace std;
const double a[]={
2.3,4.5,6.6,8.0,10.0
};

template<class T> T * endof( T parray[], long unsigned int s){
return (parray+s/sizeof(parray[0]));
};

int main (int argc, char * const argv[]) {
const double * pEnd=endof(a,sizeof(a));
cout<<"Returned pointer: "< cout<<"Outside the function: "< cout<<"Array size: "< cout<<" Element size: "< cout<<"Pointer to array start: "< int numElements=sizeof(a)/sizeof(a[0]);
const double * pEnd2= a+numElements;
cout <<"Pointer to one-past-end: "< cout<<"Pointer to a[5]"<<&(a[5])< return 0;
}

.....

This still allows me to add extra 'magic numbers' in my const array at
the top of the file without changing any subsequent code. I'm not
entirely happy that I have to pass both 'a' and 'sizeof(a)' since it
feels like the 'sizeof' should be redundant, but I understand why it
isn't.

cheers

shaun