How to have struct members accessible in different ways
- by Paul J. Lucas
I want to have a structure token that has start/end pairs for position, sentence, and paragraph information. I also want the members to be accessible in two different ways: as a start/end pair and individually. Given:
struct token {
struct start_end {
int start;
int end;
};
start_end pos;
start_end sent;
start_end para;
typedef start_end token::*start_end_ptr;
};
I can write a function, say distance(), that computes the distance between any of the three start/end pairs like:
int distance( token const &i, token const &j, token::start_end_ptr mbr ) {
return (j.*mbr).start - (i.*mbr).end;
}
and call it like:
token i, j;
int d = distance( i, j, &token::pos );
that will return the distance of the pos pair. But I can also pass &token::sent or &token::para and it does what I want. Hence, the function is flexible.
However, now I also want to write a function, say max(), that computes the maximum value of all the pos.start or all the pos.end or all the sent.start, etc.
If I add:
typedef int token::start_end::*int_ptr;
I can write the function like:
int max( list<token> const &l, token::int_ptr p ) {
int m = numeric_limits<int>::min();
for ( list<token>::const_iterator i = l.begin(); i != l.end(); ++i ) {
int n = (*i).pos.*p; // NOT WHAT I WANT: It hard-codes 'pos'
if ( n > m )
m = n;
}
return m;
}
and call it like:
list<token> l;
l.push_back( i );
l.push_back( j );
int m = max( l, &token::start_end::start );
However, as indicated in the comment above, I do not want to hard-code pos. I want the flexibility of accessible the start or end of any of pos, sent, or para that will be passed as a parameter to max().
I've tried several things to get this to work (tried using unions, anonymous unions, etc.) but I can't come up with a data structure that allows the flexibility both ways while having each value stored only once.
Any ideas how to organize the token struct so I can have what I want?
Attempt at clarification
Given struct of pairs of integers, I want to be able to "slice" the data in two distinct ways:
By passing a pointer-to-member of a particular start/end pair so that the called function operates on any pair without knowing which pair. The caller decides which pair.
By passing a pointer-to-member of a particular int (i.e., only one int of any pair) so that the called function operates on any int without knowing either which int or which pair said int is from. The caller decides which int of which pair.
Another example for the latter would be to sum, say, all para.end or all sent.start.
Also, and importantly: for #2 above, I'd ideally like to pass only a single pointer-to-member to reduce the burden on the caller. Hence, me trying to figure something out using unions.
