Sorting by class value

[Michael]

Suppose I have:
#include <vector>
using std::vector;

class C
{
public:
int a,b;
};

int main()
{

vector<C> vecC;
//Populate vecC

how do i now sort according to a or b?? do I pass in a function pointer
to sort(.....)??
}
Regards

Mike

[llewelly]

"Michael" <sl***********@hotmail.com> writes:
[snip]

how do i now sort according to a or b?? do I pass in a function pointer
to sort(.....)??

You may use either a pointer to a function or a functor:

#include <vector>
#include <algorithm>
using std::vector;
using std::sort;

class C
{
public:
int a,b;
};

bool comp(C const& lhs, C const& rhs){return lhs.a < rhs.a;}

struct compare
{
bool operator()(C const& lhs, C const& rhs){return lhs.a < rhs.a;}
};
int main()
{
vector<C> vecC;
//Use pointer to function.
sort(vecC.begin(),vecC.end(),comp);
//Use functor.
sort(vecC.begin(),vecC.end(),compare());
}

[Thomas Matthews]

Michael wrote:

Suppose I have:
#include <vector>
using std::vector;

class C
{
public:
int a,b;
};

int main()
{

vector<C> vecC;
//Populate vecC

how do i now sort according to a or b?? do I pass in a function pointer
to sort(.....)??
}
Regards

Mike

Create an ordering functor:
class a_less_b
{
public:
bool operator() (const C& c1, const C& c2)
{return c1.a < c2.b;}
};

Use the function when sorting the vector:
std::sort(vecC.begin(), vecC.end(), a_less_b());
--
Thomas Matthews

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[joey tribbiani]

Michael wrote:

Suppose I have:
#include <vector>
using std::vector;

class C
{
public:
int a,b;
};

int main()
{

vector<C> vecC;
//Populate vecC

how do i now sort according to a or b?? do I pass in a function pointer
to sort(.....)??
}

You could overload the < operator for C
class C {
public:
int a,b;
bool operator < ( C c )
{ return a < c.a; }
}

[Siemel Naran]

"llewelly" <ll*********@xmission.dot.com> wrote in message

struct compare
{
bool operator()(C const& lhs, C const& rhs){return lhs.a < rhs.a;}
};

Typically it should be a const function.

bool operator()(C const& lhs, C const& rhs) const {return lhs.a < rhs.a;}