Using qsort() with class pointers

Question

I am using the in-built function qsort() to sort a vector of class item pointers.

class item {
int value;
vector<char> c;
...
...
};

//Declaration of vector
vector<item*> items;

//Function Call
qsort(&items, items.size(), sizeof(item*), value_sort);

int value_sort(const void* a, const void* b)
{
item* pa = *(item**) a;
item* pb = *(item**) b;

if (pb->value < pa->value)
    return 1;
else if (pa->value < pb->value)
    return -1;
return 0;
}

In the debugger mode, pointers neither pa nor pb point to a valid location. Set of all data members of the class items pointed by either pa or pb contain garbage values. Where am I making a mistake? I am also not certain on the usage of double pointers.

Thanks.

Are you populating the vector? It would help to see that code. As it is, you appear to be sorting an empty vector. — Fred Larson
– Fred Larson, Commented Feb 7, 2011 at 19:45

Fred Larson · Accepted Answer · 2011-02-07 19:56:49Z

6

I agree with the answers that advise using std::sort. But ignoring that for the moment, I think the reason for your problem is that you're passing the address of the vector object, not the contents of the vector. Try this:

//Function Call
qsort(&items[0], items.size(), sizeof(item*), value_sort);

Then after you try that, go back and use std::sort instead. 8v)

edited Feb 7, 2011 at 19:56

answered Feb 7, 2011 at 19:48

Fred Larson

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2 Comments

Fred Larson Over a year ago

@ajmartin: Cool. But I hope you are heeding the advice about using std::sort. It is cleaner, easier, and less error-prone. Used with a function object, it's likely faster as well. The compiler can take advantage of inlining, which it can't do with the function pointer approach.

ajmartin Over a year ago

Advice taken, and implemented. Thanks :)

Mark B · Accepted Answer · 2011-02-07 19:45:04Z

4

Don't use qsort in C++, use std::sort instead:

int value_sort(item* pa, item* pb)
{
    return pa->value < pb->value;
}

std::sort(items.begin(), items.end(), value_sort);

answered Feb 7, 2011 at 19:45

Mark B

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1 Comment

ajmartin Over a year ago

This looks must neater. Thanks

UmmaGumma · Accepted Answer · 2011-02-07 20:02:38Z

3

Use std::sort from algorithm. It is easy to use, type safe and faster than qsort and haven't problems with pointers :).

#include <algorithm>

inline bool comparisonFuncion( item *  lhs,item  * rhs)
{
    return lhs->value<rhs->value;
}

std::sort(items.begin(),items.end(),comparisonFunction);

edited Feb 7, 2011 at 20:02

answered Feb 7, 2011 at 19:41

UmmaGumma

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2 Comments

Fred Larson Over a year ago

lhs and rhs aren't pointers (so you'll have a syntax error), but they will need to be to match the contents of the vector.

UmmaGumma Over a year ago

@Fred Larson I changed in function . to -> but forgot to change references to pointers. Thanks for pointing :).

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