compilation error with sort(Comp)

[June Ye]

Hi,

I have a compilation error with the list sort(Comp). Following is the description of the error:

error C2664: 'void __thiscall std::list<struct people,class std::allocator<struct people> >::sort(struct std::greater<struct people>)' : cannot convert parameter 1 from 'bool (const struct people &,const struct people &)' to 'struct std::greater<struct people>'
No constructor could take the source type, or constructor overload resolution was ambiguous

********************************************
//my code
#include <stdlib.h>
#include <stdio.h>
#include <list>
 
 
using namespace std;
 
typedef struct people
{
    int height;
    int weight;
} people;
 
 
bool compare_height(const people& first, const people& second)
{
    if (first.height > second.height)
        return true;
    else
        return false;
 
}
 
bool compare_weight(const people& first, const people& second)
{
    if (first.weight > second.weight)
        return true;
    else
        return false;
 
}
 
void print(list<people>);
 
 
int main(void)
{
    int count = 0;
 
    list<people> group;
    people a;
 
    printf("please input the data \n");
 
    scanf("%d, %d", &a.height, &a.weight);
 
    while(a.height != 0){
        group.push_back(a);
        scanf("%d, %d", &a.height, &a.weight);
    }
 
    print(group);
 
    printf("after soring \n");
    group.sort();
    print(group);
 
 
    printf("after soring with height\n");
    group.sort(compare_height);
    print(group);
 
    printf("after soring with weight\n");
    group.sort(compare_weight);
    print(group);
 
    return 0;
}
 
void print(list<people> group)
{
    for(list<people>::iterator i = group.begin(); i != group.end(); ++i){
            people m = *i;
            printf("%d, %d \n", m.height, m.weight);
    }
}
 
***********************************************

THANK YOU.

[weaknessforcats]

list::sort requires an operator< for people. In order to sort a call is made to a function that returns true if the frst argument is less than the second argument.

You need to write:

bool operator<(people& lhs, people& rhs)
{
    //TODO: the guts
}

or don't use list::sort but use std::sort instead because there you can specify your own comparator function which means you don't need to name it operator<.

[June Ye]

Thank you.
It seems only allowing one sort function. The default sort() does not work after I overloaded the function. Here is the new code after change.

************************************************
#include <stdlib.h>
#include <stdio.h>
#include <list>
 
 
using namespace std;
 
typedef struct people
{
    int height;
    int weight;
} people;
 
 
bool operator>(const people& first, const people& second)
{
    if (first.height > second.height)
        return true;
    else
        return false;
 
}
 
 
 
void print(list<people>);
 
 
int main(void)
{
    int count = 0;
 
    list<people> group;
    people a;
 
    printf("please input the data \n");
 
    scanf("%d, %d", &a.height, &a.weight);
 
    while(a.height != 0){
        group.push_back(a);
        scanf("%d, %d", &a.height, &a.weight);
    }
 
    print(group);
 
 
    printf("after soring with height\n");
    group.sort( greater<people>());
    print(group);
 
 
    return 0;
}
 
void print(list<people> group)
{
    for(list<people>::iterator i = group.begin(); i != group.end(); ++i){
        people m = *i;
        printf("%d, %d \n", m.height, m.weight);
    }
}
*************************************************

[weaknessforcats]

list::sort calls your operator< not your operator>.

[June Ye]

Thank you, weaknessforcats.
Could you elaborate a little more of your last post?

[June Ye]

weaknessforcats, I guess you mean I should change the code in the compare function to be

if (first.height < second.height)

instead of

if (first.height > second.height)

[weaknessforcats]

What I mean is that list::sort has this code:

if (left < right)
{
etc...

That is a call to operator<() that returns true if left is less that right.

It makes no difference what the code inside operator<() does. For example, if your operator<() returns true if 9 is less than 3 you will find that you have a sort in descending sequence. Otherwise, you have a sort in ascending sequence.

So, your operator<() returns true based on how you define "lessness". Many of the STL containers n algorithms let you pass a function pointer to a comparator function that acts like an operator<(). Therefore, you can have all manner of ascending, descending sorts, including those with inner and outer keys so long as you have a comparator function that knows what "less" means today.

[June Ye]

Thank you.
It is working now.

bool operator<(const people& first, const people& second)
{
if (first.height < second.height)
return true;
else
return false;

}

bool operator>(const people& first, const people& second)
{
if (first.height > second.height)
return true;
else
return false;

}

int main()
{
...
group.sort( );

group.sort( greater<people>());
...
}

Thank you so much for the detail explanation of sort comparison function!