Sorting an array with own template compare function in C++

Question

template <class T>
bool cmp(const T &a, const T &b){
    return a <= b;
}

template <class T>
void bubble_sort(T tablica[], int size, bool compare(T,T)){
    bool change = true;

    while(change){
        change = false;
        for(int i=0; i < size-1; ++i){
            if(compare(tablica[i+1], tablica[i])){
                zamien(tablica[i+1], tablica[i]);
                change = true;
            }
        }
    }
}

It doesn't work, I have errors:

'void bubble_sort(T [],int,bool (__cdecl *)(T,T))' : 
 could not deduce template argument for 'T []' from 'int [10]'  
'void bubble_sort(T [],int,bool (__cdecl *)(T,T))' :
 cannot use function template 'bool cmp(const T,const T)' as a function argument'

but when I replace a cmp function with that:

bool cmp(const int a, const int b){
    return a <= b;
}

Everything works fine. How to change my cmp function to work with templates?

C++ does not have generics, it has templates. The two may look the same, but they work in fundamentally different ways. — Jasper
– Jasper, Commented Apr 21, 2012 at 14:36
Mine compiled when I changed compare(T,T) to compare (const T &, const T &) and commented out the zamien line. Changing zamien to std::swap produced correct results when the numbers were different at least. — Qaz
– Qaz, Commented Apr 21, 2012 at 14:40
I suppose. But C++ Templates is an ugly-ass bodybuilder who can lift cars, while generics are just an average Joe that happens to be good at dressing up... — Jasper
– Jasper, Commented Apr 21, 2012 at 14:47

mfontanini · Accepted Answer · 2012-04-21 14:53:01Z

2

The problem is that the "compare" function parameter that bubble_sort expects is of type:

bool compare(T,T)

While the "cmp" function is of type:

bool compare(const T&,const T&)

In order to fix it, modify the "compare" parameter's type:

template <class T>
void bubble_sort(T tablica[], int size, bool compare(const T&,const T&)){
    /* ... */
}

edited Apr 21, 2012 at 14:53

answered Apr 21, 2012 at 14:44

mfontanini

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2 Comments

Finley Osbert Over a year ago

I have still that error: 'void bubble_sort(T [],int,bool (__cdecl *)(const T &,const T &))' : could not deduce template argument for 'T []' from 'int [10]' when I want to use it like: bubble_sort(tab, size, cmp), where tab[] = {0,1,2,3,4,5,6,7,8,9}.

Finley Osbert Over a year ago

Ok, on ideone it works perfectly, but when I try to compile it in MS Visual 2010 I have errors.

Finley Osbert · Accepted Answer · 2012-05-19 08:31:42Z

0

This is how I handled with this problem:

int (*cmp_int)(int,int) = compare<int>;
bubble_sort(in, 5, cmp_int);

Now it should work fine in MS Visual.

answered May 19, 2012 at 8:31

Finley Osbert

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