C++ : Can't pass generic function to another one as a parameter - c++

Question

I want to pass multiple compare functions to the selection sort function as shown below but i get the fallowing error :
Error 1 error C2664: 'sort' : cannot convert parameter 3 from 'bool (__cdecl *)(int,int)' to 'bool *(__cdecl *)(T,T)' c:\users\milad\documents\visual studio 2008\projects\functionpass\functionpass\passcompare.cpp 49 FunctionPass
Code :
bool integerCompare (int a , int b)
{
return(a<b);
}
bool charCompare (char a , char b)
{
return(a<b);
}
bool stringCompare (string a , string b)
{
if(a.compare(b)<0) return true;
else return false;
}
template <class T>
void sort(T x[], int n , bool(*whichCompare(T,T))) // n=size of the array
{
for (int pass=0; pass<n-1; pass++) {
int potentialSmallest = pass;
for (int i=pass+1; i<n; i++) {
if ((*whichCompare)(x[i],x[potentialSmallest])) {
potentialSmallest = i;
}
}
int temp = x[pass];
x[pass] = x[potentialSmallest];
x[potentialSmallest] = temp;
}
}
template <typename T>
void printArray(T a[], int size)
{
for(int i=0;i<size;i++)
cout<<" "<<a[i];
}
int main()
{
int intArray[] = {1,7,-8,-14,46,33,4};
sort <int>(intArray , 7 , integerCompare);
printArray<int>(intArray,7);
}

Answer 1

You have this:
template <class T> void sort(T x[], int n , bool(*whichCompare(T,T)))
{ /*...*/ }
The parameter declaration for a pointer to a function returning a bool and two arguments of type T is wrong. You probably really wanted:
template <class T> void sort(T x[], int n , bool (*whichCompare)(T,T))
{ /*...*/ }
Although typically functions like these are written like this:
template <class T, typename Functor>
void sort(T x[], int n , Functor whichCompare)
{
// ...
bool result = whichCompare(x[i], x[potentialSmallest]);
// ...
}
This way, users can not only pass in function pointers, but function objects that provides an operator()():
struct MyIntegerCompareFunctor
{
bool operator()(int a, int b) { return a < b; }
};
sort(intArray, 7, MyIntegerCompareFunctor());
sort(intArray, 7, &integerCompare); // Works too
Some of the algorithms provided by the C++ standard libraries are written like this.

Answer 2

See my corrections in the lines that have whichCompare in them.
template <class T>
void sort(T x[], int n , bool(*whichCompare)(T,T)) // n=size of the array
{
for (int pass=0; pass<n-1; pass++) {
int potentialSmallest = pass;
for (int i=pass+1; i<n; i++) {
if (whichCompare(x[i],x[potentialSmallest])) {
potentialSmallest = i;
}
}
int temp = x[pass];
x[pass] = x[potentialSmallest];
x[potentialSmallest] = temp;
}
}
You could also templatise the function itself thus:
template< typename T, typename Pred >
void sort( T x[], int n, Pred whichCompare )
{ // etc.
}
I used to do this initially just because it was easier but also allows functors / boost-function/boost-bind etc. to be used with your algorithm.

Answer 3

there is a much more sexy solution:
bool integerCompare (int a , int b)
{
return(a<b);
}
bool charCompare (char a , char b)
{
return(a<b);
}
bool stringCompare (string a , string b)
{
return (a.compare(b)<0);
}
template <typename T, size_t n >
void sort(T (&x)[n], bool whichCompare (T,T) ) // n=size of the array
{
for (int pass=0; pass<n-1; pass++) {
int potentialSmallest = pass;
for (int i=pass+1; i<n; i++) {
if (whichCompare(x[i],x[potentialSmallest])) {
potentialSmallest = i;
}
}
std::swap(x[pass], x[potentialSmallest]);
}
}
template <typename T, size_t n>
void printArray(T (&a)[n])
{
for(int i=0;i<n;i++)
cout<<" "<<a[i];
}
int main()
{
int intArray[] = {1,7,-8,-14,46,33,4};
sort (intArray, integerCompare);
printArray(intArray);
}