Code resides in memory just as data does. You can use pointers to point to functions just as you can use pointers to point to data.
int x = 0;//some data
int* ptr = &x;//ptr points to the address of variable 'x'
int func() { return 0; }//a function
int (*fPtr)() = &func;//a pointer to func
fPtr();//calling the function pointed to by the function pointer
We can use function pointers to allow STL functions to operate in custom ways. Assume we have a struct:
struct Coords
{
int x;
int y;
};
and we want to sort a collection of Coords in a custom way. We can do so by passing a function pointer into the 3rd parameter of the std::sort function (it needs to be a function with the following signature):
bool compareFunc(Coords lhs, Coords rhs);
A vector to sort:
std::vector someCoords;
Assuming some data was placed in someCoords, we then sort it like this:
std::sort(someCoords.begin(), someCoords.end(),compareFunc);
compareFunc must be a global function or a static member function:
struct Coords
{
int x;
int y;
static bool compareFunc(Coords lhs, Coords rhs);
};
Lastly, it is probably more efficient to pass the parameters by const reference rather than by value:
bool compareFunc(const Coords& lhs, const Coords& rhs);
Now, if we wanted to pass a function pointer into another function to do the comparison we would define the function prototype like this:
void doSort(std::vector<Coords> someCoords,
bool(*funcPtr)(const Coords&, const Coords&));
The body of the sort function could be as short as this:
doSort(std::vector<Coords> someCoords, bool(*funcPtr)(const Coords&, const Coords&))
{
std::sort(someCoords.begin(), someCoords.end(), funcPtr);
}
and you could call sort like this:
doSort(someCoords, compareFunc);
Useful links:
https://www.cprogramming.com/tutorial/function-pointers.html
https://stackoverflow.com/questions/1902311/problem-sorting-using-member-function-as-comparator
https://www.geeksforgeeks.org/functors-in-cpp/
