C++ Function Pointers


Example

Function pointers are the most basic way of passing functions around, which can also be used in C. (See the C documentation for more details).

For the purpose of callable objects, a function pointer can be defined as:

typedef returnType(*name)(arguments);                       // All
using name = returnType(*)(arguments);                      // <= C++11
using name = std::add_pointer<returnType(arguments)>::type; // <= C++11
using name = std::add_pointer_t<returnType(arguments)>;     // <= C++14

If we would be using a function pointer for writing our own vector sort, it would look like:

using LessThanFunctionPtr = std::add_pointer_t<bool(int, int)>;
void sortVectorInt(std::vector<int>&v, LessThanFunctionPtr lessThan) {
    if (v.size() < 2)
        return;
    if (v.size() == 2) {
        if (!lessThan(v.front(), v.back())) // Invoke the function pointer
            std::swap(v.front(), v.back());
        return;
    }
    std::sort(v, lessThan);
}

bool lessThanInt(int lhs, int rhs) { return lhs < rhs; }
sortVectorInt(vectorOfInt, lessThanInt); // Passes the pointer to a free function

struct GreaterThanInt {
   static bool cmp(int lhs, int rhs) { return lhs > rhs; }
};
sortVectorInt(vectorOfInt, &GreaterThanInt::cmp); // Passes the pointer to a static member function

Alternatively, we could have invoked the function pointer one of following ways:

  • (*lessThan)(v.front(), v.back()) // All
  • std::invoke(lessThan, v.front(), v.back()) // <= C++17