Dev Notes

Various Cheat Sheets and Resources by David Egan/Carawebs.

Function Pointers in C++


C++, Functional, Pointers
David Egan

In C++ like many other languages, functions can be assigned to variables. Functions can be passed into other functions as parameters.

Full Example

#include <iostream>
#include <vector>

typedef std::vector<int> intVec; // Define std::vector type for brevity

// Declare functions
// ---------------------------------------------------------------------------------------------------------------------
// This function receives the function pointer as third parameter
intVec combineVecs(intVec vec1, intVec vec2, int(*func)(int a, int b));

// These functions will be assigned to function pointers:
int multiply(int a, int b);
int xorInts(int a, int b);

int main(int argc, char const *argv[])
{
    intVec v1 = { 1, 1, 1, 0 };
    intVec v2 = { 0, 1, 0, 1, 10 };

    // Set up function pointers
    // -----------------------------------------------------------------------------------------------------------------
    // Declare a function pointer variable.
    // Defines a symbol xorEls, pointer to a function that takes two ints as arguments and returns an int
    int (*xorEls)(int a, int b);

    // Set it with the address of a function (the use of '&' is optional).
    xorEls = &xorInts;

    // Pass the function pointer for use in another function
    // -----------------------------------------------------------------------------------------------------------------
    // Function pointer is the third function argument.
    intVec xored = combineVecs(v1, v2, xorEls);

    for(int& val : xored) {
        std::cout << val << std::endl; // Output result
    }

    // Repeat with a different function to demonstrate the point of function pointers
    // -----------------------------------------------------------------------------------------------------------------
    // Output a new vector which holds the product of each element in two input vectors.
    int (*multiplyEls)(int a, int b);
    multiplyEls = &multiply;

    intVec multiplied = combineVecs(v1, v2, multiplyEls);

    for(int& val : multiplied) {
        std::cout << val << std::endl;
    }

    return 0;
}

// Function definitions
// ---------------------------------------------------------------------------------------------------------------------
/**
 * Combine two integer vectors - operate on elements of input vectors in place
 * by means of a provided function, creating a new vector. Performs the operation
 * up to the length of the shortest provided vector.
 * @param  vec1 std::vector<int>
 * @param  vec2 std::vector<int>
 * @param  func A function pointer
 * @return zipped A std::vector<int> object with each element operated on in-place.
 */
intVec combineVecs(intVec vec1, intVec vec2, int(*func)(int a, int b))
{
    intVec combined;
    typedef typename std::vector<int>::size_type const containerSize;
    containerSize vec1Size = vec1.size();
    containerSize vec2Size = vec2.size();
    containerSize zipLen = (vec1Size >= vec2Size)
        ? vec2Size
        : vec1Size;
    for (size_t i = 0; i < zipLen; i++) {
        // Append the result of the passed-in function to `combined`
        combined.push_back(func(vec1.at(i), vec2.at(i)));
    }
    return combined;
}

int multiply(int a, int b)
{
    return a * b;
}

int xorInts(int a, int b)
{
    return a ^ b;
}

References


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