Function Pointers and Templates

Function pointers and function templates can interact in interesting and powerful ways, but they also present some challenges. Let's explore this relationship in detail.

Basic Function Templates

First, let's remind ourselves what a function template looks like:

1#include <iostream>
2
3template<typename T>
4T Add(T a, T b) {
5  return a + b;
6}
7
8int main() {
9  std::cout << Add(5, 3) << "\n";
10  std::cout << Add(5.5, 3.2);
11}

18
28.7

The compiler generates specific functions based on the types used when calling the template function.

Function Pointers to Template Functions

When using function pointers with template functions, we need to be explicit about which instantiation of the template we're referring to:

1#include <iostream>
2
3template <typename T>
4T Multiply(T a, T b) { return a * b; }
5
6int main() {
7  int (*intMultiply)(int, int) = Multiply<int>;
8  
9  double (*doubleMultiply)(double, double) =
10    Multiply<double>; 
11
12  std::cout << intMultiply(5, 3) << "\n";
13  std::cout << doubleMultiply(5.5, 3.2);
14  return 0;
15}

115
217.6

Here, we're creating function pointers to specific instantiations of the Multiply template function.

Challenges with Template Deduction

One challenge is that template argument deduction doesn't work with function pointers. This won't compile:

1template<typename T>
2T Square(T x) {
3  return x * x;
4}
5
6int main() {
7  auto funcPtr = Square;
8}

1error: unable to deduce 'auto' from 'Square'

The compiler can't deduce which instantiation of Square to use because there's no context for type deduction.

Solutions to Template Deduction

We can solve this in a few ways. We can specify the template arguments explicitly:

1#include <iostream>
2
3template<typename T>
4T Square(T x) {
5  return x * x;
6}
7
8int main() {
9  auto funcPtr = Square<int>;
10  std::cout << funcPtr(5);
11}

125

Alternatively, use a type alias with a specific type:

1#include <iostream>
2
3template<typename T>
4T Square(T x) {
5  return x * x;
6}
7
8using IntSquare = int(*)(int);
9
10int main() {
11  IntSquare funcPtr = Square;
12  std::cout << funcPtr(5);
13}

125

Function Templates with Non-Type Parameters

Function templates can also have non-type parameters, which can be used with function pointers:

1#include <array>
2#include <iostream>
3
4template <typename T, size_t Size>
5void PrintArray(
6  const std::array<T, Size>& arr) {
7  for (const auto& item : arr) {
8    std::cout << item << " ";
9  }
10  std::cout << "\n";
11}
12
13int main() {
14  std::array<int, 3> intArr = {1, 2, 3};
15  std::array<double, 2> doubleArr = {1.1, 2.2};
16
17  void (*intPrinter)(const std::array<int, 3>&)
18    = PrintArray; 
19  void (*doublePrinter)(
20    const std::array<double, 2>&) = PrintArray;
21  
22
23  intPrinter(intArr);
24  doublePrinter(doubleArr);
25}

11 2 3
21.1 2.2

In this case, both the type and the non-type template parameters are part of the function pointer type.

Understanding these interactions is crucial when working with both function templates and function pointers in C++. While they provide a lot of flexibility, they also require careful handling to ensure type safety and avoid compilation errors.