Function Pointers and Default Arguments

Function pointers can indeed be used with functions that have default arguments, but there are some important considerations to keep in mind. Let's explore this topic in detail.

First, let's remind ourselves how default arguments work:

1#include <iostream>
2
3void Greet(const char* name = "World") {
4  std::cout << "Hello, " << name << "!";
5}
6
7int main() {
8  Greet();
9  Greet("Alice");
10}

1Hello, World!
2Hello, Alice!

In this example, Greet() can be called with or without an argument.

When using function pointers, it's important to understand that default arguments are not part of the function's type. They are resolved at the call site, not where the function is defined. This means that when you use a function pointer, you lose the default argument information:

1#include <iostream>
2
3void Greet(const char* name = "World") {
4  std::cout << "Hello, " << name << "!";
5}
6
7int main() {
8  void (*greetPtr)(const char*) = Greet;
9
10  // greetPtr();  // This would be an error 
11  greetPtr("Bob");  // This works fine 
12}

1Hello, Bob!

If we were to uncomment the greetPtr(); line, we'd get a compilation error because the function pointer doesn't know about the default argument.

There are a few ways to work around this limitation. We can always provide the argument explicitly:

1#include <iostream>
2
3void Greet(const char* name = "World") {
4  std::cout << "Hello, " << name << "!";
5}
6
7int main() {
8  void (*greetPtr)(const char*) = Greet;
9
10  // Explicitly provide the default
11  greetPtr("World");
12}

1Hello, World!

Alternatively, create a wrapper that is responsible for wrangling arguments and then deferring implementation to the primary function:

1#include <iostream>
2
3void Greet(const char* name = "World") {
4  std::cout << "Hello, " << name << "!\n";
5}
6
7int main() {
8  auto greetFunc{[](const char* name = nullptr){
9    name ? Greet(name) : Greet();
10  }};
11
12  // This uses the default argument
13  greetFunc();
14
15  // This uses "Alice" as the argument
16  greetFunc("Alice");
17}

1Hello, World!
2Hello, Alice!

While function pointers can be used with functions that have default arguments, it's important to remember that the default arguments are not carried over to the function pointer.

You'll need to handle the default values yourself, either by always providing them explicitly or by using one of the workarounds described above.

This behavior is actually a feature of C++, as it allows you to use a function pointer to call a function with different default values than those specified in the original function declaration.

However, it does require extra care when working with function pointers and default arguments together.