Perfect Forwarding and Overload Resolution

Perfect forwarding using std::forward() can have an impact on overload resolution when forwarding arguments to a function with multiple overloads. The forwarded arguments maintain their value category and const-ness, which can influence which overload gets selected.

Consider the following example:

1#include <iostream>
2
3void process_data(int& x) {
4  std::cout << "process_data(int&): "
5    << x << "\n";
6}
7
8void process_data(const int& x) {
9  std::cout << "process_data(const int&): "
10    << x << "\n";
11}
12
13void process_data(int&& x) {
14  std::cout << "process_data(int&&): "
15    << x << "\n";
16}
17
18template <typename T>
19void forward_data(T&& arg) {
20  process_data(std::forward<T>(arg));
21}
22
23int main() {
24  int a = 10;
25  const int b = 20;
26
27  forward_data(a);
28  forward_data(b);
29  forward_data(30);
30}

1process_data(int&): 10
2process_data(const int&): 20
3process_data(int&&): 30

In this example, process_data() has three overloads:

1. process_data(int& x): Takes a non-const lvalue reference.
2. process_data(const int& x): Takes a const lvalue reference.
3. process_data(int&& x): Takes an rvalue reference.

The forward_data() function template takes a forwarding reference arg and forwards it to process_data() using std::forward<T>(arg).

When forward_data() is called with different arguments:

1. forward_data(a): a is an lvalue, so T deduces to int&. The overload process_data(int&) is selected.
2. forward_data(b): b is a const lvalue, so T deduces to const int&. The overload process_data(const int&) is selected.
3. forward_data(30): 30 is an rvalue, so T deduces to int. The overload process_data(int&&) is selected.

Perfect forwarding with std::forward() ensures that the forwarded arguments maintain their value category and const-ness, allowing the appropriate overload of process_data() to be selected based on the argument type.

This demonstrates how perfect forwarding can impact overload resolution when forwarding arguments to a function with multiple overloads, enabling the selection of the most suitable overload based on the forwarded argument type.