Perfect Forwarding Multiple Arguments

When you have a function that takes multiple arguments of different types and you want to perfectly forward them to another function, you can use std::forward() with a variadic template.

Here's an example:

1#include <iostream>
2#include <string>
3#include <utility>
4
5void process_data(int id,
6  const std::string& name, double value) {
7  std::cout
8    << "ID: " << id
9    << ", Name: " << name
10    << ", Value: " << value << "\n";
11}
12
13template <typename... Args>
14void forward_data(Args&&... args) {
15  process_data(std::forward<Args>(args)...);  
16}
17
18int main() {
19  int id = 42;
20  std::string name = "John";
21  double value = 3.14;
22
23  forward_data(
24    id, name, value);
25  forward_data(
26    std::move(id), name, std::move(value));
27}

1ID: 42, Name: John, Value: 3.14
2ID: 42, Name: John, Value: 3.14

In this example, forward_data() is a variadic template function that takes any number of arguments of different types. It forwards these arguments to process_data() using std::forward<Args>(args)..., which expands the parameter pack args and applies std::forward() to each argument.

By using std::forward<Args>(args)..., each argument is perfectly forwarded to process_data(), preserving its value category (lvalue or rvalue) and const-ness.

In the main() function, forward_data() is called twice:

1. forward_data(id, name, value): Forwards id, name, and value as lvalues.
2. forward_data(std::move(id), name, std::move(value)): Forwards id and value as rvalues using std::move(), while name is forwarded as an lvalue.

This demonstrates how std::forward() can be used with a variadic template to perfectly forward multiple arguments of different types, preserving their value categories and const-ness.