Passing Parameter Packs by Value or Reference

The choice of passing parameters by value, reference, const reference, or forwarding reference depends on your specific use case:

Pass by value when:

• The parameter is a small, cheap-to-copy type like int, double, bool, etc.
• You want to work with a copy of the argument and avoid modifying the original

Pass by reference (&) when:

• The parameter is a large, expensive-to-copy type like a vector or string
• You need to modify the original argument and have those changes persist after the function call

Pass by const reference (const &) when:

• The parameter is a large, expensive-to-copy type
• You don't need to modify the original argument
• You want to avoid the overhead of copying

Pass by forwarding reference (&&) when:

• You need to forward the arguments to another function while preserving their value category (lvalue/rvalue)
• You want to avoid unnecessary copying and enable perfect forwarding

For example:

1template <typename... Types>
2void LogByValue(Types... Args) {
3  // Each arg copied into the function
4  (std::cout << ... << Args) << '\n';
5}
6
7template <typename... Types>
8void LogByReference(Types&... Args) {
9  // Each arg passed by reference
10  (std::cout << ... << Args) << '\n';
11  Args...;  // Potential to modify arguments
12}
13
14template <typename... Types>
15void LogByConstRef(const Types&... Args) {
16  // Each arg passed by const reference
17  (std::cout << ... << Args) << '\n';
18  // Args...; // Can't modify arguments
19}
20
21template <typename... Types>
22void LogAndForward(Types&&... Args) {
23  (std::cout << ... << Args) << '\n';
24  SomeOtherFunction(std::forward<Types>(Args)...);
25}

In general, prefer passing by value for small types and const reference for large types. Use non-const references when you need to modify arguments, and forwarding references when you need to forward arguments while preserving their original value category.