Parameter pack

Learn parameter pack and expansion.

Parameter pack

A parameter pack holds zero or more values under a single name. A type pack is written typename... Args and a value pack Args... args. The pack is expanded at compile time; separate code is generated for each element.

Example: template <typename... Args> void f(Args... args) takes any number of arguments. The call f(1, 2.0, 'x') produces Args = {int, double, char} and args = {1, 2.0, 'x'}. The pack is expanded inside the function body.

Pack expansion

A pack is expanded automatically when used in a function body. sizeof...(args) gives the number of values. To process pack elements one by one, use a fold expression or a recursive template.

Recursive approach: a function takes the first argument and passes the rest to itself. An overload for the empty pack acts as the base case. This classic technique was common before C++17; fold expressions now offer a shorter form.

Exercise count_args function

Write template <typename... Args> int count_args(Args... args). The body should be return sizeof...(Args);. In main call count_args(1, 2.0, "x") and print the result via std::cout.

Show example solution

#include <iostream>

template <typename... Args>
int count_args(Args... args) {
    return sizeof...(Args);
}

int main() {
    std::cout << count_args(1, 2.0, "x");
    return 0;
}

Test yourself

Complete the value pack

template <typename... Args>
void f(____) { }

Show answer

template <typename... Args>
void f(Args... args) { }

What does sizeof...(args) return?
```
template <typename... Args>
void f(Args... args) { return sizeof...(args); }
```
- The number of values in the pack
- The first type of the pack
- Byte count, not type count
- The address of the first value
Show answer

The number of values in the pack — sizeof...(args) gives the number of values in the pack.
Match each expression to its purpose

typename... Args

Type pack definition
Args... args

Value pack definition
sizeof...(args)

Value count
sizeof...(Args)

Type count

Next lesson: Fold expressions

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