Variadic templates

Learn variable number of types with variadic templates.

Variadic template

A variadic template takes zero or more type parameters. The syntax is template <typename... Args>. The ellipsis (...) marks a parameter pack and lets Args hold any number of types.

This is the foundation of standard tools like std::make_shared, std::tuple and std::function. A single template handles both foo() and foo(int, double, std::string). The pack is expanded at compile time with no run-time cost.

Pack size

To learn how many elements a parameter pack holds, use sizeof...(Args). The result is a compile-time constant. Example: sizeof...(Args) returns 3 for Args = {int, double, char}.

This count can drive compile-time branches. For example, when the pack is empty sizeof...(Args) == 0 runs different code. The pack size is fixed by the type list the template is called with; the user does not count it manually.

Exercise Write the Count template

Write template <typename... Args> struct Count with static const int value = sizeof...(Args); inside. In main write std::cout << Count<int, double, char>::value;.

Show example solution

#include <iostream>

template <typename... Args>
struct Count {
    static const int value = sizeof...(Args);
};

int main() {
    std::cout << Count<int, double, char>::value;
    return 0;
}

Test yourself

Which line is a variadic template?
- template <int N>
- template <typename T>
- template <typename... Args>
- template <>
Show answer

template <typename... Args> — typename... Args takes zero or more types.

Complete the pack size

template <typename... Args>
struct Count {
    static const int value = ____;
};

Show answer

template <typename... Args>
struct Count {
    static const int value = sizeof...(Args);
};

Match each piece to its role

typename... Args

Type parameter pack
sizeof...(Args)

Pack size
template <typename... Args>

Variadic template
Args... args

Value parameter pack

Next lesson: Parameter pack

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