Variadic Template

• Variadic Templates are used when don’t know how many templates are needed (can take 0 ~ n)
• We can do this by using parameter packs

In functions (T… args is called function parameter pack)

template<typename... T>
void function(T... args) { }

In class/struct (T is called template parameter pack)

template<typename... T>
class SampleClass{};

How to use them

• For classes, parameter packs need to be the final parameter in the parameter list
• For functions, parameter packs can be placed earlier if everything can be deduced from the function arguments

template<typename... Ts, typename U, typename=void>
void valid(U, Ts...);

Above is valid because we know the first argument will be U and the rest will be Ts

void valid(Ts..., U);

Above is not valid because when parameter pack is placed in the front, Ts cannot be deduced.

Pack Expansion

• Pack Expansion can be done recursively
• Which means we need to take care of base case

auto calc() {
    return 0;
}

template<typename T1, typename... T2>
auto calc(T1 t1, T2... t2) {
    return t1 + calc(t2...);
}

calc(1,2,3);

1. 1,2,3 are passed into the second method since it has more than one arg.
2. t1 = 1 and t2 = 2,3
3. then we need to unpack t2 into 2 and 3
4. call(2,3) which makes t1 = 2 and t2 = 3
5. this continues until we no longer have any args

Sources

• parameter pack: https://en.cppreference.com/w/cpp/language/parameter_pack
• pack expansion: https://www.modernescpp.com/index.php/more-arbout-variadic-templates
• pack expansion: https://kevinushey.github.io/blog/2016/01/27/introduction-to-c++-variadic-templates/
• parameter pack placement: https://stackoverflow.com/questions/38629601/function-template-parameter-pack-not-at-the-end-of-the-parameter-list#comment64648723_38629697