How Does C 17's `auto` Keyword Simplify Template Parameter Deduction?

Advantages of Automatic Template Parameter Deduction in C 17

C 17 introduces the keyword in template parameters, a significant addition that offers numerous advantages.

Natural Extension of auto for Template Instantiation

Similar to the auto keyword used for variable declaration, in template parameters allows you to deduce the type of non-type parameters at instantiation time. It eliminates the need to explicitly specify the parameter type, as seen in the example below:

auto v1 = constant<5>;      // v1 == 5, decltype(v1) is int
auto v2 = constant<true>;   // v2 == true, decltype(v2) is bool
auto v3 = constant<'a'>;    // v3 == 'a', decltype(v3) is char

Enhanced Convenience

Replacing explicit type declarations with greatly simplifies template code, making it more readable and maintainable. Consider the following code:

template <typename Type, Type value> constexpr Type constant = value;
constexpr auto const IntConstant42 = constant<int, 42>;

This code can be rewritten with as follows:

template <auto value> constexpr auto constant = value;
constexpr auto const IntConstant42 = constant<42>;

Improved Code Conciseness

is particularly useful when working with variadic template parameters. For instance, creating a compile-time list of constant values becomes more concise and straightforward:

template <auto ... vs> struct HeterogenousValueList {};
using MyList1 = HeterogenousValueList<42, 'X', 13u>;

template <auto v0, decltype(v0) ... vs> struct HomogenousValueList {};
using MyList2 = HomogenousValueList<1, 2, 3>;

In comparison, achieving the same functionality in pre-C 17 would require more verbose and convoluted constructs involving additional templates.

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