How Does `auto` in C 17 Template Parameters Improve Code and Type Safety?

Benefits of Auto in C 17 Template Parameters

C 17 introduces the highly practical new feature of auto in template parameters. While leveraging the familiarity of using auto when instantiating templates, as seen in the example code provided, this feature extends its applications in numerous ways.

Type Deduction at Instantiation Point

Unlike defining parameters with fixed types, auto in template parameters allows for type deduction at the point of instantiation. This simplifies the code, eliminating the need to explicitly specify types, as seen in the modified example:

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

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

Handy for Variadic Templates

The template feature shines especially when combined with variadic templates. For instance, creating compile-time value lists simplifies code, such as:

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>;

Enhancing Type Safety

By using auto with template parameters, type errors are caught during compilation rather than relying on runtime checks. This approach promotes safer code and improves error detection.

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