Suppose I have this 2 definitions.
template<typename... Ts>
struct S{};
template<auto... Ts>
struct S{};
What's the difference between this 2 definitions?
When we should opt for the one or another?
Difference between typename parameter pack and auto parameter pack?
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2 Answers
They have different purpose, the former allows
S<int, char, float> s;
the later (only available since C++17) allows
S<42, '*', 42U> s;
Templates have 3 kinds of parameters:
- Template type parameters
- Template non-type parameters
- Template template parameters
template<typename... Ts>
struct S{};
Here Ts is a pack of type parameters. It accepts data types like in S<int, long>.
template<auto... Ts>
struct S{};
Here Ts is a pack of non-type parameters with a placeholder type. It accepts values like in S<12, 34L>.
auto allows mixing of values of different types.
template<int... Ts>
struct S{};
This is similar, but all values are of fixed type int.
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