CODEWITHSUNDEEP
c++templatesc++17template-argument-deductiondeduction-guide
Andreas DM
Andreas DM

Reputation: 11008

What are my options for adding deduction guides for stl types

This post already explains how adding deduction guides in the std namespace is undefined.
Now, what I would really like to do is this:

namespace std { // undefined behavior
template <class... U>
array(char const*, U...) -> array<string, 1 + sizeof...(U)>;
}

So this is what I've tried:

template <typename T, std::size_t N>
struct array : std::array<T, N> {};

template <class... U>
array(char const*, U...) -> array<std::string, 1 + sizeof...(U)>;

template <typename T, class... U>
array(T, U...) -> array<T, 1 + sizeof...(U)>;

And it works

auto const arr = array{ "hello", "world" };
// array<std::string, 2ul>

My question now is:
Q: Is this my only option for adding deduction guides for stl types? Are there other options?

Upvotes: 3

Views: 171

Answers (1)

Barry
Barry

Reputation: 303217

Is this my only option for adding deduction guides for stl types?

Yes - in the sense that what you did was not adding a deduction guide for a type in the standard library, it was adding a deduction guide for your own type (that happens to inherit from a standard library type). You can always add deduction guides to your own types.

Are there other options?

This doesn't have to use CTAD. You could also write a function:

auto const arr = make_array("hello", "world");

Such that make_array gives you an std::array<T, N> where T is the decayed type of the first element or, if that type is char const*, string instead.

Upvotes: 2

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