What are the Applications of `decltype(auto)` in C ?

What are some uses of decltype(auto)?

Introduced in C 14, the decltype(auto) idiom allows auto declarations to use the decltype rules on the given expression. While its primary use is for a function's return type deduction, there are other valuable applications.

Return Type Forwarding in Generic Code

In generic code, decltype(auto) enables perfect forwarding of a return type without knowing its underlying type. This is particularly useful for generic functions:

template<class Fun, class... Args>
decltype(auto) Example(Fun fun, Args&&... args) {
    return fun(std::forward<Args>(args)...);
}

Delaying Return Type Deduction in Recursive Templates

decltype(auto) can also delay return type deduction in recursive templates, preventing infinite recursion. For example:

template<int i>
struct Int {};

constexpr auto iter(Int<0>) -> Int<0>;

template<int i>
constexpr auto iter(Int<i>) -> decltype(auto) {
    return iter(Int<i-1>{});
}

Other Uses

Beyond these primary applications, decltype(auto) has various other uses, as outlined in the C draft Standard (N3936):

• Variable initialization:

  decltype(auto) x3d = i;  // decltype(x3d) is int

• Pointer declaration:

  decltype(auto)*x7d = &i;  // decltype(x7d) is int*

• Representing a placeholder type in specific contexts, such as function declarators, type-specifier-seqs, and conversion-function-ids.

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