How Does `decltype(auto)` Expand C Type Deduction Capabilities?

Expanding the Capabilities of decltype(auto)

The introduction of decltype(auto) in C 14 brought a transformative feature that extended the possibilities of type deduction. Beyond its primary purpose of allowing auto declarations to conform to decltype rules, decltype(auto) offers a myriad of other valuable applications.

Return Type Forwarding in Generic Code

In generic code, flawlessly forwarding return types is crucial. Unlike non-generic code, where return types can be manually specified to obtain a reference type, decltype(auto) provides the flexibility to forward return types, regardless of their reference type, without any prior knowledge.

template
decltype(auto) Example(Fun fun, Args&&... args)
{

return fun(std::forward<Args>(args)...); 

}

Delaying Return Type Deduction

In recursive templates, infinite recursion can occur during template instantiation if the return type is specified based on a previous iteration. By utilizing decltype(auto), the return type deduction is postponed until template instantiation is finished, ensuring proper type resolution.

template
struct Int {};

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

template
constexpr auto iter(Int) -> decltype(auto)
{ return iter(Int{}); }

int main() { decltype(iter(Int<10>{})) a; }

Additional Applications

The versatility of decltype(auto) extends to other contexts as well, as outlined in the C draft standard (N3936):

• Variable initialization: decltype(auto) can infer the type of a variable from its initializer, enabling concise and type-safe code.
• Conversion function: decltype(auto) can derive the return type of a conversion function, ensuring proper conversion semantics.
• Lambda expressions: decltype(auto) can deduce the return type of lambda expressions, simplifying generic programming.

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