How Does `decltype(auto)` Improve Return Type Deduction and Template Usage in C ?

Uses of the 'decltype(auto)' Language Feature

Introduced in C 14, the 'decltype(auto)' idiom allows 'auto' declarations to leverage 'decltype' rules on specified expressions, primarily for return type deduction in functions. However, its utility extends beyond this scenario.

Return Type Forwarding in Generic Code

For non-generic code, explicit selection of return types as references is possible. However, in generic code, 'decltype(auto)' enables accurate return type forwarding regardless of whether the return type is a reference or a value.

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

When defining recursive templates, 'decltype(auto)' can postpone return type deduction until after template instantiation.

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>{}); }

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

Additional Applications

According to the draft Standard N3936, 'decltype(auto)' can appear in various contexts, including function declarators, type specifiers, and variable initialization. These contexts provide utility for actions such as return type deduction, type placeholder designation, and safe value assignment.

int i;
int&& f();
auto x3a = i;              // decltype(x3a) is int
decltype(auto) x3d = i;    // decltype(x3d) is int
auto x4a = (i);            // decltype(x4a) is int
decltype(auto) x4d = (i);  // decltype(x4d) is int&
auto x5a = f();            // decltype(x5a) is int
decltype(auto) x5d = f();  // decltype(x5d) is int&&

The above is the detailed content of How Does `decltype(auto)` Improve Return Type Deduction and Template Usage in C ?. For more information, please follow other related articles on the PHP Chinese website!