Why Does `decltype` Behavior Change with Double Parentheses?

Advanced Applications of decltype

In C , the decltype operator is a powerful tool for determining the type of an expression. While it typically operates as you might expect, there are circumstances where the use of double parentheses can make a significant difference.

As stated in the FCD (§7.6.1.2/4):

const int&&& foo();
int i;
struct A { double x; };
const A* a = new A();

Consider the following example:

decltype(foo()) x1 = i;     // type is const int&&&
decltype(i) x2;             // type is int
decltype(a->x) x3;          // type is double
decltype((a->x)) x4 = x3;   // type is const double&

The question arises: why do the parentheses make a difference in the last line? Intuitively, one might assume that it should simply be double, similar to the line above.

However, the FCD provides some insight:

• If e is an unparenthesized id-expression or a class member access (5.2.5), decltype(e) is the type of the entity named by e.
• If e is an lvalue, decltype(e) is T&, where T is the type of e.

In this case, decltype(a->x) is a class member access, and thus, x3 has the type of the entity named by a->x, which is double. However, decltype((a->x)) is an lvalue, and therefore, x4 has the type of a->x, which is const double&.

This distinction is subtle but critical in understanding the nuances of decltype. By leveraging the power of double parentheses, programmers can manipulate and determine types with greater precision, allowing for more robust and flexible code.

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