When and Why Do Double Parentheses Matter in `decltype((...))` in C ?

Clarifying the Role of Parentheses in decltype((...))

In C , decltype is employed to determine the data type of a given expression. However, when double parentheses are used within the decltype construct, it signifies two distinct scenarios that impact the resulting type.

Case 1: Class Member Access

Without parentheses, decltype(e) derives the type of an expression e that represents either an unparenthesized identifier or class member access. For instance, decltype(a->x) considers a->x as the expression and deduces its type as the type of the class member x. In the provided example, a->x refers to a double, so x3 is declared as double.

Case 2: Lvalue Extraction

When parentheses enclose an expression e, decltype(e) retrieves the type of the lvalue (amodifiable value) represented by e. In this context, an lvalue is typically a variable or an object accessible through a reference or pointer. For instance, decltype((a->x)) encloses the class member access expression a->x within parentheses, effectively extracting the lvalue of x3. Since x3 is of type double, x4 is declared as const double & (a reference to a constant double).

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