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Type Deduction with Double Parentheses in decltype
In C , the decltype keyword can be used to determine the type of an expression. When used with double parentheses, it exhibits a subtle behavior that can lead to confusion.
Problem:
Consider the following code snippet:
<code class="cpp">const int&&& foo(); int i; struct A { double x; }; const A* a = new A(); 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&</code>
Why does adding parentheses to a->x in the fourth line change the resulting type from double to const double&?
Answer:
The explanation lies in the rules for deducing types using decltype. According to the C language standard:
In the given example:
Therefore, adding parentheses to a->x changes the type deduction from the type of the member accessed to a reference to the member itself.
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