Why is it that for
struct A { int a };
using B = const A; // or typedef const A B;
decltype(B::a) evaluates to int and not const int.
What chapter of the How to learn C++ in a life time am I missing?
constness it not propagated for dependent types
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1 Answers
Assuming you meant decltype(B::a), I do not see why you expect it to return const int.
decltype(x) roughly gives you the "type x was declared as", which in this case is int.
Here's an interesting example:
int main()
{
B x{};
static_assert(std::is_same_v<decltype(x.a), int>); // (0)
static_assert(std::is_same_v<decltype((x.a)), const int&>); // (1)
}
(0) tells you that the declared type of x.a is indeed int, even though x is const.
(1) tells you that the value category of x.a is an lvalue, which happens to be const.
(Note that decltype behaves differently with parenthesized expressions.)
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