I haven't been able to find a good explanation of decltype. Please tell me, as a beginning programmer, what it does and why it is useful.
For example, I am reading a book that asked the following question. Can someone explain to me the answer and why, along with some good (beginner-level) examples?
What would be the type of each variable and what value would each variable have when the code finishes?
int a = 3, b = 4; decltype(a) c = a; decltype((b)) d = a; ++c; ++d;
A line-by-line explanation would be very helpful.
decltype is a way to specify a type: You give it an expression, and decltype gives you back a type which corresponds to the type of the expression. Specifically, decltype(e) is the following type:
If e is the name of a variable, i.e. an "id-expression", then the resulting type is the type of the variable.
Otherwise, if e evaluates to an lvalue of type T, then the resulting type is T &, and if e evaluates to an rvalue of type T, then the resulting type is T.
Combining these rules with reference collapsing rules allows you to make sense of decltype(e) &&, which is always a "suitable" reference. (C++14 also adds decltype(auto) to give you the type-deduction of auto combined with the value category semantics of decltype.)
Examples:
int foo();
int n = 10;
decltype(n) a = 20; // a is an "int" [id-expression]
decltype((n)) b = a; // b is an "int &" [(n) is an lvalue]
decltype(foo()) c = foo(); // c is an "int" [rvalue]
decltype(foo()) && r1 = foo(); // int &&
decltype((n)) && r2 = n; // int & [& && collapses to &]
It might be worth stressing the difference between auto and decltype: auto works on types, and decltype works on expressions.
You shouldn't be seeing or using decltype in "day-to-day" programming. It is most useful in generic (templated) library code, where the expression in question is not known and depends on a paramater. (By contrast, auto may be used generously all over the place.) In short, if you're new to programming, you probably won't need to use decltype for some time.
Collected from the Internet
Please contact [email protected] to delete if infringement.
Comments