c++ - What is decltype and how is it used?

Question

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.

Answer 1

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.