auto and typeof

auto #

At times, we have to declare variables that need type declaration on both sides of the = operator. For example, when declaring a file variable (don’t get confused with the type of the variable, we will cover this variable type in a later chapter), we have to repeat the name of the type on both sides of the = operator:

File file = File("student_records", "w");

This looks redundant. Also, it would be cumbersome and error-prone if the type names were long.

VeryLongTypeName var = VeryLongTypeName(/* ... */);

However, specifying the type on the left-hand side is not necessary if we use the auto keyword. In this case, the compiler infers the type of the left-hand side from the expression on the right-hand side. For the compiler to infer the type, the auto keyword can be used:

auto var = VeryLongTypeName(/* ... */);

Few examples of auto:

auto duration = 42;
auto distance = 1.2;
auto greeting = "Hello";
auto vehicle = BeautifulBicycle("blue");

Although auto is the abbreviation of automatic, it does not come from the automatic type inference. It represents an automatic storage class, which is a concept regarding the lifetime of variables. auto is used when no other specifier is appropriate. For example, the following definition does not use auto:

immutable i = 42;

Above, the compiler infers the type of i as an immutable int above. (We will learn about the immutable type in a later chapter.)

typeof #

typeof provides the type of expression (including single variables, objects, literals, etc.) without actually evaluating that expression.

The following is an example of how typeof can be used to specify a type without explicitly spelling it out:

int value = 100; // already defined as 'int'
typeof(value) value2; // means "type of value" 
typeof(100) value3; // means "type of literal 100"

The last two variable definitions above are equivalent to the following:

int value2;
int value3;

It is obvious that typeof is not needed in situations when the actual types are known. Instead, you would typically use typeof in more elaborate scenarios, where you want the type of your variables to be consistent with some other piece of code whose type can vary. This keyword is especially useful in templates and mixins.