Compile time function execution (CTFE)

CTFE is a mechanism which allows the compiler to execute functions at compile time. There is no special set of the D language necessary to use this feature. The only requirement is that the function just depends on values known at compile time.

Keywords like static, immutable or enum instruct the compiler to use CTFE.

int sum(int a, int b)
{
    return a + b;
}

void main()
{
    enum a = 5;
    enum b = 7;
    enum c = sum(a, b);
}

In the above example, sum(a, b) is evaluated at compile time. In the object file, no call to sum is issued.

TODO: quiz

:warning: If the type of a or b is changed to int, the compiler will issue an error. Why?

Practice

Pragmas are compile time instructions used to pass or ask the compiler for specific information. pragma(msg) is used as a compile time debugging tool that allows printing of compile time known variables:

class Foo {}
Foo a;
enum b = 2;
pragma(msg, "hello");     // prints "hello" at compile time
pragma(msg, Foo);         // prints Foo at compile time
pragma(msg, a);           // error, a cannot be read at compile time
pragma(msg, typeof(a));   // prints Foo at compile time
pragma(msg, b);           // prints 2 at compile time

Use pragma(msg) inside the code snippet above to observe the output when compiling the code.