Hi, in C and C++ you can use #define to substitute a value in place of an identifier while preprocessing. If you initialize a new string and don't change its value after that, will the compiler substitute the string identifier with its value, like #define in C, or will it make a string in memory and refer to that?

On Tuesday, 14 March 2023 at 05:47:35 UTC, Jeremy wrote:

Manifest constants in D have a similar effect as #defined values, e.g.:

enum someVal = 10;

writeln(someVal);

Here, the effect is the same as if you'd written writeln(10). The difference is that with the preprocessor, it's a text replacement on the source before the compiler gets ahold of it, but in D the compiler handles the substitution internally.

The same is true for string literals:

enum str = "Hello";
writeln(str);

String literals get special treatment from the compiler in that they are "interned".

auto s1 = "What's up?";
auto s2 = "What's up?";

The literal "What's up?" should be stored in the binary only once, so both s1 and s2 will point to the same location. Substitute a manifest constant and the effect should be the same:

enum greeting = "What's up?";
auto s1 = greeting;
auto s2 = greeting;

Just be aware that there's a consequence for array literals:

enum vals = [1, 2, 3];

auto a1 = vals;
auto a2 = vals;

This allocates two dynamic arrays, not one. Everywhere you use vals it's just like using the literal directly, which usually means an allocation.

On Tuesday, 14 March 2023 at 05:47:35 UTC, Jeremy wrote:

In D, you can get #define-like behavior by declaring the string as a manifest constant, like this:

enum myString = "hello";

For more tips on how to translate C preprocessor idioms to D, take a look at "The C Preprocessor vs D" in the "Articles" section of dlang.org.

On Tuesday, 14 March 2023 at 06:20:29 UTC, Mike Parker wrote:

Thanks a lot!

Just out of curiosity:
Can you explain to me why this is called an enum although it's clearly not an enumeration?
Seems like a random abuse of a keyword...

On Wednesday, 15 March 2023 at 16:40:52 UTC, bomat wrote:

It's shorthand for defining an unnamed enum with a single member:

enum
{
    myString = "hello"
}

D lets you leave out the {} in this case, for convenience.

On Wednesday, 15 March 2023 at 19:27:19 UTC, Paul Backus wrote:

Ah, I see. Thanks!