[pcwalton]

I think another way to put it is that the fact that Zig reuses the keyword "comptime" to denote type-level parameters and to denote compile-time evaluation doesn't mean that there's only one feature. There are still two features (templates and CTFE), just two features that happen to use the same keyword.

[pron]

Maybe you can insist that these are two features (although I disagree), but calling one of them templates really misses the mark. That's because, at least in C++, templates have their own template-level language (of "metafunctions"), whereas that's not the case in Zig. E.g. that C++'s `std::enable_if` is just the regular `if` in Zig makes all the difference (and also shows why there may not really be two features here, only one).

[TinkersW]

std::enable_if is not the correct comparison, I think you mean "if constexpr"

enable_if is mostly deprecated, and was used for overloading not branching, you can use concepts now instead

[sekao]

Agreed. Zig's approach re-uses the existing machinery of the language far more than C++ templates do. Another example of this is that Zig has almost no restrictions on what kinds of values can be `comptime` parameters. In C++, "non-type template parameters" are restricted to a small subset of types (integers, enums, and a few others). Rust's "const generics" are even more restrictive: only integers for now.

In Zig I can pass an entire struct instance full of config values as a single comptime parameter and thread it anywhere in my program. The big difference here is that when you treat compile-time programming as a "special" thing that is supported completely differently in the language, you need to add these features in a painfully piecemeal way. Whereas if it's just re-using the machinery already in place in your language, these restrictions don't exist and your users don't need to look up what values can be comptime values...they're just another kind of thing I pass to functions, so "of course" I can pass a struct instance.

[WalterBright]

> Zig has almost no restrictions on what kinds of values can be `comptime` parameters.

Neither does D. The main restriction is the CTFE needs to be pure. I.e. you cannot call the operating system in CTFE (this is a deliberate restriction, mainly to avoid clever malware).

CTFE isn't "special" in D, either. CTFE is triggered for any instance of a "constant expression" in the grammar, and doesn't require a keyword.

[edflsafoiewq]

std::enable_if exists to disable certain overloads during overload resolution. Zig has no overloading, so it has no equivalent.

[matklad]

I'd flip it over and say that C++ has overloading&SFINAE to enable polymorphism which it otherwise can't express.

[edflsafoiewq]

Such as? The basic property of overloading is it's open. Any closed set of overloads can be converted to a single function which does the same dispatch logic with ifs and type traits (it may not be very readable).

[edflsafoiewq]

They are the same thing though. Conceptually there's a partial evaluation pass whose goal is to eliminate all the comptimes by lowering them to regular runtime values. The apparent different "features" just arise from its operation on the different kinds of program constructs. To eliminate a expression, it evaluates the expression and replaces it with its value. To eliminate a loop, it unrolls it. To eliminate a call to a function with comptime arguments, it generates a specialized function for those arguments and replaces it with a call to the specialized function.