D语言中固定长度char数组为何支持直接字符串赋值？

Great question! Let's unpack the D language's type rules and array semantics here to understand what's going on.

First, a quick recap to set the stage:

• In D, string is just an alias for immutable(char[]), meaning it's a dynamic array of immutable characters.
• You can't implicitly assign a string to a mutable char[] (dynamic array) because that would let you modify immutable data, which breaks type safety—hence the error when you try char[] baba = "Baba Yaga";. Using .dup creates a mutable copy of the string's data, which is safe.

Now, why does the fixed-size static array char[9] work? Here's the key breakdown:

1. Exact length match
   The string literal "Baba Yaga" has exactly 9 characters (count them: B,a,b,a, ,Y,a,g,a). This matches the size of the static array char[9] perfectly.

2. Static arrays are value types, not references
   Unlike dynamic arrays (char[] or string), static arrays in D store their data directly (on the stack or in static memory) instead of pointing to a separate data buffer. When you initialize a static array with a string literal of the same length, the compiler doesn't try to convert an immutable dynamic array to a mutable one—it directly copies the characters from the literal into the static array's memory.

3. No type safety risk
   Since the static array gets its own copy of the data, there's no connection to the original immutable string literal. You can even modify the static array's contents safely:

   char[9] baba = "Baba Yaga";
   baba[0] = 'b'; // Works perfectly—this modifies the static array's own mutable data
   

To contrast, if you tried to use a static array with a mismatched length (like char[8] baba = "Baba Yaga";), the compiler would throw an error, because it can't fit the 9-character literal into an 8-character static array.

In short: The static array initialization is a direct value copy, not a reference conversion—so the immutability of the original string literal doesn't get in the way.

内容的提问来源于stack exchange，提问作者tastyminerals