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P H Kaznowski's questions

Martin Hope
P H Kaznowski
Asked: 2024-08-05 07:08:14 +0800 CST
  • 5

我在 Zig 中有以下内容

 const std = @import("std");

 var messages = [2]*[]u8{undefined, undefined};

 pub fn main() !void {
     // Allocate some strings
     const msg1 = "First message";
     const msg2 = "Second message has different length";
     const allocator = std.heap.page_allocator;
     const msg1_ptr = try allocator.alloc(u8, msg1.len);
     defer allocator.free(msg1_ptr);
     const msg2_ptr = try allocator.alloc(u8, msg2.len);
     defer allocator.free(msg2_ptr);
     messages[0] = @as(*[]u8, &msg1_ptr);
     messages[1] = @as(*[]u8, &msg2_ptr);

它给出了以下错误

src/main.zig:14:30: error: expected type '*[]u8', found '*const []u8'
    messages[0] = @as(*[]u8, &msg1_ptr);
                             ^~~~~~~~~
src/main.zig:14:30: note: cast discards const qualifier
referenced by:
    callMain: /usr/local/Cellar/zig/0.13.0/lib/zig/std/start.zig:524:32
    callMainWithArgs: /usr/local/Cellar/zig/0.13.0/lib/zig/std/start.zig:482:12
    remaining reference traces hidden; use '-freference-trace' to see all reference traces

为了解决这个问题，只需将指针的 const 改为 var 即可

     var msg1_ptr = try allocator.alloc(u8, msg1.len);
     defer allocator.free(msg1_ptr);
     var msg2_ptr = try allocator.alloc(u8, msg2.len);
     defer allocator.free(msg2_ptr);

为什么 const 指针不能强制转换为 const 无关的指针，但将其更改为 var 是有效的？是不是因为 const 意味着不可变，而 var 则不然，所以限制较少？这是否意味着 const 指针永远不能被强制转换？

zig