Io.Writer.Allocating: .init vs .fromArrayList

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1

I’m trying to update tools/doctest to use the new Io.Writer interface.

Here is an example of the current code: zig/tools/doctest.zig at master · ziglang/zig · GitHub

The problem is that I’m not sure what I should use between .init and .fromArrayList.

The documentation is not very useful.

Thanks.

2

Its very simple, do you need to use the Arraylist before turning it into a writer? Then use fromArrayList if not, use init preferably initCapacity if you have at least a reasonable assumption required space.

1 Like
3

Here’s how I’d update that test code:

var buffer = std.array_list.Managed(u8).init(test_allocator);
defer buffer.deinit();

try printShell(buffer.writer(), shell_out, false);
try testing.expectEqualSlices(u8, expected, buffer.items);

:down_arrow:

var aw: std.Io.Writer.Allocating = .init(test_allocator);
defer aw.deinit();

try printShell(&aw.writer, shell_out, false);
try testing.expectEqualSlices(u8, expected, aw.written());
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4

This is the same code that I’m currently used.

        var a: Io.Writer.Allocating = .init(test_allocator);
        defer a.deinit();
        const writer = &a.writer;

I got the code from test Allocating test.

Thanks for the confirmation.

Just to be sure, are both .init and .fromArrayList have the same performance?

Is .fromArrayList primary used for ArrayList.print?

5

It just uses the existing allocation in the array list.

It also sets the array list to .empty so you can’t step on the writer’s toes.

6

But, checking the code, Allocating.sendFile creates a new ArrayList each time it is called, so it seems to me that .init is less efficient compared to .fromArrayList.

Update: this is incorrect, since the ArrayList is replaced with empty. Still not sure when you need to use .fromArrayList or .empty.

Thanks

7

I’m having some problems with updating the code in zig/tools/doctest.zig at master · ziglang/zig · GitHub.

Here is the diff with the new code using Io.Writer.Allocating.
The doctest.zig is from the current master.

diff --git a/tools/doctest.zig b/tools/doctest.zig
index a762b8d0b9..266aa2143f 100644
--- a/tools/doctest.zig
+++ b/tools/doctest.zig
@@ -1011,10 +1011,16 @@ fn termColor(allocator: Allocator, input: []const u8) ![]u8 {
     const supported_sgr_colors = [_]u8{ 31, 32, 36 };
     const supported_sgr_numbers = [_]u8{ 0, 1, 2 };
 
-    var buf = std.array_list.Managed(u8).init(allocator);
-    defer buf.deinit();
+    //var buf = std.array_list.Managed(u8).init(allocator);
+    //defer buf.deinit();
+    //
+    //var out = buf.writer();
+
+    var buf: std.ArrayList(u8) = .empty;
+    var a: std.Io.Writer.Allocating = .fromArrayList(allocator, &buf);
+    defer a.deinit();
+    const out = &a.writer;
 
-    var out = buf.writer();
     var sgr_param_start_index: usize = undefined;
     var sgr_num: u8 = undefined;
     var sgr_color: u8 = undefined;
@@ -1118,7 +1124,7 @@ fn termColor(allocator: Allocator, input: []const u8) ![]u8 {
             },
         }
     }
-    return try buf.toOwnedSlice();
+    return try buf.toOwnedSlice(allocator);
 }
 
 // Returns true if number is in slice.

I used .fromArrayList due to the code constraint.
When running zig test tool/doctest.zig, the test fails because the termColor function returns an empty string.

Probably I’m doing something wrong, but I have no idea about
how to fix it.

Update: I fixed it, using a.writer.end in

and

Thanks