Can somone explane @Type and how it works

Help
,
1

this give me an error

 error: type 'void' does not support struct initialization syntax
            const ret: type = @Type(.{ .Type = .{ .Struct = .{
                                               ~^

pub fn frame(comptime _scema: type) !type {
    switch (@typeInfo(_scema)) {
        .Struct => |st| {
            var Struc_fields: [st.fields.len]std.builtin.Type.StructField = undefined;
            for (st.fields, 0..) |F, i| {
                Struc_fields[i] = .{
                    .name = F.name,
                    .type = column(F.type),
                    .default_value = undefined,
                    .is_comptime = true,
                    .alignment = 4,
                };
            }
            const ret: type = @Type(.{ .Type = .{ .Struct = .{
                .fields = &Struc_fields,
                .layout = .auto,
                .is_tuple = false,
                .decls = &.{},
            } } });
            return ret;
        },
        else => {
            @compileError("Not a struct");
        },
    }
}
2

This part is wrong. Type is the type, not a field.

3

Documentation for @Type(): Documentation - The Zig Programming Language

In essence, you give it a type specification and it reifies (“materializes”) and returns that type for you to use. It takes a std.builtin.Type, which is a tagged union, so you initialize its members by setting the active tag, and those have lowercase names. So, for example:

const ret = @Type(.{
    .@"struct" = .{
        // ...
    },
};
1 Like
5

i am aware of what it does but this is the problem

 error: use of undefined value here causes undefined behavior
            const ret: type = @Type(.{ .Struct = .{
                                    ~^

Why is it telling me this causes undefined behavior? When I try to be more explicit, it gives me the error above.

6

what is the correct way of doing It?

7

you are giving @Type a struct that contains undefined entries:

.default_value = undefined,

You probably meant to use null (denoting no default value) here.
From the documentation here is an explanation what undefined does:

Translated into English, undefined means “Not a meaningful value. Using this value would be a bug. The value will be unused, or overwritten before being used.”

8

For zig 0.13.0 Type use .Struct (the dotted name of the field):

const ret: type = @Type(.Struct = .{
                .fields = &Struc_fields,

In the latest zig (master/0.14.0) the name of the field is lower case struct and .@"struct" is used because struct is a keyword.

9

the default value can’t be null if .comptime = true
this is the “working” solution

pub fn frame(comptime _scema: type) !type {
    switch (@typeInfo(_scema)) {
        .Struct => |st| {
            var Struc_fields: [st.fields.len]std.builtin.Type.StructField = undefined;
            for (st.fields, 0..) |F, i| {
                Struc_fields[i] = .{
                    .name = F.name,
                    .type = column(F.type),
                    .default_value = null,
                    .is_comptime = false,
                    .alignment = 4,
                };
            }
            const ret: type = @Type(.{ .Struct = .{
                .fields = &Struc_fields,
                .layout = .auto,
                .is_tuple = false,
                .decls = &.{},
            } });
            return ret;
        },
        else => {
            @compileError("Not a struct");
        },
    }
}