Function hasUniqueRepresentation [src]

True if every value of the type T has a unique bit pattern representing it. In other words, T has no unused bits and no padding. Result is always comptime-known.

Prototype

pub inline fn hasUniqueRepresentation(comptime T: type) bool

Parameters

T: type

Example

 test hasUniqueRepresentation {
    const TestStruct1 = struct {
        a: u32,
        b: u32,
    };

    try testing.expect(hasUniqueRepresentation(TestStruct1));

    const TestStruct2 = struct {
        a: u32,
        b: u16,
    };

    try testing.expect(!hasUniqueRepresentation(TestStruct2));

    const TestStruct3 = struct {
        a: u32,
        b: u32,
    };

    try testing.expect(hasUniqueRepresentation(TestStruct3));

    const TestStruct4 = struct { a: []const u8 };

    try testing.expect(!hasUniqueRepresentation(TestStruct4));

    const TestStruct5 = struct { a: TestStruct4 };

    try testing.expect(!hasUniqueRepresentation(TestStruct5));

    const TestStruct6 = packed struct(u8) {
        @"0": bool,
        @"1": bool,
        @"2": bool,
        @"3": bool,
        @"4": bool,
        @"5": bool,
        @"6": bool,
        @"7": bool,
    };

    try testing.expect(hasUniqueRepresentation(TestStruct6));

    const TestUnion1 = packed union {
        a: u32,
        b: u16,
    };

    try testing.expect(!hasUniqueRepresentation(TestUnion1));

    const TestUnion2 = extern union {
        a: u32,
        b: u16,
    };

    try testing.expect(!hasUniqueRepresentation(TestUnion2));

    const TestUnion3 = union {
        a: u32,
        b: u16,
    };

    try testing.expect(!hasUniqueRepresentation(TestUnion3));

    const TestUnion4 = union(enum) {
        a: u32,
        b: u16,
    };

    try testing.expect(!hasUniqueRepresentation(TestUnion4));

    inline for ([_]type{ i0, u8, i16, u32, i64 }) |T| {
        try testing.expect(hasUniqueRepresentation(T));
    }
    inline for ([_]type{ i1, u9, i17, u33, i24 }) |T| {
        try testing.expect(!hasUniqueRepresentation(T));
    }

    try testing.expect(hasUniqueRepresentation(*u8));
    try testing.expect(hasUniqueRepresentation(*const u8));
    try testing.expect(hasUniqueRepresentation(?*u8));
    try testing.expect(hasUniqueRepresentation(?*const u8));

    try testing.expect(!hasUniqueRepresentation([]u8));
    try testing.expect(!hasUniqueRepresentation([]const u8));
    try testing.expect(!hasUniqueRepresentation(?[]u8));
    try testing.expect(!hasUniqueRepresentation(?[]const u8));

    try testing.expect(hasUniqueRepresentation(@Vector(std.simd.suggestVectorLength(u8) orelse 1, u8)));
    try testing.expect(@sizeOf(@Vector(3, u8)) == 3 or !hasUniqueRepresentation(@Vector(3, u8)));

    const StructWithComptimeFields = struct {
        comptime should_be_ignored: u64 = 42,
        comptime should_also_be_ignored: [*:0]const u8 = "hope you're having a good day :)",
        field: u32,
    };

    try testing.expect(hasUniqueRepresentation(StructWithComptimeFields));
}

Source

  pub inline fn hasUniqueRepresentation(comptime T: type) bool {
    return switch (@typeInfo(T)) {
        else => false, // TODO can we know if it's true for some of these types ?

        .@"anyframe",
        .@"enum",
        .error_set,
        .@"fn",
        => true,

        .bool => false,

        .int => |info| @sizeOf(T) * 8 == info.bits,

        .pointer => |info| info.size != .slice,

        .optional => |info| switch (@typeInfo(info.child)) {
            .pointer => |ptr| !ptr.is_allowzero and switch (ptr.size) {
                .slice, .c => false,
                .one, .many => true,
            },
            else => false,
        },

        .array => |info| hasUniqueRepresentation(info.child),

        .@"struct" => |info| {
            if (info.layout == .@"packed") return @sizeOf(T) * 8 == @bitSizeOf(T);

            var sum_size = @as(usize, 0);

            inline for (info.fields) |field| {
                if (field.is_comptime) continue;
                if (!hasUniqueRepresentation(field.type)) return false;
                sum_size += @sizeOf(field.type);
            }

            return @sizeOf(T) == sum_size;
        },

        .vector => |info| hasUniqueRepresentation(info.child) and
            @sizeOf(T) == @sizeOf(info.child) * info.len,
    };
}  