# Protozero Advanced Topics This documentation contains some mixed advanced topics for Protozero users. Read the [tutorial](tutorial.md) first if you are new to Protozero. ## Limitations of Protozero * A protobuf message has to fit into memory completely, otherwise it can not be parsed with this library. There is no streaming support. * The length of a string, bytes, or submessage can't be more than 2^31-1. * There is no specific support for maps but they can be used as described in the "Backwards compatibility" section of https://developers.google.com/protocol-buffers/docs/proto3#maps. ## Checking the Protozero version number If `protozero/version.hpp` is included, the following macros are set: | Macro | Example | Description | | -------------------------- | ------- | ---------------------------------------------- | | `PROTOZERO_VERSION_MAJOR` | 1 | Major version number | | `PROTOZERO_VERSION_MINOR` | 3 | Minor version number | | `PROTOZERO_VERSION_PATCH` | 2 | Patch number | | `PROTOZERO_VERSION_CODE` | 10302 | Version (major * 10,000 + minor * 100 + patch) | | `PROTOZERO_VERSION_STRING` | "1.3.2" | Version string | ## Changing Protozero behaviour with macros The behaviour of Protozero can be changed by defining the following macros. They have to be set before including any of the Protozero headers. ### `PROTOZERO_STRICT_API` If this is set, you will get some extra warnings or errors during compilation if you are using an old (deprecated) interface to Protozero. Enable this if you want to make sure your code will work with future versions of Protozero. ### `PROTOZERO_USE_VIEW` Protozero uses the class `protozero::data_view` as the return type of the `pbf_reader::get_view()` method and a few other functions take a `protozero::data_view` as parameter. If `PROTOZERO_USE_VIEW` is unset, `protozero::data_view` is Protozero's own implementation of a *string view* class. Set this macro if you want to use a different implementation such as the C++17 `std::string_view` class. In this case `protozero::data_view` will simply be an alias to the class you specify. #define PROTOZERO_USE_VIEW std::string_view ## Repeated fields in messages The Google Protobuf spec documents that a non-repeated field can actually appear several times in a message and the implementation is required to return the value of the last version of that field in this case. `pbf_reader.hpp` does not enforce this. If this feature is needed in your case, you have to do this yourself. The [spec also says](https://developers.google.com/protocol-buffers/docs/encoding#packed) that you must be able to read a packed repeated field where a not-packed repeated field is expected and vice versa. Also there can be several (packed or not-packed) repeated fields with the same tag and their contents must be concatenated. It is your responsibility to do this, Protozero doesn't do that for you. ### Using `tag_and_type()` The `tag_and_type()` free function and the method of the same name on the `pbf_reader` and `pbf_message` classes can be used to access both packed and unpacked repeated fields. (It can also be used to check that you have the right type of encoding for other fields.) Here is the outline: ```cpp enum class ExampleMsg : protozero::pbf_tag_type { repeated_uint32_x = 1 }; std::string data = ... pbf_message message{data}; while (message.next()) { switch (message.tag_and_type()) { case tag_and_type(ExampleMsg::repeated_uint32_x, pbf_wire_type::length_delimited): { auto xit = message.get_packed_uint32(); ... // handle the repeated field when it is packed } break; case tag_and_type(ExampleMsg::repeated_uint32_x, pbf_wire_type::varint): { auto x = message.get_uint32(); ... // handle the repeated field when it is not packed } break; default: message.skip(); } } ``` All this works on `pbf_reader` in the same way as with `pbf_message` with the usual difference that `pbf_reader` takes a numeric field tag and `pbf_message` an enum field. If you only want to check for one specific tag and type you can use the two-argument version of `pbf_reader::next()`. In this case `17` is the field tag we are looking for: ```cpp std::string data = ... pbf_reader message{data}; while (message.next(17, pbf_wire_type::varint)) { auto foo = message.get_int32(); ... } ``` See the test under `test/t/tag_and_type/` for a complete example. ## Reserving memory when writing messages If you know beforehand how large a message will become or can take an educated guess, you can call the usual `std::string::reserve()` on the underlying string before you give it to an `pbf_writer` or `pbf_builder` object. Or you can (at any time) call `reserve()` on the `pbf_writer` or `pbf_builder`. This will reserve the given amount of bytes *in addition to whatever is already in that message*. (Note that this behaviour is different then what `reserve()` does on `std::string` or `std::vector`.) In the general case it is not easy to figure out how much memory you will need because of the varint packing of integers. But sometimes you can make at least a rough estimate. Still, you should probably only use this facility if you have benchmarks proving that it actually makes your program faster. ## Using the low-level varint and zigzag encoding and decoding functions Protozero gives you access to the low-level functions for encoding and decoding varint and zigzag integer encodings, because these functions can sometimes be useful outside the Protocol Buffer context. ### Using low-level functions To use the low-level functions, add this include to your C++ program: ```cpp #include ``` ### Functions The following functions are then available: ```cpp decode_varint() write_varint() encode_zigzag32() encode_zigzag64() decode_zigzag32() decode_zigzag64() ``` See the reference documentation created by `make doc` for details. ## Vectored input for length-delimited fields Length-delimited fields (like string fields, byte fields and messages) are usually set by calling `add_string()`, `add_message()`, etc. These functions have several forms, but they basically all take a *tag*, a *size*, and a *pointer to the data*. They write the length of the data into the message and then copy the data over. Sometimes you have the data not in one place, but spread over several buffers. In this case you have to consolidate those buffers first, which needs an extra copy. Say you have two very long strings that should be concatenated into a message: ```cpp std::string a{"very long string..."}; std::string b{"another very long string..."}; std::string data; protozero::pbf_writer writer{data}; a.append(b); // expensive extra copy writer.add_string(1, a); ``` To avoid this, the function `add_bytes_vectored()` can be used which allows vectored (or scatter/gather) input like this: ```cpp std::string a{"very long string..."}; std::string b{"another very long string..."}; std::string data; protozero::pbf_writer writer{data}; writer.add_bytes_vectored(1, a, b); ``` `add_bytes_vectored()` will add up the sizes of all its arguments and copy over all the data only once. The function takes any number of arguments. The arguments must be of a type supporting the `data()` and `size()` methods like `protozero::data_view()`, `std::string` or the C++17 `std::string_view`. Note that there is only one version of the function which can be used for any length-delimited field including strings, bytes, messages and repeated packed fields. The function is also available in the `pbf_builder` class. ## Internal handling of varints When varints are decoded they are always decoded as 64bit unsigned integers and after that casted to the type you are requesting (using `static_cast`). This means that if the protocol buffer message was created with a different integer type than what you are reading it with, you might get wrong results without any warning or error. This is the same behaviour as the Google Protocol Buffers library has. In normal use, this should never matter, because presumably you are using the same types to write that data as you are using to read it later. It can happen if the data is corrupted intentionally or unintentionally in some way. But this can't be used to feed you any data that it wasn't possible to feed you without this behaviour, so it doesn't open up any potential problems. You always have to check anyway that the integers are in the range you expected them to be in if the expected range is different than the range of the integer type. This is especially true for enums which protozero will return as `int32_t`. ## How many items are there in a repeated packed field? Sometimes it is useful to know how many values there are in a repeated packed field. For instance when you want to reserve space in a `std::vector`. ```cpp protozero::pbf_reader message{...}; message.next(...); const auto range = message.get_packed_sint32(); std::vector myvalues; myvalues.reserve(range.size()); for (auto value : range) { myvalues.push_back(value); } ``` It depends on the type of range how expensive the `size()` call is. For ranges derived from packed repeated fixed sized values the effort will be constant, for ranges derived from packed repeated varints, the effort will be linear, but still considerably cheaper than decoding the varints. You have to benchmark your use case to see whether the `reserve()` (or whatever you are using the `size()` for) is worth it.