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Protozero Advanced Topics

This documentation contains some mixed advanced topics for Protozero users. Read the tutorial 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 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:

enum class ExampleMsg : protozero::pbf_tag_type {
    repeated_uint32_x = 1
};

std::string data = ...
pbf_message<ExampleMsg> 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:

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:

#include <protozero/varint.hpp>

Functions

The following functions are then available:

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:

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:

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.