Maps
If you want to create an associative map as part of your data definition, protocol buffers provides a handy shortcut syntax:
map<key_type, value_type> map_field = N;
…where the key_type can be any integral or string type (so, any scalar type except for floating point types and bytes). Note that enum is not a valid key_type. The value_type can be any type except another map.
So, for example, if you wanted to create a map of projects where each Project message is associated with a string key, you could define it like this:
map<string, Project> projects = 3;
The generated map API is currently available for all proto2 supported languages. You can find out more about the map API for your chosen language in the relevant API reference.
Maps Features
- Extensions are not supported for maps.
- Maps cannot be
repeated,optional, orrequired. - Wire format ordering and map iteration ordering of map values is undefined, so you cannot rely on your map items being in a particular order.
- When generating text format for a
.proto, maps are sorted by key. Numeric keys are sorted numerically. - When parsing from the wire or when merging, if there are duplicate map keys the last key seen is used. When parsing a map from text format, parsing may fail if there are duplicate keys.
Backwards compatibility
The map syntax is equivalent to the following on the wire, so protocol buffers implementations that do not support maps can still handle your data:
message MapFieldEntry {
optional key_type key = 1;
optional value_type value = 2;
}
repeated MapFieldEntry map_field = N;
Any protocol buffers implementation that supports maps must both produce and accept data that can be accepted by the above definition.