Skip to content

ROS 2 Interfaces

What is a Message Type

In ROS 2 (Robot Operating System 2), a msg type refers to a message definition that contains data structures used for communication between nodes. These messages are the building blocks for publishing and subscribing to topics, which enable nodes to exchange information with each other. Think of a message as a container that holds specific types and amounts of data, like a digital package being sent between nodes in a ROS 2 nodes. Each type of message corresponds to a particular set of data fields and their data types (e.g., int8, float32, String). When you define a message, you're specifying the structure of the data that will be sent or received by nodes. This allows for more organized and efficient communication between nodes.

How to find them

When creating a publisher you have to figure out what data type it is going to publish. To find examples of the different data types in ROS 2 you can run a special command.

ros2 interface show example_interfaces/msg/String #(1)
  1. In this case it was the String type that I used but it could have been any other type. To get a list use Tabbefore typing String for a autocomplete list of options.

Including Message Types

Once you have found the message type you will need to make sure that it is imported. This can be pretty easily in both C++ and Python.

Below you can see that we use the String as the message type. This example works for other types as well of messages as well. You can just swap String for whatever other data type that is available. 

from example_interfaces.msg import String 

#include "example_interfaces/msg/string.hpp"

Variables in ROS 2 Interfaces

You can find more information at the ROS 2 Interface Documentation.

Type name C++ Python DDS type
bool bool builtins.bool boolean
byte uint8_t builtins.bytes* octet
char char builtins.int* char
float32 float builtins.float* float
float64 double builtins.float* double
int8 int8_t builtins.int* octet
uint8 uint8_t builtins.int* octet
int16 int16_t builtins.int* short
uint16 uint16_t builtins.int* unsigned short
int32 int32_t builtins.int* long
uint32 uint32_t builtins.int* unsigned long
int64 int64_t builtins.int* long long
uint64 uint64_t builtins.int* unsigned long long
string std::string builtins.str string
wstring std::u16string builtins.str wstring

Every built-in-type can be used to define arrays:

Type name C++ Python DDS type
static array std::array builtins.list* T[N]
unbounded dynamic array std::vector builtins.list sequence
bounded dynamic array custom_class builtins.list* sequence
bounded string std::string builtins.str* string

All types that are more permissive than their ROS definition enforce the ROS constraints in range and length by software.