etl/docs/Messaging/message-packet.md

---
title: "message_packet"
---

{{< callout type="info">}}
  Header: `message_packet.h`  
{{< /callout >}}

A container for more than one type of message.  
The messages must have been derived from `etl::imessage`.

The messages types that the packet may contain are listed as template parameters.  
e.g. `etl::message_packet<Message1, Message2, Message3>`  

From `20.40.0` message types are not required to have a virtual destructor.

---

```cpp
message_packet()
```
Default constructs a message packet.  
`is_valid()` returns `false`.

---

```cpp
explicit message_packet(const etl::imessage&)
```
Constructs a message packet from an `etl::imessage` reference.  
Asserts an `etl::unhandled_message_exception` error if the parameter is not one listed in the template parameter list.  

---

```cpp
explicit message_packet(etl::imessage&&) C++11
```
Move constructs a message packet from an `etl::imessage` rvalue reference.  
Emits an `etl::unhandled_message_exception` error if the parameter is not one listed in the template parameter list.

---

```cpp
template <typename TMessage>
explicit message_packet(const TMessage&)
```
Constructs a message packet from a `TMessage` reference.
Emits a compile time static assert if the parameter is not one listed in the template parameter list.  
From: `20.22.0`

---

```cpp
template <typename TMessage>
explicit message_packet(TMessage&&)
```
Move constructs a message packet from a `TMessage` rvalue reference.
Emits a compile time static assert if the parameter is not one listed in the template parameter list.  
From: `20.22.0`

---

```cpp
message_packet(const message_packet&)
```
Constructs a message packet from an `message_packet` reference.  
Emits an `etl::unhandled_message_exception` error if the parameter is not one listed in the template parameter list.

---

```cpp
message_packet(message_packet&&)
```
Move constructs a message packet from an `message_packet` rvalue reference.  
Emits an `etl::unhandled_message_exception` if the parameter is not one listed in the template parameter list.

---

```cpp
message_packet& operator =(const message_packet&)
```
Assigns a message packet from an `message_packet` reference.  
Emits an `etl::unhandled_message_exception` error if the parameter is not one listed in the template parameter list.

---

```cpp
message_packet& operator =(message_packet&&)
```
Move assigns a message packet from an `message_packet` rvalue reference.  
Emits an `etl::unhandled_message_exception` error if the parameter is not one listed in the template parameter list.

---

```cpp
etl::imessage& get()
const etl::imessage& get() const
```
Returns a reference to an `etl::imessage`.

---

```cpp
bool is_valid() const
```
Returns `true` if the packet contains a valid message, otherwise `false`.

---

## Constants

`SIZE`      The size of the largest message.
`ALIGNMENT` The largest message alignment.

---

## Example
```cpp
enum
{
  MESSAGE1,
  MESSAGE2,
  MESSAGE3
};

struct Message1 : public etl::message<MESSAGE1>
{
};

struct Message2 : public etl::message<MESSAGE2>
{
};

struct Message3 : public etl::message<MESSAGE3>
{
};

using Packet = etl::message_packet<Message1, Message2>

Message1 message1;
Message2 message2;
Message3 message3;

Packet packet1(message1);
Packet packet2(message2);
Packet packet3(message3); // Runtime time error! Packet does not support Message3 type.

etl::imessage& m1 = message1;
Packet packet4(m1); // Construct from an etl::imessage reference.

etl::imessage& m3 = message3;
Packet packet4(m3); // Asserts etl::unhandled_message_exception! Packet does not support Message3 type.

etl::imessage& m = packet2.get(); // Get a reference to an etl::imessage from the packet.
```