TechOverflow Logo
TechOverflow

NanoPB: How to handle oneof (union) types in C++

See also: C version: How to handle oneof/union types in C

NanoPB is a code-size optimized Protocol Buffers implementation for embedded systems. This post shows how to handle oneof (union) types in C++ with NanoPB.

Proto definition

First, create a .proto file with oneof fields:

oneof.proto
syntax = "proto3";

package example;

message OneofMessage {
  oneof value {
    uint32 int_value = 1;
    float float_value = 2;
    string string_value = 3;
  }
}

Generate NanoPB code

Generate the NanoPB code with a .options file to specify string buffer sizes:

Create oneof.options:

example.txt
example.OneofMessage.string_value max_size:32

Then generate:

example.sh
protoc --nanopb_out=. oneof.proto

This will generate oneof.pb.h and oneof.pb.c.

C++ example

Here’s a complete C++ example handling oneof fields:

oneof_example.cpp
#include <stdio.h>
#include <string.h>
#include "oneof.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

int main() {
    // Buffer for encoded message
    uint8_t buffer[128];
    size_t message_length;
    
    // --- ENCODING (int_value) ---
    example_OneofMessage message = example_OneofMessage_init_zero;
    
    // Set oneof field (only one can be set at a time)
    message.which_value = example_OneofMessage_int_value_tag;
    message.value.int_value = 42;
    
    // Create stream for encoding
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    
    // Encode the message
    if (!pb_encode(&ostream, example_OneofMessage_fields, &message)) {
        printf("Encoding failed: %s\n", PB_GET_ERROR(&ostream));
        return 1;
    }
    
    message_length = ostream.bytes_written;
    printf("Encoded %zu bytes (int_value)\n", message_length);
    
    // Print hex dump
    printf("Encoded data: ");
    for (size_t i = 0; i < message_length; i++) {
        printf("%02x ", buffer[i]);
    }
    printf("\n");
    
    // --- DECODING ---
    example_OneofMessage decoded = example_OneofMessage_init_zero;
    
    // Create stream for decoding
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    
    // Decode the message
    if (!pb_decode(&istream, example_OneofMessage_fields, &decoded)) {
        printf("Decoding failed: %s\n", PB_GET_ERROR(&istream));
        return 1;
    }
    
    // Print decoded value
    printf("Decoded value:\n");
    switch (decoded.which_value) {
        case example_OneofMessage_int_value_tag:
            printf("  int_value: %u\n", decoded.value.int_value);
            break;
        case example_OneofMessage_float_value_tag:
            printf("  float_value: %f\n", decoded.value.float_value);
            break;
        case example_OneofMessage_string_value_tag:
            printf("  string_value: %s\n", decoded.value.string_value);
            break;
        default:
            printf("  (none)\n");
            break;
    }
    
    return 0;
}

Compile command

Compile the example with nanopb. NanoPB is typically used by including the source files directly in your project:

example.sh
g++ -o oneof_example oneof_example.cpp oneof.pb.c pb_common.c pb_encode.c pb_decode.c -I.

Note: NanoPB source files (pb_common.c, pb_encode.c, pb_decode.c) need to be compiled directly with your project. You can obtain these from the NanoPB GitHub repository.

Python test script

To verify the encoding, you can use Python’s protobuf library:

test_oneof.py
import oneof_pb2

# Read the binary data
with open('encoded.bin', 'rb') as f:
    data = f.read()

# Decode
msg = oneof_pb2.OneofMessage()
msg.ParseFromString(data)

print("Python decoded values:")
print(f"  Which oneof: {msg.WhichOneof('value')}")
if msg.HasField('int_value'):
    print(f"  int_value: {msg.int_value}")
if msg.HasField('float_value'):
    print(f"  float_value: {msg.float_value}")
if msg.HasField('string_value'):
    print(f"  string_value: {msg.string_value}")

First, compile the Python protobuf definitions:

example.sh
protoc --python_out=. oneof.proto

Then modify the C++ example to save the encoded data to a file:

example.cpp
// After encoding, add this:
FILE *f = fopen("encoded.bin', "wb");
fwrite(buffer, 1, message_length, f);
fclose(f);

Example with all oneof types

Here’s an example showing all oneof types:

oneof_all_example.cpp
#include <stdio.h>
#include <string.h>
#include "oneof.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

void encode_decode_int_value() {
    uint8_t buffer[128];
    size_t message_length;
    
    example_OneofMessage message = example_OneofMessage_init_zero;
    message.which_value = example_OneofMessage_int_value_tag;
    message.value.int_value = 42;
    
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    if (!pb_encode(&ostream, example_OneofMessage_fields, &message)) {
        printf("Encoding failed\n");
        return;
    }
    message_length = ostream.bytes_written;
    
    example_OneofMessage decoded = example_OneofMessage_init_zero;
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    if (!pb_decode(&istream, example_OneofMessage_fields, &decoded)) {
        printf("Decoding failed\n");
        return;
    }
    
    printf("int_value: %u\n", decoded.value.int_value);
}

void encode_decode_float_value() {
    uint8_t buffer[128];
    size_t message_length;
    
    example_OneofMessage message = example_OneofMessage_init_zero;
    message.which_value = example_OneofMessage_float_value_tag;
    message.value.float_value = 3.14159f;
    
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    if (!pb_encode(&ostream, example_OneofMessage_fields, &message)) {
        printf("Encoding failed\n");
        return;
    }
    message_length = ostream.bytes_written;
    
    example_OneofMessage decoded = example_OneofMessage_init_zero;
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    if (!pb_decode(&istream, example_OneofMessage_fields, &decoded)) {
        printf("Decoding failed\n");
        return;
    }
    
    printf("float_value: %f\n", decoded.value.float_value);
}

void encode_decode_string_value() {
    uint8_t buffer[128];
    size_t message_length;
    
    example_OneofMessage message = example_OneofMessage_init_zero;
    message.which_value = example_OneofMessage_string_value_tag;
    strncpy(message.value.string_value, "Hello", sizeof(message.value.string_value) - 1);
    
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    if (!pb_encode(&ostream, example_OneofMessage_fields, &message)) {
        printf("Encoding failed\n");
        return;
    }
    message_length = ostream.bytes_written;
    
    example_OneofMessage decoded = example_OneofMessage_init_zero;
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    if (!pb_decode(&istream, example_OneofMessage_fields, &decoded)) {
        printf("Decoding failed\n");
        return;
    }
    
    printf("string_value: %s\n", decoded.value.string_value);
}

int main() {
    printf("Testing all oneof types:\n");
    encode_decode_int_value();
    encode_decode_float_value();
    encode_decode_string_value();
    return 0;
}

Key points

When to use oneof

Expected output

example.txt
Encoded 2 bytes (int_value)
Encoded data: 08 2a 
Decoded value:
  int_value: 42

Expected output (all types)

example.txt
Testing all oneof types:
int_value: 42
float_value: 3.141590
string_value: Hello

More NanoPB posts

Check out similar posts by category: Embedded, C/C++, Protocol Buffers