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NanoPB: How to handle basic scalar types in C++

See also: C version: How to handle basic scalar types in C

NanoPB is a code-size optimized Protocol Buffers implementation for embedded systems. This post shows how to handle basic scalar types (integers, floats, booleans) in C++ with NanoPB.

Proto definition

First, create a simple .proto file with basic scalar types:

scalars.proto
syntax = "proto3";

package example;

message ScalarMessage {
  uint32 uint32_value = 1;
  uint64 uint64_value = 2;
  int32 int32_value = 3;
  int64 int64_value = 4;
  float float_value = 5;
  bool bool_value = 6;
}

Generate NanoPB code

Generate the C++ NanoPB code using the nanopb generator:

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

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

C++ example

Here’s a complete C++ example that encodes and decodes the message:

scalars_example.cpp
#include <stdio.h>
#include <string.h>
#include "scalars.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 ---
    example_ScalarMessage message = example_ScalarMessage_init_zero;
    
    // Set field values
    message.uint32_value = 42;
    message.uint64_value = 1234567890ULL;
    message.int32_value = -100;
    message.int64_value = -9876543210LL;
    message.float_value = 3.14159f;
    message.bool_value = true;
    
    // Create stream for encoding
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    
    // Encode the message
    if (!pb_encode(&ostream, example_ScalarMessage_fields, &message)) {
        printf("Encoding failed: %s\n", PB_GET_ERROR(&ostream));
        return 1;
    }
    
    message_length = ostream.bytes_written;
    printf("Encoded %zu bytes\n", message_length);
    
    // Print hex dump of encoded data
    printf("Encoded data: ");
    for (size_t i = 0; i < message_length; i++) {
        printf("%02x ", buffer[i]);
    }
    printf("\n");
    
    // --- DECODING ---
    example_ScalarMessage decoded = example_ScalarMessage_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_ScalarMessage_fields, &decoded)) {
        printf("Decoding failed: %s\n", PB_GET_ERROR(&istream));
        return 1;
    }
    
    // Print decoded values
    printf("Decoded values:\n");
    printf("  uint32_value: %u\n", decoded.uint32_value);
    printf("  uint64_value: %llu\n", (unsigned long long)decoded.uint64_value);
    printf("  int32_value: %d\n", decoded.int32_value);
    printf("  int64_value: %lld\n", (long long)decoded.int64_value);
    printf("  float_value: %f\n", decoded.float_value);
    printf("  bool_value: %s\n", decoded.bool_value ? "true" : "false");
    
    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 scalars_example scalars_example.cpp scalars.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_scalars.py
import scalars_pb2

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

# Decode
msg = scalars_pb2.ScalarMessage()
msg.ParseFromString(data)

print("Python decoded values:")
print(f"  uint32_value: {msg.uint32_value}")
print(f"  uint64_value: {msg.uint64_value}")
print(f"  int32_value: {msg.int32_value}")
print(f"  int64_value: {msg.int64_value}")
print(f"  float_value: {msg.float_value}")
print(f"  bool_value: {msg.bool_value}")

First, compile the Python protobuf definitions:

example.sh
protoc --python_out=. scalars.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);

Key points

Expected output

example.txt
Encoded 28 bytes
Encoded data: 08 2a 10 d2 95 89 04 18 9c ff ff ff ff ff ff ff 01 20 c6 ef be ad de 05 2d 15 49 0e 40 30 01 
Decoded values:
  uint32_value: 42
  uint64_value: 1234567890
  int32_value: -100
  int64_value: -9876543210
  float_value: 3.141590
  bool_value: true

More NanoPB posts

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