NanoPB：如何在 C++ 中处理基本标量类型

NanoPB 是一个面向嵌入式系统的代码体积优化的 Protocol Buffers 实现。本文介绍如何在 C++ 中使用 NanoPB 处理基本标量类型（整数、浮点数、布尔值）。

Proto 定义

首先，创建一个包含基本标量类型的简单 .proto 文件：

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;
}

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;
}

生成 NanoPB 代码

使用 nanopb 生成器生成 C++ NanoPB 代码：

generate_nanopb_scalars.sh

protoc --nanopb_out=. scalars.proto

protoc --nanopb_out=. scalars.proto

这将生成 scalars.pb.h 和 scalars.pb.c。

C++ 示例

以下是一个编码和解码消息的完整 C++ 示例：

scalars_example.cpp

#include <stdio.h>
#include <string.h>
#include "scalars.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

int main() {
    // 编码消息的缓冲区
    uint8_t buffer[128];
    size_t message_length;
    
    // --- 编码 ---
    example_ScalarMessage message = example_ScalarMessage_init_zero;
    
    // 设置字段值
    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;
    
    // 创建编码流
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    
    // 编码消息
    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);
    
    // 打印编码数据的十六进制转储
    printf("Encoded data: ");
    for (size_t i = 0; i < message_length; i++) {
        printf("%02x ", buffer[i]);
    }
    printf("\n");
    
    // --- 解码 ---
    example_ScalarMessage decoded = example_ScalarMessage_init_zero;
    
    // 创建解码流
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    
    // 解码消息
    if (!pb_decode(&istream, example_ScalarMessage_fields, &decoded)) {
        printf("Decoding failed: %s\n", PB_GET_ERROR(&istream));
        return 1;
    }
    
    // 打印解码后的值
    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;
}

#include <stdio.h>
#include <string.h>
#include "scalars.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

int main() {
    // 编码消息的缓冲区
    uint8_t buffer[128];
    size_t message_length;
    
    // --- 编码 ---
    example_ScalarMessage message = example_ScalarMessage_init_zero;
    
    // 设置字段值
    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;
    
    // 创建编码流
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    
    // 编码消息
    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);
    
    // 打印编码数据的十六进制转储
    printf("Encoded data: ");
    for (size_t i = 0; i < message_length; i++) {
        printf("%02x ", buffer[i]);
    }
    printf("\n");
    
    // --- 解码 ---
    example_ScalarMessage decoded = example_ScalarMessage_init_zero;
    
    // 创建解码流
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    
    // 解码消息
    if (!pb_decode(&istream, example_ScalarMessage_fields, &decoded)) {
        printf("Decoding failed: %s\n", PB_GET_ERROR(&istream));
        return 1;
    }
    
    // 打印解码后的值
    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;
}

编译命令

使用 nanopb 编译示例。NanoPB 通常通过将源文件直接包含到项目中来使用：

compile_scalars_example.sh

g++ -o scalars_example scalars_example.cpp scalars.pb.c pb_common.c pb_encode.c pb_decode.c -I.

g++ -o scalars_example scalars_example.cpp scalars.pb.c pb_common.c pb_encode.c pb_decode.c -I.

**注意：**NanoPB 源文件（pb_common.c、pb_encode.c、pb_decode.c）需要直接与你的项目一起编译。你可以从 NanoPB GitHub 仓库获取这些文件。

Python 测试脚本

要验证编码结果，可以使用 Python 的 protobuf 库：

test_scalars.py

import scalars_pb2

# 读取二进制数据
with open('encoded.bin', 'rb') as f:
    data = f.read()

# 解码
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}")

import scalars_pb2

# 读取二进制数据
with open('encoded.bin', 'rb') as f:
    data = f.read()

# 解码
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}")

首先，编译 Python protobuf 定义：

compile_python_scalars.sh

protoc --python_out=. scalars.proto

protoc --python_out=. scalars.proto

然后修改 C++ 示例，将编码后的数据保存到文件：

save_encoded_scalars.cpp

// 编码后，添加以下代码：
FILE *f = fopen("encoded.bin", "wb");
fwrite(buffer, 1, message_length, f);
fclose(f);

// 编码后，添加以下代码：
FILE *f = fopen("encoded.bin", "wb");
fwrite(buffer, 1, message_length, f);
fclose(f);

要点

使用 *_init_zero 以默认值初始化消息
pb_ostream_from_buffer 创建用于编码的输出流
pb_istream_from_buffer 创建用于解码的输入流
pb_encode 和 pb_decode 处理实际的编码/解码
检查返回值并使用 PB_GET_ERROR 进行错误处理

预期输出

scalars_expected_output.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

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