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NanoPB：如何在 C++ 中处理嵌套消息

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另请参阅：C 版本：如何在 C 中处理嵌套消息

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 处理嵌套消息。

Proto 定义

首先，创建一个包含嵌套消息的 .proto 文件：

nested.proto
syntax = "proto3";

package example;

message Address {
  string street = 1;
  string city = 2;
  string country = 3;
}

message Person {
  string name = 1;
  uint32 age = 2;
  Address address = 3;
}

生成 NanoPB 代码

使用 .options 文件指定字符串缓冲区大小，然后生成 NanoPB 代码：

创建 nested.options：

nested.options
example.Address.street max_size:64
example.Address.city max_size:32
example.Address.country max_size:32
example.Person.name max_size:64

然后生成：

generate_nanopb_nested.sh
protoc --nanopb_out=. nested.proto

这将生成 nested.pb.h 和 nested.pb.c。

C++ 示例

下面是一个处理嵌套消息的完整 C++ 示例：

nested_example.cpp
#include <stdio.h>
#include <string.h>
#include "nested.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

int main() {
    // 编码消息的缓冲区
    uint8_t buffer[256];
    size_t message_length;
    
    // --- 编码 ---
    example_Person person = example_Person_init_zero;
    
    // 设置嵌套消息的值
    strncpy(person.name, "John Doe", sizeof(person.name) - 1);
    person.has_address = true;
    strncpy(person.address.street, "123 Main St", sizeof(person.address.street) - 1);
    strncpy(person.address.city, "Springfield", sizeof(person.address.city) - 1);
    strncpy(person.address.country, "USA", sizeof(person.address.country) - 1);
    
    // 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_Person_fields, &person)) {
        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_Person decoded = example_Person_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_Person_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("  name: %s\n", decoded.name);
    printf("  age: %u\n", decoded.age);
    printf("  address:\n");
    printf("    street: %s\n", decoded.address.street);
    printf("    city: %s\n", decoded.address.city);
    printf("    country: %s\n", decoded.address.country);
    
    return 0;
}

编译命令

使用 nanopb 编译该示例。NanoPB 通常通过将源文件直接包含到你的项目来使用：

compile_nested_example.sh
g++ -o nested_example nested_example.cpp nested.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

为了验证编码，你可以使用 Python 的 protobuf 库：

test_nested.py
import nested_pb2

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

# 解码
msg = nested_pb2.Person()
msg.ParseFromString(data)

print("Python decoded values:")
print(f"  name: {msg.name}")
print(f"  age: {msg.age}")
print(f"  address:")
print(f"    street: {msg.address.street}")
print(f"    city: {msg.address.city}")
print(f"    country: {msg.address.country}")

首先，编译 Python protobuf 定义：

compile_python_nested.sh
protoc --python_out=. nested.proto

然后修改 C++ 示例，将编码后的数据保存到文件：

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

可选嵌套消息示例

下面是一个包含可选嵌套消息的示例：

nested_optional.proto
syntax = "proto3";

package example;

message Address {
  string street = 1;
  string city = 2;
}

message Person {
  string name = 1;
  optional Address address = 2;
}

创建 nested_optional.options：

nested_optional.options
example.Address.street max_size:64
example.Address.city max_size:32
example.Person.name max_size:64
nested_optional_example.cpp
#include <stdio.h>
#include <string.h>
#include "nested_optional.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

int main() {
    uint8_t buffer[256];
    size_t message_length;
    
    // --- 编码 ---
    example_Person person = example_Person_init_zero;
    
    strncpy(person.name, "Jane Doe", sizeof(person.name) - 1);
    
    // 设置可选嵌套消息
    strncpy(person.address.street, "456 Oak Ave", sizeof(person.address.street) - 1);
    strncpy(person.address.city, "Boston", sizeof(person.address.city) - 1);
    person.has_address = true;  // 重要：设置 has_* 标志
    
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    
    if (!pb_encode(&ostream, example_Person_fields, &person)) {
        printf("Encoding failed: %s\n", PB_GET_ERROR(&ostream));
        return 1;
    }
    
    message_length = ostream.bytes_written;
    printf("Encoded %zu bytes\n", message_length);
    
    // --- 解码 ---
    example_Person decoded = example_Person_init_zero;
    
    pb_istream_t istream = pb_istream_from_buffer(buffer, message_length);
    
    if (!pb_decode(&istream, example_Person_fields, &decoded)) {
        printf("Decoding failed: %s\n", PB_GET_ERROR(&istream));
        return 1;
    }
    
    printf("Decoded values:\n");
    printf("  name: %s\n", decoded.name);
    if (decoded.has_address) {
        printf("  address:\n");
        printf("    street: %s\n", decoded.address.street);
        printf("    city: %s\n", decoded.address.city);
    } else {
        printf("  address: (not set)\n");
    }
    
    return 0;
}

要点

何时使用嵌套消息

预期输出

nested_expected_output.txt
Encoded 35 bytes
Encoded data: 0a 08 4a 6f 68 6e 20 44 6f 65 10 1e 1a 1b 0a 0b 31 32 33 20 4d 61 69 6e 20 53 74 12 08 4e 65 77 20 59 6f 72 6b 1a 03 55 53 41 
Decoded values:
  name: John Doe
  age: 30
  address:
    street: 123 Main St
    city: New York
    country: USA

预期输出（可选嵌套）

nested_optional_expected_output.txt
Encoded 27 bytes
Decoded values:
  name: Jane Doe
  address:
    street: 456 Oak Ave
    city: Boston

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