如何从 C++ 中的 protobuf 获取信息？

Question

基本上我要做的是从类似于 SHPObject 类型的 ProtoBuf 消息中获取 x 和 y 坐标。我知道使用 SHPObject 我可以做 double* x = obj->padfX 和 double* y = obj->padfY 之类的事情。但是，我不完全确定如何从我的 ProtoBuf 中提取相同的信息（代码如下所示）。感谢您的帮助！

到目前为止我已经尝试过：

myProject::protobuf::NewShape _NewShape;

auto obj = _NewShape.shape(0);

double* x = obj.polygon(0).point(0);

报错：

cannot initialize a variable of the type 'double' with an rvalue of type 'unsigned int'

然后我尝试了这个编译但没有做任何事情（没有给我想要的输出）：

double x_coordinate = obj.polygon(0).point(0);
double *x_ptr = &x_coordinate;

这是我的 ProtoBuf 文件：

newShape.proto

syntax = "proto2";

package myProject.protobuf;

message NewShape {

  message Polygon 
  {
    enum PolygonType {
      POLY_TYPE_OUTER = 1;
      POLY_TYPE_INNER = 2;
    };

    optional PolygonType type = 1 [default = POLY_TYPE_OUTER];

    // x, y coordinates 
    repeated uint32 point     = 2 [packed = true];
  }

  message Shape
  {
    repeated Polygon polygon  = 1;
  }

  repeated Shape shape = 2;
}

Answer 1

根据您的格式，您可以像这样从正确填充和反序列化的对象访问点：

// newShape is the deserialized object here

const auto s0p0p0 = newShape.shape(0).polygon(0).point(0);
const auto s0p0p1 = newShape.shape(0).polygon(0).point(1);

类似地，shape(1) 会给你第二个形状来访问它在多边形对象中的点。您应该在访问索引之前检查 *_size() 方法以确保有效访问，例如newShape.shape_size()、shape.polygon_size() 和 polygon.point_size()。

如果您打算修改消息，可以使用 mutable_* 方法获取指向相应对象的指针，然后您可以更改它们。

例如（第一个形状的第一个多边形的变化点）：

auto p = newShape2.mutable_shape(0);
p->mutable_polygon(0)->set_point(0, 123);
p->mutable_polygon(0)->set_point(1, 345);

这是序列化和反序列化的完整工作示例：

#include <iostream>
#include "newShape.pb.h"

int main()
{
    // Serailization

    ::myProject::protobuf::NewShape newShape1;

    auto shape1 = newShape1.add_shape();
    auto polygon1 = shape1->add_polygon();
    polygon1->add_point(1);
    polygon1->add_point(2);

    auto shape2 = newShape1.add_shape();
    auto polygon2 = shape2->add_polygon();
    polygon2->add_point(3);
    polygon2->add_point(4);

    const auto serializedData = newShape1.SerializeAsString();

    std::cout << "Serialized Data Size: " << serializedData.size() << "\n\n";

    // Send/Store Serialized Data

    // Deserialization

    ::myProject::protobuf::NewShape newShape2;

    if ( !newShape2.ParseFromString( serializedData ) )
    {
        std::cerr << "Deserialization failed!\n";
        return -1;
    }

    std::cout << "Deserialized Data Size: " << newShape2.ByteSize() << "\n\n";

    std::cout << "NewShape [Shapes: " << newShape2.shape_size() << "]\n";

    for ( int i {0}; i < newShape2.shape_size(); ++i )
    {
        std::cout << " Shape # " << i << '\n';

        const auto& shape = newShape2.shape( i );
        for ( int j {0}; j < shape.polygon_size(); ++j )
        {
            std::cout << "  Polygon # " << j << '\n';

            const auto& polygon = shape.polygon( j );
            for ( int k {0}; k < polygon.point_size(); ++k )
            {
                const auto& point = polygon.point( k );
                std::cout << "   Point # " << k << ": " << point << '\n';
            }
        }
    }

    return 0;
}

输出：

Serialized Data Size: 16

Deserialized Data Size: 16

NewShape [Shapes: 2]
 Shape # 0
  Polygon # 0
   Point # 0: 1
   Point # 1: 2
 Shape # 1
  Polygon # 0
   Point # 0: 3
   Point # 1: 4

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