How to Parse DWG and DXF Formats Using libdxfrw in C++

libdxfrw is a free, open-source C++ library designed to read and write DXF files (both ASCII and binary) and read DWG files from Release 14 up to version 2015. It serves as the parsing backend for prominent open-source CAD software like LibreCAD and FreeCAD.

Parsing a file with libdxfrw relies on the Event-Driven (Callback) Pattern. Instead of returning a complex object tree directly, the library acts as a SAX-style parser: it reads the file stream and invokes custom callback functions whenever it encounters an entity (like a line, circle, or layer). Core Architecture and Header Files

To parse files, you interact primarily with two main interfaces:

dxfReader / dwgReader (or the unified DRW::Dxf class): The classes responsible for reading the physical file stream and orchestrating the parse.

DRW_Interface: An abstract interface class provided by libdxfrw. You must inherit from this class and override its virtual methods to capture and handle the geometric or structural data. Step-by-Step Implementation Guide

To implement libdxfrw in a C++ application, you need to write a custom interface class, instantiate it, and pass it to the parser reader. 1. Define Your Custom Class Inheriting DRW_Interface

You must override specific methods inside DRW_Interface to capture layout data and entity attributes as the file is processed.

#include #include // Ensure your include paths match your build setup class MyCADParser : public DRW_Interface { public: // Callback triggered when a line entity is parsed virtual void addLine(const DRW_Line& data) override { std::cout << “Found Line: ” << “Start(” << data.basePoint.x << “, ” << data.basePoint.y << “) -> ” << “End(” << data.secPoint.x << “, ” << data.secPoint.y << “) “; } // Callback triggered when a circle entity is parsed virtual void addCircle(const DRW_Circle& data) override { std::cout << “Found Circle: ” << “Center(” << data.basePoint.x << “, ” << data.basePoint.y << “), ” << “Radius: ” << data.radious << “ “; } // Callback triggered when text or MText is parsed virtual void addText(const DRW_Text& data) override { std::cout << “Found Text: ” << data.text << “ “; } // Overriding mandatory interface lifecycle callbacks virtual void addHeader(const DRW_Headerdata) override {} virtual void addLayer(const DRW_Layer& data) override {} virtual void addLtype(const DRW_LType& data) override {} // (Implement other pure virtuals as empty stubs if not needed) }; Use code with caution. 2. Execute the Parsing Logic

Once your callback interface is built, pass a reference of your class along with the target filename into the reader instance.

int main() { std::string filename = “architectural_blueprint.dxf”; // Can also be a .dwg file // Create an instance of your custom event listener MyCADParser myParserInterface; // Instantiate the libdxfrw unified DXF/DWG management object DRW::Dxf dxfParser; // The .in() function accepts the file path and a pointer to the interface handler std::cout << “Attempting to parse: ” << filename << std::endl; bool success = dxfParser.in(filename, &myParserInterface); if (success) { std::cout << “File parsed successfully!” << std::endl; } else { std::cerr << “Failed to parse the file structure.” << std::endl; } return 0; } Use code with caution. Important Considerations

DWG Limitations: libdxfrw parses DWG files by internally mapping elements to equivalent DXF data structures. It supports files natively created by AutoCAD version R14 up to version 2015. If you attempt to feed it newer versions (like DWG 2018 or later), the parsing sequence will fail.

Completeness: DRWInterface features dozens of virtual methods corresponding to specialized AutoCAD structures (addLWPolyline, addBlock, addInsert, addDimension). To avoid compilation errors regarding pure virtual functions, ensure your subclass implements all required pure virtual overrides, even if they remain empty stubs.

Data Types: Entities pass complex data through custom structs prefixes with DRW (e.g., DRW_Coord represents coordinates containing .x, .y, and .z parameters).

codelibs/libdxfrw: C++ library to read and write DXF/DWG files