Generating Complete C++ Inheritance Diagrams Across Multiple Projects with Doxygen

Solving Incomplete Inheritance Diagrams in Multi-Project C++ Documentation

When working on large-scale C++ projects, developers often split code across multiple repositories or modules. To document relationships between classes, tools like Doxygen are widely used. However, an issue arises when inheritance diagrams fail to display derived classes from external projects. 📌

This problem occurs even when using tag files to enable cross-referencing. While base classes from external projects appear correctly, derived classes are often missing, leading to incomplete diagrams. Imagine documenting a core framework where child classes from other modules are invisible—frustrating, right?

For instance, consider a project where Class A exists in Project 1, while its derived classes Class D, E, and F reside in Project 2. Despite linking both projects with tag files, only Class A is displayed in the inheritance graph, leaving developers in the dark about its full hierarchy. 🔍

So, how can we ensure Doxygen generates complete inheritance diagrams, spanning multiple projects? This article explores possible solutions, configurations, and best practices to overcome this challenge effectively.

Enhancing Doxygen Inheritance Diagrams Across Multiple C++ Projects

When documenting large-scale C++ projects with Doxygen, one of the major challenges developers face is ensuring that inheritance diagrams display all related classes, even those spread across multiple repositories. Our solution involves configuring Doxygen's tag files correctly, merging external references, and verifying the completeness of the output using custom scripts. These steps allow us to generate an accurate representation of class relationships across different projects. 🔍

The first approach involves configuring Doxygen’s TAGFILES setting. This enables cross-referencing between different projects by linking external tag files. Each project must generate its own tag file, and these files must be referenced correctly in the respective Doxygen configurations. By doing so, base classes and associated metadata become visible, but derived classes from external projects might still be missing. This is where additional XML parsing comes into play.

To solve the missing derived class issue, we developed a Python script that parses and merges multiple Doxygen tag files. Using the ElementTree library, we extract relevant class definitions from one tag file and append them to another, ensuring that all relationships are preserved. For example, if Class A exists in Project 1 and Class D inherits from it in Project 2, our script ensures that Project 1's documentation properly includes Class D in its inheritance diagram.

Finally, we validate our solution by scanning the generated XML files for missing class references. A script systematically checks whether each expected class appears in the documentation, ensuring correctness. This approach not only enhances the completeness of inheritance graphs but also improves maintainability across large codebases. By combining Doxygen's built-in features with automated XML manipulation, we provide a scalable solution for documenting complex, multi-repository C++ projects. 🚀

# Step 1: Generate tag files for each project
doxygen -g Doxyfile_proj1
doxygen -g Doxyfile_proj2
# Step 2: Modify Doxyfile in Project 1 to include Project 2’s tag
TAGFILES = "proj2.tag=path/to/proj2/html"
# Step 3: Modify Doxyfile in Project 2 to include Project 1’s tag
TAGFILES = "proj1.tag=path/to/proj1/html"
# Step 4: Ensure that both projects generate the XML output
GENERATE_XML = YES
# Step 5: Generate documentation for both projects
doxygen Doxyfile_proj1
doxygen Doxyfile_proj2

import xml.etree.ElementTree as ET
# Load both tag files
proj1 = ET.parse("proj1.tag").getroot()
proj2 = ET.parse("proj2.tag").getroot()
# Merge classes
for elem in proj2.findall(".//compound"):  # Find all class definitions
    proj1.append(elem)  # Append to Project 1's tag file
# Save merged file
proj1_tree = ET.ElementTree(proj1)
proj1_tree.write("merged.tag")

import os
def check_class_exists(class_name, xml_dir):
    for file in os.listdir(xml_dir):
        if file.endswith(".xml"):
            with open(os.path.join(xml_dir, file), 'r') as f:
                if class_name in f.read():
                    return True
    return False
# Example usage
print(check_class_exists("ClassD", "proj1/xml"))  # Should return True

Maximizing Doxygen's Potential for Multi-Project Inheritance Diagrams

One often overlooked aspect of using Doxygen for documenting multi-project C++ codebases is its ability to generate not only class diagrams but also detailed relationship graphs. While our previous discussion focused on inheritance graphs, another important feature is collaboration diagrams, which help visualize dependencies between classes. These diagrams can be essential for understanding how different components of a large software system interact. 📌

To enhance Doxygen’s output, developers can enable features like UML-style diagrams, which improve readability by making complex hierarchies clearer. The setting HAVE_DOT = YES ensures that Graphviz is used to render visually appealing and fully detailed diagrams. Additionally, the option CALL_GRAPH = YES helps document function calls across projects, adding another layer of clarity when understanding software architecture.

Another valuable technique involves extending documentation with EXTRACT_ALL = YES. By default, Doxygen ignores undocumented classes and methods, potentially hiding critical parts of the inheritance tree. Enabling this option ensures that every class, including those inherited from external tag files, is fully documented. This is particularly useful when working on projects where documentation is incomplete but still needs to be generated in full.