Generating sequence diagrams

• Sequence diagram overview
• Specifying diagram location constraints
• Grouping free functions by file
• Lambda expressions in sequence diagrams
• Customizing participants order
• Generating return types
• Generating condition statements
• Injecting call expressions manually through comments
• Including comments in sequence diagrams

The minimal config required to generate a sequence diagram is presented below:

# Path to the directory where `compile_commands.json` can be found
compilation_database_dir: _build
# Output directory for the diagrams
output_directory: diagrams
# Diagrams definitions
diagrams:
  # Diagram name
  my_class_diagram:
    # Type of diagram (has to be `sequence`)
    type: sequence
    # Include only translation units matching the following patterns
    glob:
      - src/*.cc
    # Include only classes and functions from files in `src` directory
    include:
      paths:
        - src
    # Exclude calls to/from `std` namespace
    exclude:
      namespaces:
        - std
    start_from:
      - function: "main(int,const char**)"

Sequence diagram overview

Consider the following diagram:

clang-uml generated sequence diagrams are not strictly speaking conforming to the UML specification. In order to make them more useful for documenting modern C++ code, the following assumptions were made:

• Free functions are included in the sequence diagrams as standalone participants (in fact clang-uml can be used to generate sequence diagrams from plain old C code). Functions can also be aggregated into file participants, based on their place of declaration
• Call expressions in conditional expressions in block statements (e.g. if or while) are rendered inside the PlantUML or MermaidJS alt or loop blocks but wrapped in [, ] brackets
• Lambda expressions are generated as standalone participants, whose name comprises the parent context where they are defined and the exact source code location

Specifying diagram location constraints

Sequence diagrams require specification of location constraints in order to determine, which call chains should be included in the diagram. Currently, there are 3 types of constraints:

• from - will include all message call chains, which start at the locations specified in this constraint (this was previously named start_from)
• to - will include all message call chains, which end at the specified locations
• from_to - will include all call chains, which start and end at the specified locations

Currently, the constraints can be a method or a free function, both specified using the full signature of the function, e.g.:

from:
  - function: "main(int,const char**)"

or

to:
  - function: "clanguml::sequence_diagram::visitor::translation_unit_visitor::VisitCXXRecordDecl(clang::CXXRecordDecl *)"

The locations must be fully qualified, and they must match exactly the string representation of a given function or method in the clang-uml model.

In case of the from_to constraint, it is necessary to provide both from and to locations as follows:

from_to:
  - [function: "clanguml::t20034::D::d2()",
     function: "clanguml::t20034::A::a2()"]

To find the exact function signature, which can be used as a from location, run clang-uml as follows (assuming the function of interest is called main):

clang-uml --print-from -n main_sequence | grep main

or to get all possible to locations, run:

clang-uml --print-to -n main_sequence | grep main

Command line flags --print-from and --print-to will print on stdout all functions and methods available in the diagram model, and each line of this output can be directly used as a value of from, from_to or to properties in the config file.

Since that list can be quite large, it's best to filter the output to limit the number of lines to a subset of possible candidates.

Grouping free functions by file

By default, clang-uml will generate a new participant for each call to a free function (not method), which can lead to a very large number of participants in the diagram. If it's an issue, an option can be provided in the diagram definition:

combine_free_functions_into_file_participants: true

which will aggregate free functions per source file where they were declared thus minimizing the diagram size. An example of such diagram is presented below:

Lambda expressions in sequence diagrams

Lambda expressions in sequence diagrams are... tricky. There is currently tentative support, which follows the following rules:

• If lambda expression is called within the scope of the diagram, the calls from the lambda will be placed at the lambda invocation and not declaration
• If lambda expression is passed to some function or method, which is outside the scope of the diagram (e.g. used in std::transform call) the call will not be generated
• If the lambda is passed as template argument in instantiation it will not be generated

Another issue is the naming of lambda participants. Currently, each lambda is rendered in the diagram as a separate class whose name is composed of the lambda location in the code (the only unique way of identifying lambdas I was able to find). For example the following code:

#include <algorithm>
#include <functional>
#include <memory>
#include <optional>
#include <utility>
 
namespace clanguml {
namespace t20012 {
struct A {
    void a() { aa(); }
 
    void aa() { aaa(); }
 
    void aaa() { }
};
 
struct B {
    void b() { bb(); }
 
    void bb() { bbb(); }
 
    void bbb() { }
 
    void eb() { }
};
 
struct C {
    void c() { cc(); }
 
    void cc() { ccc(); }
 
    void ccc() { }
};
 
struct D {
    int add5(int arg) const { return arg + 5; }
};
 
class E {
    std::optional<std::shared_ptr<B>> maybe_b;
    std::shared_ptr<A> a;
 
public:
    template <typename F> void setup(F &&f) { f(maybe_b); }
};
 
template <typename F> struct R {
    R(F &&f)
        : f_{std::move(f)}
    {
    }
 
    void r() { f_(); }
 
    F f_;
};
 
void tmain()
{
    A a;
    B b;
    C c;
 
    // The activity shouldn't be marked at the lambda definition, but
    // wherever it is actually called...
    auto alambda = [&a, &b]() {
        a.a();
        b.b();
    };
 
    // ...like here
    alambda();
 
    // There should be no call to B in the sequence diagram as the blambda
    // is never called
    [[maybe_unused]] auto blambda = [&b]() { b.b(); };
 
    // Nested lambdas should also work
    auto clambda = [alambda, &c]() {
        c.c();
        alambda();
    };
    clambda();
 
    R r{[&c]() { c.c(); }};
 
    r.r();
 
    D d;
 
    std::vector<int> ints{0, 1, 2, 3, 4};
    std::transform(ints.begin(), ints.end(), ints.begin(),
        [&d](auto i) { return d.add5(i); });
}
}
}

results in the following diagram:

In case lambda expressions are redundant and we are only interested in the calls generate from the lambda expressions, it is possible to inline lambda expressions in the generated diagrams by specifying the following option:

inline_lambda_messages: true

For example compare the test cases t20012 and t20052.

Customizing participants order

The default participant order in the sequence diagram can be suboptimal in the sense that consecutive calls can go right, then left, then right again depending on the specific call chain in the code. It is however possible to override this order in the diagram definition using participants_order property, for instance like this:

diagrams:
  t20029_sequence:
    type: sequence
    glob:
      - t20029.cc
    include:
      namespaces:
        - clanguml::t20029
    exclude:
      access:
        - private
    using_namespace: clanguml::t20029
    start_from:
      - function: clanguml::t20029::tmain()
    participants_order:
      - "clanguml::t20029::tmain()"
      - "clanguml::t20029::Encoder<clanguml::t20029::Retrier<clanguml::t20029::ConnectionPool>>"
      - "clanguml::t20029::Retrier<clanguml::t20029::ConnectionPool>"
      - "clanguml::t20029::ConnectionPool"
      - "clanguml::t20029::encode_b64(std::string &&)"

Generating return types

By default, return messages do not contain the return type information from the function or method. Instead, if the result is void there is no return arrow from the activity representing the function body.

It is however possible to enable rendering of return types, by adding the following configuration option:

generate_return_types: true

This option only affects the plantuml and mermaid generators, in json generator return_type property is always present in the message nodes.

The diagram below presents what it looks like in a PlantUML generated diagram:

Generating condition statements

Sometimes, it is useful to include actual condition statements (for instance contents of the if() condition in the alt or loop blocks in the sequence diagrams, to make them more readable.

This can be enabled using the following option:

generate_condition_statements: true

An example of a diagram with this feature enabled is presented below:

Injecting call expressions manually through comments

In some cases, clang-uml is not yet able to discover a call expression target in some line of code. This can include passing function or method address to some executor (e.g. thread), async calls etc.

However, a call expression can be injected manually through a comment directive

// \uml{note CALLEE}

It should be placed in the comment just before such line of code, for example:

// \uml{call clanguml::t20038::B::bbb()}
auto bbb_future = std::async(std::launch::deferred, &B::bbb, b);

also see the t20038 test case.

Please note that the callee must have fully qualified name including complete namespace.

In order to enable this, the .clang-uml must contain the following option:

add_compile_flags:
  - -fparse-all-comments

otherwise Clang will skip these comments during AST traversal.

Including comments in sequence diagrams

clang-uml can add code comments placed directly before or next to a call expression as notes in the diagram (see for instance t20038).

This however is not enabled by default. In order to enable this feature it is necessary to first of all force Clang to parse all comments in the source code by adding the following compile flag at the top of .clang-uml:

add_compile_flags:
  - -fparse-all-comments

or adding it to the compile_commands.json database somehow directly.

Another option needed to generate these comments in the diagram is to set

generate_message_comments: true

for each sequence diagram, which should include these comments.

In case only selected messages should have some specific comments, instead of enabling the generate_message_comments option, it is possible to use \\uml{note TEXT} directive in the comment above the expression, see t20001.