Header lexy/visualize.hpp

Functions to visualize lexy’s data structures for debugging purposes.

Enum bit flags lexy::visualization_flags

namespace lexy
    enum visualization_flags


            = visualize_use_unicode | visualize_use_color | visualize_use_symbols,


    constexpr visualization_flags operator|(visualization_flags lhs,
                                            visualization_flags rhs) noexcept;

Flags for controlling the visualization.


The default behavior; no flag is set.


Visualization can use Unicode characters for meta data (e.g. the box drawing characters for the parse tree); non-ASCII characters in the content are still escaped. It will write UTF-8 encoded code points.


Visualization can use the ANSI escape codes to add color.


Visualization can use Unicode symbols for e.g. newline and tab characters, which will be written as UTF-8 encoded code points. It is separate from visualize_use_unicode.


Enable Unicode, color, and symbols.


Visualization will display space as a visible symbol instead of the literal space character. Useful to indicate trailing spaces; best combined with visualize_use_symbols.

Use visualize_fancy for visualizing to modern terminals; use visualize_use_unicode | visualize_use_symbols for visualizing to a file.

Struct lexy::visualization_options

namespace lexy
    struct visualization_options
        static constexpr unsigned char max_tree_depth_limit = 32;

        visualization_flags flags = visualize_default;

        unsigned char max_tree_depth   = max_tree_depth_limit;
        unsigned char max_lexeme_width = 0;
        unsigned char tab_width        = 0;

        constexpr bool is_set(visualization_flags f) const noexcept;

        constexpr auto reset(visualization_flags f) const noexcept
          -> visualization_options;

    constexpr visualization_options operator|(visualization_options lhs,
                                              visualization_flags   rhs) noexcept;

Options for controlling visualization.

It combines the lexy::visualization_flags with other options.


When visualizing a lexy::parse_tree, stop visualization at this depth. Child nodes whose depth exceeds max_tree_depth are omitted, which is indicated by a symbol. It must be ⇐ max_tree_depth_limit.


When visualizing a lexy::lexeme, stop visualization after this many "characters". Here a "character" depends on the encoding, e.g. for text based encodings, it is code points. If it is 0, an unlimited amount of characters is printed.


How many spaces are printed for a tab character. If it is 0, tabs are printed as escaped control characters (e.g. \t).

Function lexy::visualize_to

namespace lexy
    template <std::output_iterator<char> OutputIt>
    OutputIt visualize_to(OutputIt out, code_point cp,
                      visualization_options opts = {});                        (1)

    template <std::output_iterator<char> OutputIt, reader Reader>
    OutputIt visualize_to(OutputIt out, lexeme<Reader> lexeme,
                      visualization_options opts = {});                        (2)

    template <std::output_iterator<char> OutputIt,
              reader Reader, typename TokenKind, typename MemoryResource>
    OutputIt visualize_to(OutputIt out,
                      const parse_tree<Reader, TokenKind, MemoryResource>& tree,
                      visualization_options opts = {});                        (3)
  1. Visualize a lexy::code_point.

  2. Visualize a lexy::lexeme by visualizing the individual code points.

  3. Visualize a lexy::parse_tree by visualizing each node.

Visualize a data structure by writing it to out using the lexy::visualization_options.

The format is designed to be human-readable only; it is not documented exactly and subject to change. By default, it writes ASCII characters only. If opts has set the flags visualize_use_unicode or visualize_use_symbols sets, it can also write UTF-8 encoded code points.

To visualize a code point, it writes it directly if it is a printable ASCII character, and otherwise writes its code point value or another escape sequence. Non-ASCII or control code points are never written directly for clarity in debugging.

Visualization of a lexeme depends on the encoding. For the text-based encodings it writes the individual code points. For lexy::byte_encoding, it writes the bytes as hexadecimal.

To visualize a parse tree, it visualizes each node in a tree format. The maximal depth can be controlled by the options.

Use lexy::trace to visualize the parsing process itself.

Function lexy::visualize

namespace lexy
    struct cfile_output_iterator;

    template <typename T>
    void visualize(std::FILE* file, const T& obj,
                   visualization_options opts = {})
        visualize_to(cfile_output_iterator{file}, obj, opts);

Visualizes a data structure by writing it to file.

It uses cfile_output_iterator, which is an output iterator that repeatedly calls std::fputc, and then forwards to lexy::visualize_to.

Example 1. Visualize a lexy::parse_tree
int main()
    auto input = lexy_ext::compiler_explorer_input();

    lexy::parse_tree_for<decltype(input)> tree;
    auto result = lexy::parse_as_tree<production>(tree, input, lexy_ext::report_error);

    lexy::visualize(stdout, tree, {lexy::visualize_fancy});

    if (!result)
        return 1;

Function lexy::visualization_display_width

namespace lexy
    template <typename T>
    std::size_t visualization_display_width(const T& obj,
                                            visualization_options opts = {});

Computes the display width for visualizing obj using opts, which is the number of Unicode code points.

As the only non-ASCII Unicode characters are carefully chosen to occupy one display cell in a monospace font, this is the number of characters necessary to underline the visualization result or indent past it.

See also