How to get glyph unicode using freetype?

One would expect the FT_CharMap to hold this info:

[...] The currently active charmap is available as face->charmap.

but unfortunately it only defines the kind of encoding (Unicode, MacRoman, Shift-JIS etc.). Apparently the act of looking up a code is done elsewhere – and .notdef simply gets returned when that character is unavailable after all.

Looking in one of my own FreeType-based OpenType renderers which reports 'by name', where possible, I found in the initialization sequence some code that stores the name of a glyph if it has one, the Unicode else. But that code was based on the presence of glyph names.

Thinking further: you can test every possible Unicode codepoint and see if it returns 0 (.notdef) or a valid glyph index. So initialize an empty table for all possible glyphs and only fill in each one's Unicode if the following routine finds it.

For a moderately modern font you need only check up to Unicode U+FFFF; for something like a heavy Chinese font (up to U+2F9F4 for Heiti SC) or Emoji (up to U+1FA95 for Segoe UI Emoji) you need quite a larger array. (Getting that max number out of a font is an entirely different story, alas. Deciding what to do depends on what you want to use this for.)

printf ("num glyphs: %u\n", face->num_glyphs);
for (code=1; code<=0xFFFF; code++)
{
    glyph_index = FT_Get_Char_Index(face, code);
    /* 0 = .notdef */
    if (glyph_index)
    {
        printf ("%d -> %04X\n", glyph_index, code);
    }
}

This short C snippet prints out the translation table from font glyph index to a corresponding Unicode. Beware that (1) not all glyphs in a font need to have a Unicode associated with them. Some fonts have tons of 'extra' glyphs, to be used in OpenType substitutions (such as alternative designs and custom ligatures) or other uses (such as aforementioned Segoe UI Emoji; it contains color masks for all of its emoji). And (2) some glyphs may be associated with multiple Unicode characters. The glyph design for A, for example, can be used as both a Latin Capital Letter A and a Greek Capital Letter Alpha.

The Unicode codepoint is not technically stored together with the glyph in the TrueType/OpenType font. One has to iterate the font cmap table in the font to get the mapping, which could also be a non-Unicode one and also multiple mappings pointing to the same glyph may exist. The good news is that FreeType provides facilities in the API to iterate the glyphs codepoints in the currently selected character map, which are very well documented. So, with code:

// Ensure an unicode characater map is loaded
FT_Select_Charmap(face, FT_ENCODING_UNICODE);

FT_ULong charcode;
FT_UInt gid;

charcode = FT_Get_First_Char(face, &gid);
while (gid != 0)
{
    std::cout << std::format("Codepoint: {:x}, gid: {}", charcode, gid) << std::endl;
    charcode = FT_Get_Next_Char(face, charcode, &gid);
}

With this information you can create a best effort map from glyphs to Unicode code points.

Not all glyphs in a font will necessarily have a Unicode code point. In OpenType text display, there is a m:n mapping that occurs between Unicode character sequences and glyph sequences. If you are interested in a relationship between Unicode code points and glyphs, the thing that makes most sense would be to use the mapping from Unicode code points to default glyph that is contained in a font's 'cmap' table.

For more background, see OpenType spec: Advanced Typographic Extensions - OpenType Layout.

As for glyph names, every glyph can have a name, regardless of whether it is mapped from a code point in the 'cmap' table or not. Glyph names are contained in the 'post' table. But not all fonts necessarily include glyph names. For example, a CJK font is unlikely to include glyph names.