代码片段
class FontCoverage
{
public:
    using SP                = std::shared_ptr<FontCoverage>;
    virtual ~FontCoverage() = default;

    virtual void                     set(int index, FontTypes::CoverageLevel level) = 0;
    virtual FontTypes::CoverageLevel get(int index)                                 = 0;
    virtual FontCoverage::SP         clone()                                        = 0;
};

freetype:
    FTCoverage::SP coverage = std::make_shared<FTCoverage>();
    FT_CharMap    *charMap  = (*m_face)->charmaps;
    for (int i = 0; i < (*m_face)->num_charmaps; i++) {
        FT_Select_Charmap((*m_face), charMap[i]->encoding);
        FT_UInt gIndex;
        for (auto charCode = FT_Get_First_Char((*m_face), &gIndex); gIndex != 0;
             charCode      = FT_Get_Next_Char((*m_face), charCode, &gIndex)) {
            coverage->set(charCode, FontTypes::CoverageLevel::COVERAGE_EXACT);
        }
    }
harfbuzz:
    FontCoverage::SP coverage = std::make_shared<HBCoverage>();
    hb_set_t        *chars    = hb_set_create();
    hb_codepoint_t   ch       = HB_SET_VALUE_INVALID;

    hb_face_collect_unicodes(m_face, chars);
    while (hb_set_next(chars, &ch))
        coverage->set(ch, FontTypes::CoverageLevel::COVERAGE_EXACT);
    hb_set_destroy(chars);
    
关键点
1.字体解析需包含完整的unicode范围
2.根据unicode，字体名，大小等最优匹配得到回退列表

C++字体库开发二-CSDN博客

创作不易，小小的支持一下吧！