使用 PoDoFo 访问 /差异数组

PoDoFo知道许多不同类型的编码类，参见编码对象工厂：

if (pObject->IsName ())
{
    const PdfName & rName = pObject->GetName ();
    if (rName == PdfName ("WinAnsiEncoding"))
        return PdfEncodingFactory::GlobalWinAnsiEncodingInstance ();
    else if (rName == PdfName ("MacRomanEncoding"))
        return PdfEncodingFactory::GlobalMacRomanEncodingInstance ();
    else if (rName == PdfName ("StandardEncoding"))      // OC 13.08.2010
        return PdfEncodingFactory::GlobalStandardEncodingInstance ();
    else if (rName == PdfName ("MacExpertEncoding"))     // OC 13.08.2010 TODO solved
        return PdfEncodingFactory::GlobalMacExpertEncodingInstance ();
    else if (rName == PdfName ("SymbolEncoding"))        // OC 13.08.2010
        return PdfEncodingFactory::GlobalSymbolEncodingInstance ();
    else if (rName == PdfName ("ZapfDingbatsEncoding"))  // OC 13.08.2010
        return PdfEncodingFactory::GlobalZapfDingbatsEncodingInstance ();
    else if (rName == PdfName ("Identity-H"))
        return new PdfIdentityEncoding ();
}
else if (pObject->HasStream ())     // Code for /ToUnicode object 
{
    return new PdfCMapEncoding(pObject);
}
else if (pObject->IsDictionary ())
{
    return new PdfDifferenceEncoding (pObject);
}

(PoDoFo/src/doc/PdfEncodingObjectFactory.cpp(

您对最后一种情况感兴趣。因此，如果您手头的编码对象是 PdfDifference Encoding 的实例，则可以使用：

/** PdfDifferenceEncoding is an encoding, which is based
 *  on either the fonts encoding or a predefined encoding
 *  and defines differences to this base encoding.
 */
class PODOFO_DOC_API PdfDifferenceEncoding : public PdfEncoding, private PdfElement {
 public:
[...]
    /** 
     * Get read-only access to the object containing the actual
     * differences.
     *
     * returns the container with the actual differences
     */
    inline const PdfEncodingDifference & GetDifferences() const;
[...]
};

(PoDoFo/src/doc/PdfDifferenceEncoding.h(

PdfDifferenceEncoding 在同一个标头类中声明，并提供一些有趣的方法：

/** A helper class for PdfDifferenceEncoding that
 *  can be used to create a differences array.
 */
class PODOFO_DOC_API PdfEncodingDifference {
    struct TDifference {
        int         nCode;
        PdfName     name;
        pdf_utf16be unicodeValue;
    };
    typedef std::vector<TDifference>                 TVecDifferences;
    typedef std::vector<TDifference>::iterator       TIVecDifferences;
    typedef std::vector<TDifference>::const_iterator TCIVecDifferences;
 public: 
    /** Create a PdfEncodingDifference object.
     */
    PdfEncodingDifference();
    /** Copy a PdfEncodingDifference object.
     */
    PdfEncodingDifference( const PdfEncodingDifference & rhs );
    /** Copy a PdfEncodingDifference object.
     */
    const PdfEncodingDifference & operator=( const PdfEncodingDifference & rhs );
    /** Add a difference to the object.
     * 
     *  param nCode unicode code point of the difference (0 to 255 are legal values)
     *
     *  see AddDifference if you know the name of the code point
     *       use the overload below which is faster
     */
    void AddDifference( int nCode );
    /** Add a difference to the object.
     * 
     *  param nCode unicode code point of the difference (0 to 255 are legal values)
     *  param rName name of the different code point or .notdef if none
     */
    void AddDifference( int nCode, const PdfName & rName );
    /** Tests if the specified code is part of the 
     *  differences.
     *
     *  param nCode test if the given code is part of the differences
     *  param rName write the associated name into this object if the 
     *               code is part of the difference
     *  param rValue write the associated unicode value of the name to this value 
     *
     *  returns true if the code is part of the difference
     */
    bool Contains( int nCode, PdfName & rName, pdf_utf16be & rValue ) const;
    /** Convert the PdfEncodingDifference to an array
     *
     *  param rArray write to this array
     */
    void ToArray( PdfArray & rArray );
    /** Get the number of differences in this object.
     *  If the user added .notdef as a difference it is 
     *  counted, even it is no real difference in the final encoding.
     *  
     *  returns the number of differences in this object
     */
    inline size_t GetCount() const;
 private:
    struct DifferenceComparatorPredicate {
        public:
          inline bool operator()( const TDifference & rDif1, 
                                  const TDifference & rDif2 ) const { 
              return rDif1.nCode < rDif2.nCode;
          }
    };
    TVecDifferences m_vecDifferences;
};

(PoDoFo/src/doc/PdfDifferenceEncoding.h(

一旦你有一个指向PoDoFo::PdfFont的指针，你就可以通过调用GetObject来访问底层对象，因为它从PoDoFo：:P dfElement继承了它。从那里调用GetIndirectKey("Encoding")以获取指向包含差异数组的编码字典的指针，并将其传递给PoDoFo::PdfDifferenceEncoding构造函数。

PoDoFo::PdfObject* fntobj = fnt->GetObject();
if (fntobj)
{
    PoDoFo::PdfObject* fntdic = fntobj->GetIndirectKey("Encoding");
    if (fntdic)
    {
        PoDoFo::PdfDifferenceEncoding diff(fntdic);
        PoDoFo::PdfArray diffarray;
        PoDoFo::PdfEncodingDifference d(diff.GetDifferences());
        d.ToArray(diffarray);
    }
}