Problem in decoding plantext using HMAC of crypto++

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cryptography

C++11

hi all
I have faced a problem in using HMAC of cryptopp library in decoding section.
I have used the flowing code for encoding the planText and it works okay
this code is described in this link [HMAC - Crypto++ Wiki[^]]

AutoSeededRandomPool prng;

    SecByteBlock key(16);
    prng.GenerateBlock(key, key.size());

    string plain = "HMAC Test";
    string mac, encoded;

    /*********************************\
    \*********************************/

    // Pretty print key
    encoded.clear();
    StringSource ss1(key, key.size(), true,
        new HexEncoder(
            new StringSink(encoded)
        ) // HexEncoder
    ); // StringSource

    cout << "key: " << encoded << endl;
    cout << "plain text: " << plain << endl;

    /*********************************\
    \*********************************/

    try
    {
        HMAC< SHA256 > hmac(key, key.size());

        StringSource ss2(plain, true,
            new HashFilter(hmac,
                new StringSink(mac)
            ) // HashFilter      
        ); // StringSource
    }
    catch (const CryptoPP::Exception& e)
    {
        cerr << e.what() << endl;
        exit(1);
    }

    /*********************************\
    \*********************************/

    // Pretty print
    encoded.clear();
    StringSource ss3(mac, true,
        new HexEncoder(
            new StringSink(encoded)
        ) // HexEncoder
    ); // StringSource
    cout << "hmac: " << encoded << endl;

What I have tried:

now for decoding that on the other side
I read this link but it didn't help me HexDecoder - Crypto++ Wiki[^]
and this section doesn't return the plaintext

string decoded;
StringSource ss(encoded, true,
    new HexDecoder(
        new StringSink(decoded)
    ) // HexDecoder
);

I Will be very grateful for any advice