getting DSA key length

[Renzo Tomaselli]

Hi all,
    I learned to get RSA key bit length as follows:

  AsnBits *y = ...
  CryptoPP::ByteQueue publicKey;
  publicKey.Put((const Byte *)y->data(), y->length());
  CryptoPP::RSAFunction f;
  f.BERDecodeKey(publicKey);
  keyBits = f.GetModulus().BitCount();

What is the equivalent for DSA, provided that I fetch a SubjectPublicKeyInfo
struct from a certificate ?
Thanks for any help,

Renzo Tomaselli