Extracting Public key from Private key using Botan

I'm trying to encrypt a message using asymmetric private-public keys. In Botan using Load_key() functions, I read the private key and want to extract it's public key from it. For constructing a RSA public key in it's constructor I'll need a "Algorithm Identifier" object and "key bits" which I have the algorithm identifier object using pcks8_algorithm_identifier() function. The problem is the "Key Bits" which returns a secure_vector instead of a vector and I encounter a "bad::alloc exception" ! Does anyone encounter such problem? If there is an alternative way of asymmetric encryption by loading keys from an input file in Botan I'll appreciate that