Forward Start Option is a Tradable that represents a forward start Vanilla Option contract with vanilla payoff. The effective strike is set to a preagreed percentage - called moneyness - of the underlying price quoted at some fixed future date. This is equivalent to having a forward start option, the strike of which is set at the future date when the forward option starts. There exist 2 payoff variations compatible with this setting: The first is the vanilla payoff given by the formula max[ S(T) - mS(t*) , 0 ] where S(T) is the underlying value at the exercise time T, S(t*) is the underlying value at the forward start time t*, when the strike is set and m is the moneyness, as agreed in the option contract.
The second is the performance payoff given by the formula max[ S(T)/S(t*) - m , 0 ]
The following features are currently not supported by QuantLib: Bermudan exercise style, discrete dividends/storage costs.