Constructs the distribution.
explicit exponential_distribution(RealType lambda = 1.0); explicit exponential_distribution(const param_type& parm);
Precondition: 0.0 < lambda
The first constructor constructs an object whose stored lambda value holds the value lambda.
The second constructor constructs an object whose stored parameters are initialized from parm. You can obtain and set the current parameters of an existing distribution by calling the param() member function.