inline keyword is telling compiler to not call function but simply copy its body into the code everytime it is exectued. double is the type of returning value FittingMod:: is the class name to which this method belong const std::vector<double>& t is constant (read only) reference to std::vector holding double types. const std::vector<double>& p is same as previous.
when you place & or * there is no need for space.
int *p is the same as int*p or int * p, although i prefer to use int* p since pointer is type of variable.