Here is a bit of explenation.
Could not convert '(operator new(104u), (<statement>, ((DoubleSolenoid*) <anonymous>)))'
from' 'DoubleSolenoid*' to 'DoubleSolenoid'
The bolded parts are the important notes. Note the asterisk (or 'star') in the from and also note that it is missing in the to.
The new operator creates a new version of the class (DoubleSolenoid here) and returns a pointer to it. The asterisk (or 'star') indicates changes the type of grabSolenoid to a "pointer to DoubleSolenoid", thus making the types on both sides the same.
Pointers are one of the more confusing aspects of C/C++, however they are used everywhere. Remember, the pointer doesn't hold the data, it just points to where it is at. For classes, use the -> operator to access methods and fields from the pointer.