Similarly, if I have: float x = 123.1; double y = 123.1; then I expect x == (float)y to be true

I don't know that that is a reasonable expectation. Conversion from double to float is not the exact same operation as the initial assignment. The first is converting the numeric string to a floating point representation. The resulting floating point representation will be very very close to but not necessarily exactly "123.1" or whatever other value you assign to it. By contrast converting double to float is reducing the number of significant digits in a representation that is already approximate. Mismatches between assignment and rounding are always possible.

In general, exact equalities on doubles and floats is not a good idea. I'm a little surprised that PDL code is assuming that it is. I was always taught instead to do x - y < EPSILON where EPSILON is some value small enough to make the difference between the two numbers irrelevant for the purposes of your computation. If exact decimal precision is needed (as is sometimes the case in financial calculations) then one needs to use one of the many C/C++ arbitrary precision libraries.

Best, beth

Update: x - y < EPSILON assumes it is known that x >= y (i.e. prior code has tested that assertion). In the general case where that assertion has not been verified, see comment by roboticus below.