How to efficiently get the Complement of a Domain and several Intervals (IntervalComplement?)

int = {{3, 7}, {17, 43}, {64, 70}};

com = Partition[#, 2]& @ {1, Sequence @@ Riffle[int[[All, 1]] - 1, int[[All, 2]] + 1], 100}

{{1, 2}, {8, 16}, {44, 63}, {71, 100}}

NumberLinePlot[{Interval @@ int, Interval @@ com}, PlotTheme -> "Detailed"]

f1 = Partition[Flatten[{0, Select[Flatten@#, Function[x, x < #2]], #2 + 1}], 
      2, 2, {1, -1}, {}, ({1, -1} + {##}) /. {1, 0} -> Nothing &]&;

f1[{{3, 7}, {17, 43}, {64, 70}}, 80]

{{1, 2}, {8, 16}, {44, 63}, {71, 80}}

Also

f2 = BlockMap[({1, -1} + #) /. {1, 0} -> Nothing &, 
      Flatten[{0, Select[Flatten@#, Function[x, x < #2]], #2 + 1}], 2] &;
f2[{{3, 7}, {17, 43}, {64, 70}}, 80]

{{1, 2}, {8, 16}, {44, 63}, {71, 80}}

Note: In versions before 10.2 you can use Developer`PartitionMap in place of BlockMap above. (thanks: @CarlWoll)