Hello,

There is an old version of the NIFTI C-library (which should
build under C++) in the NIFTI repository here:

   https://www.nitrc.org/projects/nifti

See Downloads -> nifticlib-2.0.0

For a more current version that includes NIFTI-2, see
the upstream AFNI package, where nifticlib resides:

   https://afni.nimh.nih.gov/pub/dist/tgz/a...

Within that package see the tree afni_src/nifti, under
which is niftilib and nifti2, for example.

- rick