Yalantis/uCrop · error · CImgInstanceException

select(): Empty instance.

Error message

select(): Empty instance.

What it means

CImgList::_select() (backing select() family for interactive feature selection in a display) throws CImgInstanceException when the list is empty — there are no images to visualize or pick from. It is an instance-state error, not an argument error.

Solutions

1. Check list.is_empty() / list.size() > 0 before calling select() and bail out with a message
2. Ensure the list was actually loaded/populated (verify the load call succeeded)
3. Initialize the CImgDisplay and list from a valid data source before interaction

Example fix

// before
disp.select(list); // throws on empty list
// after
if (!list.is_empty()) {
  CImg<int> sel = disp.select(list);
} else {
  std::fprintf(stderr, "nothing to select from\n");
}

if (list.is_empty()) { /* report and skip interactive selection */ }

try { sel = disp.select(list); } catch (const CImgInstanceException& e) { std::fprintf(stderr, "empty list\n"); }

When it happens

Trigger: Calling list.select(disp) (or the feature/axis variants) on a CImgList whose size is 0, typically after a failed or skipped load.

Common situations: Load returned an empty list (missing file) and the interactive selection was started anyway; a filter step removed all images before selection; wrong variable passed to select().

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• atXYZC(): Empty instance.
• _cimg_instance "max(): Empty instance."
• _cimg_instance "min(): Empty instance."
• _cimg_instance "minabs(): Empty instance."
• CImg< >:: () [%u]: cubic_atX(): Empty instance.

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/530443ea8d8f8016. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:66319

       \param feature_type Can be \c false to select a single image, or \c true to select a sublist.
       \param axis Axis along whom images are appended for visualization.
       \param align Alignment setting when images have not all the same size.
       \param exit_on_anykey Exit function when any key is pressed.
       \return A one-column vector containing the selected image indexes.
    **/
    CImg<intT> get_select(const char *const title, const bool feature_type=true,
                          const char axis='x', const float align=0,
                          const bool exit_on_anykey=false) const {
      CImgDisplay disp;
      return _select(disp,title,feature_type,axis,align,exit_on_anykey,0,false,false,false);
    }

    CImg<intT> _select(CImgDisplay &disp, const char *const title, const bool feature_type,
                       const char axis, const float align, const bool exit_on_anykey,
                       const unsigned int orig, const bool resize_disp,
                       const bool exit_on_rightbutton, const bool exit_on_wheel) const {
      if (is_empty())
        throw CImgInstanceException(_cimglist_instance
                                    "select(): Empty instance.",
                                    cimglist_instance);

      // Create image correspondence table and get list dimensions for visualization.
      CImgList<uintT> _indices;
      unsigned int max_width = 0, max_height = 0, sum_width = 0, sum_height = 0;
      cimglist_for(*this,l) {
        const CImg<T>& img = _data[l];
        const unsigned int
          w = CImgDisplay::_fitscreen(img._width,img._height,img._depth,128,-85,false),
          h = CImgDisplay::_fitscreen(img._width,img._height,img._depth,128,-85,true);
        if (w>max_width) max_width = w;
        if (h>max_height) max_height = h;
        sum_width+=w; sum_height+=h;
        if (axis=='x') CImg<uintT>(w,1,1,1,(unsigned int)l).move_to(_indices);
        else CImg<uintT>(h,1,1,1,(unsigned int)l).move_to(_indices);
      }
      const CImg<uintT> indices0 = _indices>'x';

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