Yalantis/uCrop · error · CImgArgumentException

[" cimg_appname "_math_parser] CImg<

Error message

[" cimg_appname "_math_parser] CImg<%s>::%s: %s%s Image list cannot be empty, for expression '%s'.

What it means

The math parser helper check_list throws when an expression function that operates on the image list (e.g. dynamic-array functions) is invoked while the parser's imglist is empty. CImg needs at least one image in the list to execute such functions. This prevents out-of-bounds access to a non-existent image slot.

Solutions

  1. Ensure the CImg/CImgList is assigned (non-empty) before evaluating the expression.
  2. Check that the image file load succeeded before running fill()/expression.
  3. Use an expression that does not require the image list if no images are needed.
  4. Guard with if (!img.is_empty()) before evaluating.

Example fix

// before
CImg<float> img; // empty
img.fill("da_at(I,0)");
// after
CImg<float> img(w,h,1,1,0);
img.fill("da_at(I,0)");
Defensive patterns

Strategy: validation

Validate before calling

if (img.is_empty()) throw std::runtime_error("image must be assigned before expression evaluation");

Type guard

bool usable(const CImg<T>& im) { return !im.is_empty() && im.width() > 0; }

Try / catch

try { img.fill(expr); } catch (const cimg_library::CImgArgumentException& e) { /* assign image or skip */ }

Prevention

When it happens

Trigger: Calling a function like 'da_at(...)' or other image-list-dependent MP functions inside fill()/expression on a default-constructed or empty CImgList, or an expression evaluated without assigning images.

Common situations: Invoking expression functions on a CImg that has never been assigned data; passing an empty CImgList to the math parser; chaining operations after a failed load left the list empty.

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

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/22fe3240b418f639. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:25267

            else cimg_snprintf(sb_type,sb_type._width,"'scalar' or 'vector'");
          }
          char *s0;
          _cimg_mp_strerr;
          throw CImgArgumentException("[" cimg_appname "_math_parser] "
                                      "CImg<%s>::%s: %s%s %s%s has invalid type '%s' (should be %s), "
                                      "in expression '%s'.",
                                      pixel_type(),_cimg_mp_calling_function,s_op,*s_op?":":"",
                                      s_arg,*s_op=='F'?(*s_arg?" argument":" Argument"):(*s_arg?" operand":" Operand"),
                                      s_type(arg)._data,sb_type._data,s0);
        }
      }

      // Check that imglist are not empty.
      void check_list(char *const ss, char *const se, const char saved_char) {
        if (!imglist) {
          char *s0;
          _cimg_mp_strerr;
          throw CImgArgumentException("[" cimg_appname "_math_parser] "
                                      "CImg<%s>::%s: %s%s Image list cannot be empty, for expression '%s'.",
                                      pixel_type(),_cimg_mp_calling_function,s_op,*s_op?":":"",s0);
        }
      }

      static void mp_check_list(_cimg_math_parser& mp, const char *const funcname) {
        if (!mp.imglist)
          throw CImgArgumentException("[" cimg_appname "_math_parser] "
                                      "CImg<%s>: Function '%s()': Images list cannot be empty.",
                                      pixel_type(),funcname);
      }

      // Insert constant value in memory.
      unsigned int const_scalar(const double val) {

        // Search for built-in constant.
        if (cimg::type<double>::is_nan(val)) return _cimg_mp_slot_nan;
        if (val==(double)(int)val) {

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