What causes "[" cimg_appname "_math_parser] CImg ::%s: %s: First argument cannot be a linked reference, in expression '%s'." in Yalantis/uCrop?

Pass a plain vector variable (created via 'vector(...)' or a numeric variable) as first argument to merge()

Yalantis/uCrop · error · CImgArgumentException

[" cimg_appname "_math_parser] CImg<

Error message

[" cimg_appname "_math_parser] CImg<%s>::%s: %s: First argument cannot be a linked reference, in expression '%s'.

What it means

The merge() function in the CImg math parser requires its first argument to be an assignable vector variable, not a linked reference. If compiling the first argument yields a reference (non-zero *ref), e.g. an indirection or a view of another vector, the parser throws because merge would mutate storage it does not own.

Solutions

Pass a plain vector variable (created via 'vector(...)' or a numeric variable) as first argument to merge()
Assign the referenced data to a new variable first, then merge that variable
Rewrite the expression to merge into the actual underlying variable

Example fix

// before
merge(I[0],0,+)
// after
V = I[0]; merge(V,0,+)

Defensive patterns

Strategy: validation

Validate before calling

// ensure first merge argument is a plain variable, not I[#i] or a reference
merge_target = V # plain variable only

Try / catch

try { run(expr) } catch (CImgArgumentException &e) { log(e.what()); }

Prevention

Only pass plain vector variables to merge()
Materialize references into new variables before merging
Review custom commands for indexed first arguments

When it happens

Trigger: Calling merge() with a first argument that resolves to a linked/reference vector, such as passing an indexed slice, a dereferenced named reference, or the result of a reference-returning expression instead of a plain variable.

Common situations: Passing 'I[#ind]' style indexed values, or a variable previously assigned by reference, into merge() in a G'MIC/CImg custom command.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/0ca0aa9cc3905651. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:22685

            }

            if (!std::strncmp(ss,"mse(",4)) { // Mean-squared error
              _cimg_mp_op("Function 'mse()'");
              s1 = ss4; while (s1<se1 && (*s1!=',' || level[s1 - expr._data]!=clevel1)) ++s1;
              arg1 = compile(ss4,s1,depth1,0,block_flags);
              arg2 = compile(++s1,se1,depth1,0,block_flags);
              _cimg_mp_check_type(arg2,2,is_scalar(arg1)?1:2,size(arg1));
              _cimg_mp_scalar3(mp_mse,arg1,arg2,size(arg1));
            }

            if (!std::strncmp(ss,"merge(",6)) { // Merge inter-thread variables
              _cimg_mp_op("Function 'merge()'");
              s1 = ss6; while (s1<se1 && (*s1!=',' || level[s1 - expr._data]!=clevel1)) ++s1;
              ref.assign(7);
              pos = compile(ss6,s1,depth1,ref,block_flags);
              if (*ref) {
                _cimg_mp_strerr;
                throw CImgArgumentException("[" cimg_appname "_math_parser] "
                                            "CImg<%s>::%s: %s: First argument cannot be a linked reference, "
                                            "in expression '%s'.",
                                            pixel_type(),_cimg_mp_calling_function,s_op,
                                            s0);
              }

              arg1 = ~0U; // Merge operator
                          // (0='=',1='+',2='-',3='*',4='/',5='&',6='|',7='xor',8='&&',9=='||',10='min',11='max')
              if (s1<se1) {
                ++s1;
                char st_op[4] = {};
                is_sth = false; // blank after operator?
                if (cimg_sscanf(s1," %3[=+-*/&|minaxor]%c",st_op,&sep)==2 && (sep==')' ||
                                                                              (is_sth=cimg::is_blank(sep)))) {
                  if (!is_sth || (is_sth && cimg_sscanf(s1," %*[=+-*/&|minaxor ]%c",&sep)==1 && sep==')')) {
                    cimg::strpare(st_op,' ',false,true);
                    if (!st_op[1])
                      arg1 = *st_op=='='?0:*st_op=='+'?1:*st_op=='-'?2:*st_op=='*'?3:*st_op=='/'?4:

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