Yalantis/uCrop · error · CImgArgumentException

[cimg_appname_math_parser] CImg<%s>::%s: %s: Invalid assignm

Error message

[cimg_appname_math_parser] CImg<%s>::%s: %s: Invalid assignment of %sconst variable '%s'%s, in expression '%s'.

What it means

CImg allows variables to be declared const (e.g. 'const x = ...'). Assigning a new value to a variable that is already const, or to a const scalar slot, is rejected by the expression compiler with this error.

Source

Thrown at ucrop/src/main/jni/CImg.h:18700

                  if (is_const) arg1 = arg3;
                  else {
                    arg1 = is_sth || is_comp_scalar(arg3)?arg3:scalar1(mp_copy,arg3);
                    memtype[arg1] = -1;
                  }
                }

                if (arg2!=~0U) reserved_label[arg2] = arg1;
                  else {
                    if (variable_def._width>=variable_pos._width) variable_pos.resize(-200,1,1,1,0);
                    variable_pos[variable_def._width] = arg1;
                    variable_name.move_to(variable_def);
                  }

              } else { // Variable already exists -> assign a new value
                if (is_const || is_const_scalar(arg1)) {
                  cimg::strellipsize(variable_name,64);
                  _cimg_mp_strerr;
                  throw CImgArgumentException("[" cimg_appname "_math_parser] "
                                              "CImg<%s>::%s: %s: Invalid assignment of %sconst variable '%s'%s, "
                                              "in expression '%s'.",
                                              pixel_type(),_cimg_mp_calling_function,s_op,
                                              is_const_scalar(arg1)?"":"non-",
                                              variable_name._data,
                                              !is_const_scalar(arg1) && is_const?" as a const variable":"",
                                              s0);
                }
                _cimg_mp_check_type(arg3,2,is_vector(arg1)?3:1,size(arg1));
                if (is_vector(arg1)) { // Vector
                  if (is_vector(arg3)) // From vector
                    CImg<ulongT>::vector((ulongT)mp_vector_copy,arg1,arg3,(ulongT)size(arg1)).
                      move_to(code);
                  else // From scalar
                    CImg<ulongT>::vector((ulongT)mp_vector_init,arg1,1,(ulongT)size(arg1),arg3).
                      move_to(code);
                } else // Scalar
                  CImg<ulongT>::vector((ulongT)mp_copy,arg1,arg3).move_to(code);

View on GitHub (pinned to f788b534b4)

Solutions

  1. Remove the 'const' keyword from the original declaration
  2. Use a different variable name for the new value
  3. Restructure the expression so const variables are write-once

Example fix

// before
const x = 3; x = x + 1;
// after
x = 3; x = x + 1;  // or keep const and use y = x + 1
Defensive patterns

Strategy: validation

Validate before calling

// Track const declarations in your expression strings
function findConstReassign(expr) {
  const consts = [...expr.matchAll(/const\s+([A-Za-z_]\w*)\s*=/g)].map(m => m[1]);
  return consts.filter(name =>
    new RegExp(`(?<!const\\s)\\b${name}\\s*(=[^=]|\\+=|-=|\\*=|/=)`).test(expr)
  );
}

Try / catch

try {
  img.evaluate(expr);
} catch (e) {
  if (/Invalid assignment of .*const variable/.test(String(e))) {
    // remove const or rename the assignment target
  } else throw e;
}

Prevention

When it happens

Trigger: Using assignment ('=', '+=', etc.) on a variable that was previously defined with 'const', or assigning to a memory slot holding a constant scalar (is_const_scalar(arg1)).

Common situations: Reusing a const variable name as a loop accumulator, mixing const declarations with later reassignments in long expressions.

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/42aea72e00ed7ba2. Report an issue: GitHub.