Using an Iterable std::map in Java code with SWIG 4.x #3199

trancexpress · 2025-06-16T18:09:31Z

trancexpress
Jun 16, 2025

Hi all,

we have an issue when updating some code from SWIG 2.0.10 to SWIG 4.0.2.

We have the following .i file:

%module test

%include "std_map.i"
%include "std_string.i"

using namespace std;


%typemap(javainterfaces) MapStringStringIterator "java.util.Iterator<String>"
%typemap(javacode) MapStringStringIterator %{
  public void remove() throws UnsupportedOperationException {
    throw new UnsupportedOperationException();
  }

  public String next() throws java.util.NoSuchElementException {
    if (!hasNext()) {
      throw new java.util.NoSuchElementException();
    }

    return nextImpl();
  }
%}

%javamethodmodifiers MapStringStringIterator::nextImpl "private" ;
%inline %{
  struct MapStringStringIterator {
    typedef std::map<std::string,std::string> map_t;
    MapStringStringIterator(const map_t& m) : it(m.begin()), map(m) {}
    bool hasNext() const {
      return it != map.end();
    }

    const std::string & nextImpl() {
      const std::pair<std::string,std::string>& ret = *it++;
      return ret.first;
    }
  private:
    map_t::const_iterator it;
    const map_t& map;
  };
%}

%newobject std::map<std::string,std::string>::iterator() const;
%extend std::map<std::string,std::string> {
  MapStringStringIterator *iterator() const {
    return new MapStringStringIterator(*$self);
  }
}

%typemap(javainterfaces) std::map<std::string,std::string> "Iterable<String>"

%template(map_String_String) std::map<std::string,std::string>;

We compile it with:

JAVA_HOME=/usr/lib/jvm/java-21/

all:
	swig -version
	rm -f test/*.java *.cxx libtest.so
	mkdir -p test
	swig -nopgcpp -package test -c++ -v -java -outdir test test.i
	g++ -std=c++11 -Wall -Wfatal-errors -fPIC -shared -o libtest.so test_wrap.cxx -I$(JAVA_HOME)/include -I$(JAVA_HOME)/include/linux
	$(JAVA_HOME)/bin/javac test/*.java

Using SWIG 2.0.10, this compiles fine:

$ make
swig -version

SWIG Version 2.0.10

Compiled with g++ [x86_64-redhat-linux-gnu]

Configured options: +pcre

Please see http://www.swig.org for reporting bugs and further information
rm -f test/*.java *.cxx libtest.so
mkdir -p test
swig -nopgcpp -package test -c++ -v -java -outdir test test.i
Language subdirectory: java
Search paths:
   ./
   ./swig_lib/java/
   /usr/share/swig/2.0.10/java/
   ./swig_lib/
   /usr/share/swig/2.0.10/
Preprocessing...
Starting language-specific parse...
Processing types...
C++ analysis...
Generating wrappers...
g++ -std=c++11 -Wall -Wfatal-errors -fPIC -shared -o libtest.so test_wrap.cxx -I/usr/lib/jvm/java-21//include -I/usr/lib/jvm/java-21//include/linux
/usr/lib/jvm/java-21//bin/javac test/*.java
test/MapStringStringIterator.java:24: warning: [removal] finalize() in Object has been deprecated and marked for removal
  protected void finalize() {
                 ^
test/map_String_String.java:24: warning: [removal] finalize() in Object has been deprecated and marked for removal
  protected void finalize() {
                 ^
2 warnings

When using SWIG 4.0.2, we run into errors in the generated .cxx code:

$ make
swig -version

SWIG Version 4.0.2

Compiled with g++ [x86_64-redhat-linux-gnu]

Configured options: +pcre

Please see http://www.swig.org for reporting bugs and further information
rm -f test/*.java *.cxx libtest.so
mkdir -p test
swig -nopgcpp -package test -c++ -v -java -outdir test test.i
Language subdirectory: java
Search paths:
   ./
   ./swig_lib/java/
   /usr/share/swig/4.0.2/java/
   ./swig_lib/
   /usr/share/swig/4.0.2/
Preprocessing...
Starting language-specific parse...
/usr/share/swig/4.0.2/java/std_map.i:158: Warning 302: Identifier 'Iterator' redefined by %extend (ignored),
test.i:47: Warning 302: %extend definition of 'Iterator'.
Processing types...
C++ analysis...
Processing nested classes...
Generating wrappers...
g++ -std=c++11 -Wall -Wfatal-errors -fPIC -shared -o libtest.so test_wrap.cxx -I/usr/lib/jvm/java-21//include -I/usr/lib/jvm/java-21//include/linux
test_wrap.cxx: In function ‘jlong Java_test_testJNI_map_1String_1String_1find(JNIEnv*, jclass, jlong, jstring)’:
test_wrap.cxx:413:3: error: ‘iterator’ was not declared in this scope; did you mean ‘std::iterator’?
  413 |   iterator result;
      |   ^~~~~~~~
      |   std::iterator
compilation terminated due to -Wfatal-errors.
make: *** [Makefile:8: all] Error 1

The generated code with SWIG 4.0.2 is generating iterator result instead of std::map< std::string,std::string >::iterator result:

SWIGEXPORT jlong JNICALL Java_test_testJNI_map_1String_1String_1find(JNIEnv *jenv, jclass jcls, jlong jarg1, jstring jarg2) {
  jlong jresult = 0 ;
  std::map< std::string,std::string > *arg1 = (std::map< std::string,std::string > *) 0 ;
  std::string *arg2 = 0 ;
  iterator result;
  
  (void)jenv;
  (void)jcls;
  arg1 = *(std::map< std::string,std::string > **)&jarg1; 
  if(!jarg2) {
    SWIG_JavaThrowException(jenv, SWIG_JavaNullPointerException, "null string");
    return 0;
  }
  const char *arg2_pstr = (const char *)jenv->GetStringUTFChars(jarg2, 0); 
  if (!arg2_pstr) return 0;
  std::string arg2_str(arg2_pstr);
  arg2 = &arg2_str;
  jenv->ReleaseStringUTFChars(jarg2, arg2_pstr); 
  result = (arg1)->find((std::string const &)*arg2);
  *(iterator **)&jresult = new iterator((const iterator &)result); 
  return jresult;
}

I've noticed that the generated .cxx code is fine, if I pull the definition of map_String_String before the @extend line:

%typemap(javainterfaces) std::map<std::string,std::string> "Iterable<String>"

%template(map_String_String) std::map<std::string,std::string>;

%newobject std::map<std::string,std::string>::iterator() const;
%extend std::map<std::string,std::string> {
  MapStringStringIterator *iterator() const {
    return new MapStringStringIterator(*$self);
  }
}

(no error in .cxx)

%newobject std::map<std::string,std::string>::iterator() const;
%extend std::map<std::string,std::string> {
  MapStringStringIterator *iterator() const {
    return new MapStringStringIterator(*$self);
  }
}

%typemap(javainterfaces) std::map<std::string,std::string> "Iterable<String>"

%template(map_String_String) std::map<std::string,std::string>;

10000

(error in .cxx)

Unfortunately, when I re-order the lines, the generated Java class has an error:

test/map_String_String.java:11: error: map_String_String is not abstract and does not override abstract method iterator() in Iterable
public class map_String_String extends java.util.AbstractMap<String, String> implements Iterable<String> {

How to rewrite the .i file to have the same functionality (a map that implements Iterable) with SWIG 4.x?

Best regards and thanks,
Simeon

trancexpress · 2025-06-17T08:55:23Z

trancexpress
Jun 17, 2025
Author

Possibly the change came with this PR: #1356

Up until SWIG 3.0.12 I don't see the generated errors, SWIG 4.0.0 is the first version I see them in.

And in particular this commit is the first one I see the error with: 573ddb0

0 replies

trancexpress · 2025-06-19T17:09:46Z

trancexpress
Jun 19, 2025
Author

If I rename the iterator() method in the .i file, the code is generated and the only error is the missing ... So maybe this is some collision with the type from std the Java method name for Iterable.

If there is some way to also generate the needed Java method, it would be great. E.g. rename the method:

%newobject std::map<std::string,std::string>::specialIterator() const;
%extend std::map<std::string,std::string> {
   MapStringStringIterator *specialIterator() const {
    return new MapStringStringIterator(*$self);
  }
}

%typemap(javainterfaces) std::map<std::string,std::string> "Iterable<String>"

%template(map_String_String) std::map<std::string,std::string>;

And then add this to the generated Java code (no idea how to do that unfortunately):

  public java.lang.Iterator iterator() {
    return specialIterator();
  }

0 replies

wsfulton · 2025-06-26T07:07:10Z

wsfulton
Jun 26, 2025
Maintainer

The C++ standard std::map already defines iterator so your version does indeed clash with it. Use a %rename, eg:

%newobject std::map<std::string,std::string>::custom_iterator() const;
%extend std::map<std::string,std::string> {

8000
%rename(iterator) custom_iterator() const;
  MapStringStringIterator *custom_iterator() const {
    return new MapStringStringIterator(*$self);
  }
}

Also consider dropping all your customisations and use the new SWIG wrapped version unchanged as it implements java.util.AbstractMap which has a standard Java API for iterating over both the keys and values.

0 replies

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