How to map an associative array in JavaScript to a string map in C++ using Swig?

Question

Quite similar to this question I want to wrap a function with SWIG which takes a map of strings to strings:

void foo(std::map<std::string, std::string> const& args);

For Python it's enough to create an alias for the map:

namespace std {
    %template(map_string_string) map<string, string>;
}

The code generator will create a wrapper function map_string_string and even automatically use it.

my_module.foo({'a': 'b', 'c', 'd'})

Will correctly be called and values which don't fit the signature will just be left out.

How do I do this for JavaScript?

I tried the same (of course) and the wrapper get's generated but when I try to call foo like this:

my_module.foo({'a':'b', 'c':'d'});

I get

/path/to/example.js:3
my_module.foo({'a':'b', 'c':'d'});
          ^

Error: in method 'foo', argument 1 of type 'std::map< std::string,std::string > const &'
    at Object.<anonymous> (/path/to/example.js:8:7)
    at Module._compile (module.js:653:30)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Function.Module.runMain (module.js:694:10)
    at startup (bootstrap_node.js:204:16)
    at bootstrap_node.js:625:3

Even when I try to call the wrapper function map_string_string I get this error..

Is there another way to write "string maps" in JavaScript? Or is there an easy receipt to wrap an associative array in Swig?

Edit: for completeness I've added the source files I've used:

api.h

#pragma once

#include <string>
#include <map>
#include <iostream>

static void foo(std::string const& value) noexcept {
  std::cout << value << std::endl;
}

static void bar(std::map<std::string, std::string> const& args) noexcept {
    for (auto && e : args) {
        std::cout << e.first << ": " << e.second << std::endl;
    }
}

api.i

%module api 
%include "std_string.i"
%include "std_map.i"
namespace std {
    %template(map_string_string) map<string, string>;
}
%{
    #include <api.h>
%}
%include "api.h"

This is the way I build the Python and JavaScript modules:

swig -c++ -python -o api_wrap_python.cxx api.i 
g++ -c api_wrap_python.cxx \
    -I/usr/include/python3.6m -I . \
    -fPIC -std=gnu++11
g++ -shared api_wrap_python.o -o _api.so

swig -c++ -javascript -node -o api_wrap_js.cxx api.i
g++ -c api_wrap_js.cxx \
    -I /usr/include/node -I . \
    -std=gnu++11 -fPIC -DBUILDING_NODE_EXTENSION
g++ -shared api_wrap_js.o -o api.node

and finally this is how I test them:

node -e "api = require('api.node'); api.foo('some string'); api.bar({'a':'b'});"
python3 -c "import api; api.foo('hello'); api.bar({'a':'b','c':'d'})"

In both cases - Python and JavaScript - api.foo() is beeing exectued as expected. api.bar() can be executed on Python but in JavaScript the error I've posted gets thrown.

Answer 1

I looks like the current version of swig (3.0.12) does not have built-in support for mapping a JavaScript object or primitive to a map. I think you will have to write your own transformer that takes a JavaScript object and converts it to a C++ map. See this article for a little help on that.