Question

I have working C++ code using swig which creates a struct, passes it to lua (essentially by reference), and allows manipulation of the struct such that the changes made in the lua code remain once I’ve returned to the C++ function. This all works fine until I add a std::string to the struct, as shown here:

struct stuff
{
    int x;
    int y;
    std::string z;
};

I’m unable to modify the std::string because it’s apparently passed as a const reference. If I attempt to assign a value to this string in my lua function I get this error:

Error in str (arg 2), expected ‘std::string const &’ got ‘string’

What is the proper way to address this problem? Do I have to write some custom C++ function to set z rather than using normal syntax like obj.z = "hi"? Is there some way to allow this assignment using swig?

The C++ code is

#include <stdio.h>
#include <string.h>
extern "C" {
#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"
}

#include "example_wrap.hxx"

extern int luaopen_example(lua_State* L); // declare the wrapped module

int main()
{

    char buff[256];
    const char *cmdstr = "print(33)\n";
    int error;
    lua_State *L = lua_open();
    luaL_openlibs(L);
    luaopen_example(L);

    struct stuff b;

    b.x = 1;
    b.y = 2;

    SWIG_NewPointerObj(L, &b, SWIGTYPE_p_stuff, 0);
    lua_setglobal(L, "b");

     while (fgets(buff, sizeof(buff), stdin) != NULL) {
        error = luaL_loadbuffer(L, buff, strlen(buff), "line") ||
                lua_pcall(L, 0, 0, 0);
        if (error) {
          fprintf(stderr, "%s", lua_tostring(L, -1));
          lua_pop(L, 1);  /* pop error message from the stack */
        }
      }

      printf("B.y now %d\n", b.y);
      printf("Str now %s\n", b.str.c_str());
      luaL_dostring(L, cmdstr);
      lua_close(L);
      return 0;

}

Answer 1

You need to add %include <std_string.i> to your SWIG module. Otherwise, it does not know how to map a Lua string to an C++ std::string.

---

A common problem that people encounter is that of classes/structures containing a std::string. This can be overcome by defining a typemap. For example:

%module example
%include "std_string.i"

%apply const std::string& {std::string* foo};

struct my_struct
{
  std::string foo;
};