Generic solution to get simple values from lua file into c++

Question

I'm trying to use a Lua file as a config or an ini. I have succeeded but my solution irritates me. Specifically, get_double, get_int and get_string functions needs to be done in a reusable way.

I ran into problems creating function templates that don't have arguments. Also, I'm not sure how to generalize lua_is... and lua_to.... My idea was to us if(is_same<T,double>::value) return (double)lua_isnumber(L,-1); but it didn't work.

Here is the working code:

main.cc:

#include <iostream>
#include <lua.hpp>

using namespace std;

class Lua_vm 
{
private:
  lua_State *L;
public:
  double get_double(const char *var_name) {
    lua_getglobal(L,var_name);
    if (!lua_isnumber(L,-1)) {
      cout << "error: " << var_name << " is of a wrong type\n";
    }
    return (double)lua_tonumber(L,-1);
    lua_pop(L,1);
  }
  int get_int(const char *var_name) {
    lua_getglobal(L,var_name);
    if (!lua_isnumber(L,-1)) {
      cout << "error: " << var_name << " is of a wrong type\n";
    }
    return (int)lua_tonumber(L,-1);
    lua_pop(L,1);
  }
  string get_string(const char *var_name) {
    lua_getglobal(L,var_name);
    if (!lua_isstring(L,-1)) {
      cout << "error: " << var_name << " is of a wrong type\n";
    }
    return string(lua_tostring(L,-1));
    lua_pop(L,1);
  }
  Lua_vm(const char *lua_config_filename) {
    L = lua_open();
    if (luaL_loadfile(L, lua_config_filename) || lua_pcall(L, 0,0,0)) {
      cout << "error: " << lua_tostring(L,-1) << "\n";
    }
    lua_pushnil(L);
  }
  ~Lua_vm() {
    lua_close(L);
  }
};
int main(int argc, char** argv)
{
  Lua_vm lvm("config.lua");

  cout << "vol is " << lvm.get_double("vol") << "\n";
  cout << "rho is " << lvm.get_int("rho") << "\n";
  cout << "method is " << lvm.get_string("method") << "\n";

  return 0;
}

config.lua:

method = "cube"
len = 3.21
rho = 13
vol = len*len*len
mass = vol*rho

It compiles with g++ main.C -I/usr/include/lua5.1/ -llua5.1 ; ./a.out

Answer 1

Here is code I use for that purpose (more: http://tom.rethaller.net/wiki/projects/prologue/lua)

class LuaState
{
private:
    lua_State *L;
    // [...]    

public:
    // get function
    template<typename T>
    T get(const char * varname) {
      char temp[64];
      memset(temp, 0, sizeof(temp));
      int i=0;
      int j=0;
      int n=0;
      while (varname[i] != '\0') {
        char c = varname[i];
        if (c == '.') {
          if (n == 0)
            lua_getglobal(L, temp);
          else
            lua_getfield(L, -1, temp);
          ++n;
          memset(temp, 0, sizeof(temp));
          j = 0;

          if (lua_isnil(L, -1))
            return 0;
        }
        else {
          temp[j] = c;
          ++j;
        }
        ++i;
      }
      if (n == 0)
        lua_getglobal(L, temp);
      else
        lua_getfield(L, -1, temp);
      return lua_get<T>();
    }

    // Generic get
    template<typename T>
    T lua_get() {
      return 0;
    }

    // Specializations
    template <> float lua_get<float>() {
      return lua_tonumber(L, -1);
    }
    template <> double lua_get<double>() {
      return lua_tonumber(L, -1);
    }
    template <> bool lua_get<bool>() {
      return lua_toboolean(L, -1);
    }
    template <> int lua_get<int>() {
      return lua_tointeger(L, -1);
    }
    template <> unsigned int lua_get<unsigned int>() {
      return (unsigned int)lua_tonumber(L, -1);
    }
    template <> const char * lua_get<const char *>() {
      return lua_tostring(L, -1);
    }
};

-- Edit - To illustrate how to use it, you just have to call:

bool use_blur = lua_state.get<bool>("config.render.effects.blur");

to get the value of:

config = {
    render = {
        effects = {
            blur = false,
        }
    }
}