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