Proposal for a simple new auxiliary Lua API function

[Elemer Pixard]

When coding a Lua binding to a C/C++ function which requires text
formatting parameters such as:

void cfunc(p1, p2, fmt, ...);

One simple solution is to use string.format() in Lua
or coding a Lua wrapper function which does that.

// The binding accepts only the formatted string
void lua_cfunc(lua_State* L) {
    // Get p1 and p2
    const char* fs = luaL_checkstring(L, 3)
    cfunc(p1, p2, "%s", fs);
    return 0;
}

// Call in Lua
func(p1, p2, string.format(...))

To avoid using or creating this wrapper, the following function could be
exposed to format a string directly and put the result on the stack:

int luaL_formatstring(lua_State* L, int index);

Using this function the binding becomes:

void lua_cfunc(lua_State* L) {
    // Get p1 and p2
    luaL_formatstring(L, 3);
    const char* fs = lua_tostring(L, -1);
    cfunc(p1, p2, "%s", fs);
    return 0;
}

// Call in Lua

func(p1, p2, fmt, ...)

This new function is basically str_format() from strlib.c with
an additional parameter with the index in the stack of the 'fmt' argument.

So str_format() could be changed to:

static int str_format(lua_State* L) {
    return luaL_formatstring(L, 1);
}

Please advice if I missed some other simple alternative.
Thanks and best regards.

[Denis Dos Santos Silva]

https://jdhao.github.io/2022/12/02/lua-variadic-arguments/