如何从 C++ 读取和写入嵌套的 lua 表？

Question

我的 lua 代码中有一个嵌套表，我想将它传递给 C++，以便本机代码可以对其进行操作：

-- Some persistent data in my game
local data = {
        { 44, 34, 0, 7, },
        { 4, 4, 1, 3, },
}
-- Pass it into a C++ function that can modify the input data.
TimelineEditor(data)

如何编写 C++ 代码来读取嵌套表并修改其值？

Reading Lua nested tables in C++ 和 lua c read nested tables 都描述了我如何从嵌套表中读取，但没有描述如何写入它们。

Answer 1

简答

Lua 使用堆栈来获取表中的值。要修改表值，您需要使用lua_rawgeti 推送要修改的表，使用lua_pushinteger 推送要插入的值，然后使用lua_rawseti 在表中设置值。

在编写本文时，重要的是可视化堆栈以确保您使用正确的索引：

lua_rawgeti()
    stack:
        table

lua_rawgeti()
    stack:
        number <-- top of the stack
        table

lua_tonumber()
    stack:
        number
        table

lua_pop()
    stack:
        table

lua_pushinteger()
    stack:
        number
        table

lua_rawseti()
    stack:
        table

负索引是堆栈位置，正索引是参数位置。所以我们经常会通过 -1 来访问堆栈中的表。当调用lua_rawseti 写入表时，我们将传递-2，因为表低于我们正在写入的值。

示例

我将在 lua 代码中添加inspect.lua 以打印出表值，以便我们可以看到这些值已被修改。

local inspect = require "inspect"

local data = {
        { 44, 34, 0, 7, },
        { 4, 4, 1, 3, },
}
print("BEFORE =", inspect(data, { depth = 5, }))
TimelineEditor(data)
print("AFTER =", inspect(data, { depth = 5, }))

假设你已经找到BindingCodeToLua，你可以像这样实现这个函数：

// Replace LOG with whatever you use for logging or use this:
#define LOG(...) printf(__VA_ARGS__); printf("\n")

// I bound with Lunar. I don't think it makes a difference for this example.
int TimelineEditor(lua_State* L)
{
    LOG("Read the values and print them out to show that it's working.");
    {
        int entries_table_idx = 1;
        luaL_checktype(L, entries_table_idx, LUA_TTABLE);
        int n_entries = static_cast<int>(lua_rawlen(L, entries_table_idx));
        LOG("%d entries", n_entries);
        for (int i = 1; i <= n_entries; ++i)
        {
            // Push inner table onto stack.
            lua_rawgeti(L, entries_table_idx, i);
            int item_table_idx = 1;
            luaL_checktype(L, -1, LUA_TTABLE);
            int n_items = static_cast<int>(lua_rawlen(L, -1));
            LOG("%d items", n_items);
            for (int i = 1; i <= n_items; ++i)
            {
                // Push value from table onto stack.
                lua_rawgeti(L, -1, i);
                int is_number = 0;
                // Read value
                int x = static_cast<int>(lua_tonumberx(L, -1, &is_number));
                if (!is_number)
                {
                    // fire an error
                    luaL_checktype(L, -1, LUA_TNUMBER);
                }
                LOG("Got: %d", x);
                // pop value off stack
                lua_pop(L, 1);
            }
            // pop table off stack
            lua_pop(L, 1);
        }
    }

    LOG("Overwrite the values");
    {
        int entries_table_idx = 1;
        luaL_checktype(L, entries_table_idx, LUA_TTABLE);
        int n_entries = static_cast<int>(lua_rawlen(L, entries_table_idx));
        LOG("%d entries", n_entries);
        for (int i = 1; i <= n_entries; ++i)
        {
            // Push inner table onto stack.
            lua_rawgeti(L, entries_table_idx, i);
            int item_table_idx = 1;
            luaL_checktype(L, -1, LUA_TTABLE);
            int n_items = static_cast<int>(lua_rawlen(L, -1));
            LOG("%d items", n_items);
            for (int j = 1; j <= n_items; ++j)
            {
                int x = j + 10;
                // Push new value onto stack.
                lua_pushinteger(L, x);
                // rawseti pops the value off. Need to go -2 to get to the
                // table because the value is on top.
                lua_rawseti(L, -2, j);
                LOG("Wrote: %d", x);
            }
            // pop table off stack
            lua_pop(L, 1);
        }
    }
    // No return values
    return 0;
}

输出：

BEFORE =    { { 44, 34, 0, 7 }, { 4, 4, 1, 3 } }
Read the values and print them out to show that it's working.
2 entries
4 items
Got: 44
Got: 34
Got: 0
Got: 7
4 items
Got: 4
Got: 4
Got: 1
Got: 3
Overwrite the values
2 entries
4 items
Wrote: 11
Wrote: 12
Wrote: 13
Wrote: 14
4 items
Wrote: 11
Wrote: 12
Wrote: 13
Wrote: 14
AFTER =     { { 11, 12, 13, 14 }, { 11, 12, 13, 14 } }