如何从c++中读取和写入嵌套的lua表?

Question

我在lua代码中有一个嵌套表，我想把它传递给c++，以便本机代码可以操作它:

-- 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?

在c++中读取Lua嵌套表和Lua C读取嵌套表都描述了如何从嵌套表中读取，但没有描述如何向它们写入。

Answer 1

简短回答

Lua使用堆栈从表中获取值。要修改表值，你需要用lua_rawgeti推入你想要修改的表，用lua_pushinteger推入你想要插入的值，然后用lua_rawseti设置表中的值。

在编写此代码时，可视化堆栈以确保使用正确的索引是很重要的:

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。所以我们通常会传递-1来访问堆栈上的表。当调用lua_rawseti写入表时，我们将传递-2，因为表在我们写入的值之下。

<标题>

例子我将添加inspect。使用Lua代码打印出表的值，这样我们就可以看到值被修改了。

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, }))

假设您已经了解了BindingCodeToLua，您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 } }