Lua 中混合类型的相等运算符

Question

Unlike arithmetic metamethods, relational metamethods do not support mixed types.

同时

Lua calls the equality metamethod only when the two objects being compared share this metamethod

所以我正在用 C 实现我的库，并希望能够支持像

这样的行为

a = A()
b = B()
a == b

通过提供

static const struct luaL_Reg mylib_A[] =
{
  { "__eq", my_equal }
  , <more stuff>
  , { NULL, NULL }
};

和

static const struct luaL_Reg mylib_B[] =
{
  { "__eq", my_equal }
  , <more stuff>
  , { NULL, NULL }
};

哪个似乎不起作用，是否有解决方法？
注意：my_equal 能够在其任何参数中处理类型 A 和类型 B 的用户数据

更新：元表注册：

luaL_newmetatable(lua, "B");
lua_pushvalue(lua, -1);
lua_setfield(lua, -2, "__index");
luaL_register(lua, NULL, mylib_B);

luaL_newmetatable(lua, "A");
lua_pushvalue(lua, -1);
lua_setfield(lua, -2, "__index");
luaL_register(lua, NULL, mylib_A);

luaL_register(lua, "mylib", mylib); -- where mylib is a bunch of static functions

申请代码：

require 'mylib'
a = mylib.new_A()
b = mylib.new_B()
a == b -- __eq is not called

Answer 1

编辑：另请参阅，其中有关于在 C API.

中实现 __eq 的特别警告

__eq 元方法属于您的元table，不属于 __index table.

在lua中：

function my_equal(x,y)
    return x.value == y.value
end



A = {} -- luaL_newmetatable(lua, "A");
A.__eq = my_equal

function new_A(value)
    local a = { value = value }
    return setmetatable(a, A)
end


B = {} -- luaL_newmetatable(lua, "B");
B.__eq = my_equal

function new_B(value)
    local b = { value = value }
    return setmetatable(b, B)
end


a = new_A()
b = new_B()
print(a == b) -- __eq is called, result is true

a.value = 5
print(a == b) -- __eq is called, result is false

你所做的是：

myLib_A = {}
myLib_A.__eq = my_equal

A = {} -- luaL_newmetatable(lua, "A");
A.__index = myLib_A

请注意 __eq 在 A 的 metatable 中不是，它在一个完全独立的 table 上，你恰好是使用不同的、不相关的元方法 (__index)。 Lua 在尝试解析 a.

的相等运算符时不会查看那里

Lua 手册对此有详细解释：

"eq": the == operation. The function getcomphandler defines how Lua chooses a metamethod for comparison operators. A metamethod only is selected when both objects being compared have the same type and the same metamethod for the selected operation.

 function getcomphandler (op1, op2, event)
   if type(op1) ~= type(op2) then return nil end
   local mm1 = metatable(op1)[event]
   local mm2 = metatable(op2)[event]
   if mm1 == mm2 then return mm1 else return nil end
 end

The "eq" event is defined as follows:

 function eq_event (op1, op2)
   if type(op1) ~= type(op2) then  -- different types?
     return false   -- different objects
   end
   if op1 == op2 then   -- primitive equal?
     return true   -- objects are equal
   end
   -- try metamethod
   local h = getcomphandler(op1, op2, "__eq")
   if h then
     return (h(op1, op2))
   else
     return false
   end
 end

所以当Lua遇到result = a == b时，它会做如下的事情（这是在C中做的，Lua这里用作伪代码）：

-- Are the operands are the same type? In our case they are both tables:
if type(a) ~= type(b) then
 return false
end

-- Are the operands the same object? This comparison is done in C code, so
-- it's not going to reinvoke the equality operator.
if a ~= b then
 return false
end

-- Do the operands have the same `__eq` metamethod?
local mm1 = getmetatable(a).__eq
local mm2 = getmetatable(b).__eq
if mm1 ~= mm2 then
 return false
end

-- Call the `__eq` metamethod for the left operand (same as the right, doesn't really matter)
return mm1(a,b)

您可以看到这里没有导致解析 a.__eq 的路径，它将通过您的 __index 元方法解析为 myLib_A。

Answer 2

对于将面临同样问题的所有其他人：
这是我让 Lua 意识到 my_equal 在这两种情况下从 Lua 的角度来看是完全相同的函数并因此从 getcomphandler 返回正确的运算符的唯一方法。以任何其他方式（包括单独的 luaL_Reg）注册它都不起作用，因为 my_equal 在 luaL_register 时被保存在不同的闭包下，我在这里通过只创建一次闭包来避免这种情况。

// we'll copy it further to ensure lua knows that it's the same function
lua_pushcfunction(lua, my_equal);

luaL_newmetatable(lua, "B");
// removed __index for clarity
luaL_register(lua, NULL, mylib_B);

// Now we register __eq separately
lua_pushstring(lua, "__eq");
lua_pushvalue(lua, -3); // Copy my_equal on top
lua_settable(lua, -3); // Register it under B metatable
lua_pop(lua, 1);


luaL_newmetatable(lua, "A");
// removed __index for clarity
luaL_register(lua, NULL, mylib_A);

lua_pushstring(lua, "__eq");
lua_pushvalue(lua, -3); // Copy my_equal on top
lua_settable(lua, -3); // Register it under A metatable

luaL_register(lua, "mylib", mylib); // where mylib is a bunch of static functions

Lua 中混合类型的相等运算符

Equality operator on mixed types in Lua

lua

lua-api