你能从 lua 修改 C 结构吗？

Question

我希望能够拥有这个 Lua 代码:

function myfunc(s)
    print(s.value)
    s.value = 7
end

它应该适用于这个 C(++) 代码:

struct MyStruct {
    float value;
};

void func() {
    MyStruct var;
    var.value = 5;

    lua_getglobal(L, "myfunc");
    // push `var` to lua, somehow

    lua_call(L, 1, 0); // prints "5"

    // now, var.value is 7
    assert(var.value == 7);
}

我将如何推送 var到堆栈上，以便 lua 代码可以修改其变量？

Answer 1

标准 Lua 没有内置的方式来公开 struct 的内容。 s 通过表格，因此这样做有点手动过程。这是一个完整程序的示例，它可以执行您想要的操作:

#include <assert.h>
#include <string.h>

#include <lua5.3/lua.h>
#include <lua5.3/lualib.h>
#include <lua5.3/lauxlib.h>

struct MyStruct {
    float value;
};

int MyStruct_index(lua_State *L) {
    struct MyStruct *t = (struct MyStruct *)luaL_checkudata(L, 1, "MyStruct");
    const char *k = luaL_checkstring(L, 2);
    if(!strcmp(k, "value")) {
        lua_pushnumber(L, t->value);
    } else {
        lua_pushnil(L);
    }
    return 1;
}

int MyStruct_newindex(lua_State *L) {
    struct MyStruct *t = (struct MyStruct *)luaL_checkudata(L, 1, "MyStruct");
    const char *k = luaL_checkstring(L, 2);
    if(!strcmp(k, "value")) {
        t->value = luaL_checknumber(L, 3);
        return 0;
    } else {
        return luaL_argerror(L, 2,
                             lua_pushfstring(L, "invalid option '%s'", k));
    }
}

struct MyStruct *MyStruct_new(lua_State *L) {
    struct MyStruct *var = (struct MyStruct *)lua_newuserdata(L, sizeof *var);
    luaL_setmetatable(L, "MyStruct");
    return var;
}

void func(lua_State *L) {
    // Since this is allocated inside of Lua, it will be garbage collected,
    // so we don't need to worry about freeing it
    struct MyStruct *var = MyStruct_new(L);
    var->value = 5;

    lua_getglobal(L, "myfunc");
    lua_pushvalue(L, -2); // push `var` to lua, somehow

    lua_call(L, 1, 0); // prints "5"

    // now, var->value is 7
    assert(var->value == 7);
}

int main(void) {
    lua_State *L = luaL_newstate();
    luaL_openlibs(L);

    // One-time setup that needs to happen before you first call MyStruct_new
    luaL_newmetatable(L, "MyStruct");
    lua_pushcfunction(L, MyStruct_index);
    lua_setfield(L, -2, "__index");
    lua_pushcfunction(L, MyStruct_newindex);
    lua_setfield(L, -2, "__newindex");
    lua_pop(L, 1);

    luaL_dostring(L, "function myfunc(s) print(s.value) s.value = 7 end");
    func(L);

    lua_close(L);
    return 0;
}

要支持更多字段，您必须将它们添加到 MyStruct_index和 MyStruct_newindex .如果你想要 pairs要处理它，那么您需要添加 __pairs元方法也是。不过，一般来说，公开 getter 和 setter 通常比这样的方法更受欢迎。

如果您想分配您的 struct MyStruct在 Lua 内部以外的某个地方，那么您可以在使用它的任何地方添加一个额外的间接层，如下所示:

#include <assert.h>
#include <string.h>

#include <lua5.3/lua.h>
#include <lua5.3/lualib.h>
#include <lua5.3/lauxlib.h>

struct MyStruct {
    float value;
};

int MyStruct_index(lua_State *L) {
    struct MyStruct **t = (struct MyStruct **)luaL_checkudata(L, 1, "MyStruct");
    const char *k = luaL_checkstring(L, 2);
    if(!strcmp(k, "value")) {
        lua_pushnumber(L, (*t)->value);
    } else {
        lua_pushnil(L);
    }
    return 1;
}

int MyStruct_newindex(lua_State *L) {
    struct MyStruct **t = (struct MyStruct **)luaL_checkudata(L, 1, "MyStruct");
    const char *k = luaL_checkstring(L, 2);
    if(!strcmp(k, "value")) {
        (*t)->value = luaL_checknumber(L, 3);
        return 0;
    } else {
        return luaL_argerror(L, 2,
                             lua_pushfstring(L, "invalid option '%s'", k));
    }
}

void MyStruct_push(lua_State *L, struct MyStruct *var) {
    struct MyStruct **t = (struct MyStruct **)lua_newuserdata(L, sizeof *t);
    *t = var;
    luaL_setmetatable(L, "MyStruct");
}

void func(lua_State *L) {
    struct MyStruct var;
    var.value = 5;

    lua_getglobal(L, "myfunc");
    MyStruct_push(L, &var); // push `var` to lua, somehow

    lua_call(L, 1, 0); // prints "5"

    // now, var.value is 7
    assert(var.value == 7);

    // WARNING! `var` is about to go out of scope! If Lua uses the userdata
    // that references it beyond this point, it's Undefined Behavior! Make
    // sure that it didn't keep a copy of it!
}

int main(void) {
    lua_State *L = luaL_newstate();
    luaL_openlibs(L);

    // One-time setup that needs to happen before you first call MyStruct_new
    luaL_newmetatable(L, "MyStruct");
    lua_pushcfunction(L, MyStruct_index);
    lua_setfield(L, -2, "__index");
    lua_pushcfunction(L, MyStruct_newindex);
    lua_setfield(L, -2, "__newindex");
    lua_pop(L, 1);

    luaL_dostring(L, "function myfunc(s) print(s.value) s.value = 7 end");
    func(L);

    lua_close(L);
    return 0;
}

但这很危险!如果 Lua 用户数据比它指向的对象存活时间长，就会导致未定义行为!