0

可能重复：
||=（或等于）在 Ruby 中是什么意思？

在互联网上，我在 Ruby/Rails 中看到了以下语法：

user ||= User.new

我是新手，我无法解析这个。有人可以向我解释“||=”运算符的作用吗？

4

5 回答 5

5

If user is already set this does nothing, otherwise it will assign a new User object (created with User.new).

According to David A. Black, author of "The Well-Grounded Rubyist":

x ||= y means: x || x = y

The difference is that x ||= y won't complain if x is undefined, whereas if you type x || x = y and there's no x in scope, it will.

For some added details, here's the relevant part of parse.y:

| var_lhs tOP_ASGN command_call
{
  /*%%%*/
  value_expr($3);
  if ($1) {
    ID vid = $1->nd_vid;
    if ($2 == tOROP) {
      $1->nd_value = $3;
      $$ = NEW_OP_ASGN_OR(gettable(vid), $1);
      if (is_asgn_or_id(vid)) {
        $$->nd_aid = vid;
      }
    }
    else if ($2 == tANDOP) {
      $1->nd_value = $3;
      $$ = NEW_OP_ASGN_AND(gettable(vid), $1);
    }
    else {
      $$ = $1;
      $$->nd_value = NEW_CALL(gettable(vid), $2, NEW_LIST($3));
    }
  }

NEW_OP_ASGN_OR is defined in node.h:

#define NEW_OP_ASGN_OR(i,val) NEW_NODE(NODE_OP_ASGN_OR,i,val,0)

NEW_NODE looks like this:

#define NEW_NODE(t,a0,a1,a2) rb_node_newnode((t),(VALUE)(a0),(VALUE)(a1),(VALUE)(a2))

Looking for NODE_OP_ASGN_OR leads to compile.c, where the interesting part looks like this:

case NODE_OP_ASGN_OR:{
   LABEL *lfin = NEW_LABEL(nd_line(node));
   LABEL *lassign;

   if (nd_type(node) == NODE_OP_ASGN_OR) {
     LABEL *lfinish[2];
     lfinish[0] = lfin;
     lfinish[1] = 0;
     defined_expr(iseq, ret, node->nd_head, lfinish, Qfalse);
     lassign = lfinish[1];
     if (!lassign) {
       lassign = NEW_LABEL(nd_line(node));
     }
     ADD_INSNL(ret, nd_line(node), branchunless, lassign);
   }
   else {
     lassign = NEW_LABEL(nd_line(node));
   }

   COMPILE(ret, "NODE_OP_ASGN_AND/OR#nd_head", node->nd_head);
   ADD_INSN(ret, nd_line(node), dup);

   if (nd_type(node) == NODE_OP_ASGN_AND) {
     ADD_INSNL(ret, nd_line(node), branchunless, lfin);
   }
   else {
     ADD_INSNL(ret, nd_line(node), branchif, lfin);
   }

   ADD_INSN(ret, nd_line(node), pop);
   ADD_LABEL(ret, lassign);
   COMPILE(ret, "NODE_OP_ASGN_AND/OR#nd_value", node->nd_value);
   ADD_LABEL(ret, lfin);

   if (poped) {
     /* we can apply more optimize */
     ADD_INSN(ret, nd_line(node), pop);
   }
   break;
 }

I think this is more than I ever wanted to know about assignment in Ruby, but it was rather entertaining to look this up.

于 2012-08-14T18:46:01.590 回答
3

It's basically a shortcut for:

user = user || User.new

or for better understanding:

if user.nil?
    user = User.new
end

I bet you've seen similar notations before with operators like '+'

i += 1

which can also be written out as:

i = i + 1
于 2012-08-14T18:46:58.163 回答
1

该语句等价于

user = user || User.new

这相当于

user = user ? user : User.new

当且仅当为nil时，它会将 的值分配给User.new变量。如果不是，则 的内容将保持不变。useruseruser

于 2012-08-14T18:47:48.213 回答
1

它相当于user = user || User.new.

这取决于操作员的短路行为||。如果表达式的左侧为真，那么无论右侧是什么，整个表达式都为真，因此运算符“短路”并停止计算。而不是返回一个布尔值“真”，该||运算符返回它评估的最后一个值。

因此，||=对于分配默认值很有用。如果user有一个值，则user || User.new评估为，user否则评估为User.new哪个是默认值。

一个等效的块是：

if user
    user = user
else 
    user = User.new
end
于 2012-08-14T18:51:37.543 回答
0

The statement set user to itself (if it is already an existing object) or User.new (which will create a new user if user is null). It's a logical OR that will avoid having an assigned null user object.

The code is a shorthand for

user = user || User.new

If user is null, then user will be set to User.new.

于 2012-08-14T18:45:49.867 回答