问题描述

我正在尝试在使用 boost::spirit 解析时创建具有语义规则的 AST.AST 必须只为输入的一部分构建，输入的另一部分应该在没有语法树的情况下进行解析.

I am trying to create AST with semantic rules while parsing with boost::spirit. AST must be built only for piece of the input, another part of the input should be parsed without sintax tree.

例如，对于这样的输入字符串:self.usedFoo(Bar).filter(self.baz > baz)"或self.Foo.filter(true)">)" AST 应该只为粗体部分构建.

For example, for such input strings: "self.usedFoo(Bar).filter(self.baz > baz)" or "self.Foo.filter(true)" AST should be build only for bold part.

还有一个问题:解析器运行多次解析语法并多次调用语义动作(实例化 AST 节点)，所以我得到了可怕的内存泄漏.

And there is a problem: parser runs multimple times parsing grammar and calling semantic action (instatntiating AST nodes) multimple times too, so I got terrible memory leaks.

简单的源代码:

语法:

line = stmt | stmt >> "filter.(" >> filter >> ')';
filter %= (filterterm)
filterterm %= (filterfactor)
filterfactor = value [phoenix::bind(&ValueFilterSemanticNode::Instantiate, qi::_val, qi::_1)];

实例化节点:

  static void ValueFilterSemanticNode::Instantiate(QVariant &res, QVariant &value)
    {
        qDebug() << "   Creating new Value Node...";
        ValueFilterSemanticNode *n = new ValueFilterSemanticNode();
        qDebug() << "   " << n;

        n->value = QVariant(value.toInt());
        res = QVariant::fromValue(n);
    }

输入:

self.filter(1)

self.filter(1)

调试:

   Creating new Value Node...
    0x22fdfd0
   Creating new Value Node...
    0x22fe030
   Creating new Value Node...
    0x22fde50
   [...many many lines...]
   Creating new Value Node...
    0x22fe238
   Creating new Value Node...
    0x22fe218
Running Filter test
       Value node running... 0x22fe218
Check result =  QVariant(int, 1)

因此，如您所见，节点实例化过多会导致内存泄漏.

So, as you can see, nodes instantiating too many times that causes mem leaks.

推荐答案

即使稍后有回溯，也会触发语义操作.

Semantic actions fire even if there's backtracking later.

解析器表达式可能会抛出异常.

Parser expressions might throw.

仅出于这些原因，在语义操作中进行动态分配并不是一个好主意.如果需要，请使用智能指针(尽管这仍然效率低下).

For these reasons alone, it's not a good idea to do dynamic allocations in your semantic actions. If you need to, use smart pointers (though this will still be inefficient).

见

• 提升精神:语义动作是邪恶的"?
• 如何使用多态属性boost::spirit::qi 解析器?

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