如何在LLDB中查看std::optional包裹的boost::beast::ssl_stream对象

问题场景 #

尝试打印的目标类型为std::optional包裹的boost::beast::ssl_stream，具体类型定义：

boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, 
boost::asio::any_io_executor, boost::beast::unlimited_rate_policy>>

可以正常打印std::optional包装器本身：

(lldb) p ssl_stream_

(std::optional<boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::any_io_executor, boost::beast::unlimited_rate_policy> > >) $6 =  Has Value=true  {
  Value = {
    p_ = boost::beast::flat_stream<boost::asio::ssl::stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::any_io_executor, boost::beast::unlimited_rate_policy> > > @ 0x0000000103a04080 {
      __value_ = 0x0000000103a04080
    }
  }
}

但尝试查看内部的boost对象时，执行value()或解引用操作会触发重载歧义报错：

(lldb) p ssl_stream_.value()

error: expression failed to parse:
error: <user expression 12>:1:13: call to member function 'value' is ambiguous
ssl_stream_.value()
~~~~~~~~~~~~^~~~~
note: candidate function

note: candidate function

(lldb) p *ssl_stream_

error: expression failed to parse:
error: <user expression 13>:1:1: use of overloaded operator '*' is ambiguous (operand type 'std::optional<boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::any_io_executor, boost::beast::unlimited_rate_policy>>>')
*ssl_stream_
^~~~~~~~~~~~
note: candidate function

note: candidate function

解决方法 #

1. 直接访问optional内部成员
   从之前的打印结果可以看到，std::optional的内部值存储在Value字段中，直接访问该字段即可：

(lldb) p ssl_stream_->Value

如果需要深入查看嵌套成员，还可以直接通过输出中的内存地址打印：

(lldb) p *(boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::any_io_executor, boost::beast::unlimited_rate_policy>>*)0x0000000103a04080

2. 使用frame variable命令绕过表达式解析
   LLDB的frame variable（缩写fr v）直接读取内存，不会触发C++表达式的重载解析，避免歧义：

(lldb) fr v ssl_stream_.Value

若要展开所有嵌套成员，添加-d run参数：

(lldb) fr v -d run ssl_stream_.Value

内容的提问来源于stack exchange，提问作者Zohar81