在Mongo CXX中取消设置文档元素
我有一个将bsoncxx::document::view view作为private属性的类，以及一个从Mongo数据库获取文档并将结果保存在本地bsoncxx::document::value value变量中的方法，然后从value.view()获取视图并将其保存在类bsoncxx::document::view view变量中：
const bool User::search_one_by_id(const std::string &id = "")
{
// The prototype of the method below: const std::tuple<bool, bsoncxx::document::value> search_one_by_id(mongocxx::collection &, const std::string &) const;
auto status = Crud::search_one_by_id(this->collection, id);
if (std::get<0>(status))
{
// Error
return EXIT_FAILURE;
}
else
{
// Success
bsoncxx::document::value value = std::get<1>(status);
bsoncxx::document::view view = value.view();
this->view = view;
return EXIT_SUCCESS;
}
}问题是，如果我从上面方法中的视图中获得一些元素，即在return EXIT_SUCCESS之前的下面的代码，就不会发出错误。
bsoncxx::document::element element = this->view["first_name"];
std::cout << "First Name: " << element.get_utf8().value.to_string();但是如果我得到一个视图，将它保存在bsoncxx::document::view view变量中，并尝试在另一个类方法中从视图中获取一些元素：
void get_element()
{
bsoncxx::document::element element = this->view["first_name"];
std::cout << "First Name: " << element.get_utf8().value.to_string();
}我收到以下错误：
terminate called after throwing an instance of 'bsoncxx::v_noabi::exception'
what(): unset document::element
Makefile:26: recipe for target 'run' failed
make: *** [run] Aborted (core dumped)我曾尝试使用指针来保存对search_one_by_id方法中获得的视图的引用。我已经检查了我正在获取的属性(first_name)的类型是否是获取元素值的正确类型。我已经检查了文档中是否存在该属性。我曾尝试在User::search_one_by_id中使用view中的release()方法。我通过以下命令检查视图是否为空：
if (this->view.empty())
{
std::cout << "Empty: " << this->view.empty();
}
else
{
std::cout << "Loaded: " << this->view.empty() << " length " << this->view.length();
}在get_element方法中，输出是：
# If I comment the call to search_one_by_id
$ Empty: 1
# If I enable the call to search_one_by_id
$ Loaded: 0 length 129GDB日志记录到backtrace
#0 __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1 0x00007ffff6fd7801 in __GI_abort () at abort.c:79
#2 0x00007ffff762c957 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3 0x00007ffff7632ab6 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4 0x00007ffff7632af1 in std::terminate() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#5 0x00007ffff7632d24 in __cxa_throw () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#6 0x00007ffff793985c in bsoncxx::v_noabi::document::element::type() const () from /usr/local/lib/libbsoncxx.so._noabi
#7 0x00007ffff7939abc in bsoncxx::v_noabi::document::element::get_utf8() const () from /usr/local/lib/libbsoncxx.so._noabi
#8 0x0000555555559353 in User::get_element() ()
#9 0x0000555555556668 in main ()有什么建议吗？该项目可在Github上找到
参考文献：
Unset Document::element, MongoCXX Find Option Projection
Document value
Document view
Document element
document::element::operator[] overloads should not throw when given an invalid element
回答 1
Stack Overflow用户
发布于 2019-06-25 01:49:15
我认为你需要把指向有值的缓冲区的指针放在某个地方。我想视图对象指向已删除的内存。试试这样的东西
class User
{
bsoncxx::document::value _value; <<< store at class level
};
const bool User::search_one_by_id(const std::string &id = "")
{
// The prototype of the method below: const std::tuple<bool, bsoncxx::document::value> search_one_by_id(mongocxx::collection &, const std::string &) const;
auto status = Crud::search_one_by_id(this->collection, id);
if (std::get<0>(status))
{
// Error
return EXIT_FAILURE;
}
else
{
// Success
_value = std::get<1>(status);
this->view = _value.view();
return EXIT_SUCCESS;
}
}https://stackoverflow.com/questions/56740730
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