MongoDB C++ Driver取值模版教程

项目中遇到了使用mongodb存储软件执行记录的需求，在调用mongodb的c++ driver进行数据查询时，从mongodb中取值的方法如下：

try
{
    auto cursor = collection.find({});
    for(row : cursor)
    {
        struct record;
        record.ident =     QString::fromStdString(bsoncxx::string::to_string(row["ident"].get_utf8().value));
        record.startTime = QDateTime::fromMSecsSinceEpoch(row["start_time"].get_int64().value);
        record.endTime = QDateTime::fromMSecsSinceEpoch(row["end_time"].get_int64().value);
        record.value = row["value"].get_double().value;
    }
    return true;
}
catch (std::exception e)
{
    return false;
}

但是，当mongodb的字段修改后，比如value字段没有了，那么row[“value”]将会抛出异常，但是这种情况下，我希望的是字段不存在则写入一个默认值。
不过如果每一条取值语句都用try…catch包裹起来太过繁杂，于是添加了以下取值模版。

#pragma once

#include <bsoncxx/document/view.hpp>

#include <type_traits>

//bool
template <class T>
typename std::enable_if<std::is_same<bool, T>::value, T>::type bson_get(bsoncxx::document::view view
    , const std::string& key
    , const T& default = T())
{
    try
    {
        return view[key].get_bool().value;
    }
    catch (std::exception e)
    {
        return default;
    }
}

//int, int32
template <class T>
typename std::enable_if<std::is_same<int, T>::value, T>::type bson_get(bsoncxx::document::view view
    , const std::string& key
    , const T& default = T())
{
    try
    {
        return view[key].get_int32().value;
    }
    catch (std::exception e)
    {
        return default;
    }
}

//int64_t
template <class T>
typename std::enable_if<std::is_same<int64_t, T>::value, T>::type bson_get(bsoncxx::document::view view
    , const std::string& key
    , const T& default = T())
{
    try
    {
        return view[key].get_int64().value;
    }
    catch (std::exception e)
    {
        return default;
    }
}

//double
template <class T>
typename std::enable_if<std::is_same<double, T>::value, T>::type bson_get(bsoncxx::document::view view
    , const std::string& key
    , const T& default = T())
{
    try
    {
        return view[key].get_double().value;
    }
    catch (std::exception e)
    {
        return default;
    }
}

//std::string
template <class T>
typename std::enable_if<std::is_same<std::string, T>::value, T>::type bson_get(bsoncxx::document::view view
    , const std::string& key
    , const T& default = T())
{
    try
    {
        return bsoncxx::string::to_string(view[key].get_utf8().value);
    }
    catch (std::exception e)
    {
        return default;
    }
}

//QString
template <class T>
typename std::enable_if<std::is_same<QString, T>::value, T>::type bson_get(bsoncxx::document::view view
    , const std::string& key
    , const T& default = T())
{
    try
    {
        auto&& string = bsoncxx::string::to_string(view[key].get_utf8().value);
        return QString::fromUtf8(string.c_str(), string.size());
    }
    catch (std::exception e)
    {
        return default;
    }
}

//QDateTime
template <class T>
typename std::enable_if<std::is_same<QDateTime, T>::value, T>::type bson_get(bsoncxx::document::view view
    , const std::string& key
    , const T& default = T())
{
    try
    {
        return QDateTime::fromMSecsSinceEpoch(view[key].get_int64());
    }
    catch (std::exception e)
    {
        return default;
    }
}

这样之前的取值方式就可以修改为:

try
{
    auto cursor = collection.find({});
    for(row : cursor)
    {
        struct record;
        record.ident = bson_get<QString>(row, "ident");
        record.startTime = bson_get<QDateTime>(row, "start_time");
        record.endTime = bson_get<QDateTime>(row, "end_time");
        record.value = bson_get<double>(row, "value", 0.0);
    }
    return true;
}
catch (std::exception e)
{
    return false;
}

这样在value字段不存在时，record就会获得一个默认值0.0