C++无法添加到模板内的 vector

Question

我正在学习模板。

我有:

template<typename SQLObject>
std::vector<SQLObject> executeSelectQueryReturnSQLVector(std::string _recordType,
                                                             std::string _sql,
                                                             int _rowCount)
{
    typename std::vector<SQLObject> v;

    if (_recordType == AppConstants::sSQLFieldObject)
    {
        for(int r=0; r < _rowCount; r++)
        {
            SQLFieldObject o;

            o.putFieldNumber(sqlite3_column_int(statement, 0));
            [snip]
            v.push_back(o);
        }
    }

    if (_recordType == AppConstants::sSQLNotificationObject)
    {
        for(int r=0; r < _rowCount; r++)
        {
            SQLNotificationObject o;

             o.putNotificationID(sqlite3_column_int(statement, 0));
             [snip]
             v.push_back(o);
        }
    }

    return v;
}

我在 v.push_back(o); 上遇到编译器错误，它指出:

没有匹配的成员函数来调用“push_back”

我认为这对我来说很模糊，因为这是在编译时确定的类型名，这是有道理的吗？

这是否意味着我必须实现自己的 push_back() 函数？

Vandevoorde 和 Josuttis @ 2003 的“C++ 模板完整指南”是否仍然适用于 C++11？

更新 1:考虑这个编辑:

template<typename SQLObject>
std::vector<SQLObject> executeSelectQueryReturnSQLVector(std::string _recordType,
                                                             std::string _sql,
                                                             int _rowCount)
{
    //typename std::vector<SQLObject> v;

    if (_recordType == AppConstants::sSQLFieldObject)
    {
        std::vector<SQLFieldObject> v;

        for(int r=0; r < _rowCount; r++)
        {
            SQLFieldObject o;

            o.putFieldNumber(sqlite3_column_int(statement, 0));
            [snip]
            v.push_back(o);
        }

        return v;
    }

    if (_recordType == AppConstants::sSQLNotificationObject)
    {
        std::vector<SQLNotificationObject> v;

        for(int r=0; r < _rowCount; r++)
        {
            SQLNotificationObject o;

             o.putNotificationID(sqlite3_column_int(statement, 0));
             [snip]
             v.push_back(o);
        }

        return v;
    }

    //return v;
}

我在 return v; 上都遇到了编译器错误，比如:

no viable conversion from 'vector<class SQLFieldObject>' to 'vector<class SQLNotificationObject>'

no viable conversion from 'vector<class SQLNotificationObject>' to 'vector<class SQLFieldObject>'

我这样调用它:

std::vector<SQLFieldObject> _v = executeSelectQueryReturnSQLVector<SQLFieldObject>    (AppConstants::sSQLFieldObject, getSQLToSelectFields(), rowCount);

和

std::vector<SQLNotificationObject> _v = executeSelectQueryReturnSQLVector<SQLNotificationObject>(AppConstants::sSQLNotificationObject, getSQLToSelectNotifications(), rowCount);

Answer 1

v 的类型是std::vector<SQLObject> . o 的类型是SQLFieldObject .除非有一种自动方式来转换 SQLFieldObject 类型的对象至 SQLObject ,

v.push_back(o);

这是不允许的操作。

更新

与更新代码相关的错误是:

executeSelectQueryReturnSQLVector 的返回类型是std::vector<SQLObject> . return语句返回 std::vector<SQLFieldObject>或 std::vector<SQLNotificationObject> .嗯，返回对象的类型与函数签名中的返回类型不匹配。

更优雅地处理模板:

 // A template class that returns an appropriate string based on the
 // typename used to instantiate.
 template <typename SQLObject> struct RecordTypeChooser;

 // Specialization for returning the record type for SQLFieldObjects.
 template <> struct RecordTypeChooser<SQLFieldObject>
 {
    static std::string getRecordType() { return AppConstants::sSQLFieldObject; }
 };

 // Specialization for returning the record type for SQLNotificationObjects.
 template <> struct RecordTypeChooser<SQLNotificationObject>
 {
    static std::string getRecordType() { return AppConstants::sSQLNotificationObject; }
 };

 // A template class that constructs an object and returns it.
 // The object type is based on the typename used to instantiate.
 template <typename SQLObject> struct ObjectCreator;

 // Specialization for constructing SQLFieldObjects.
 template <> struct ObjectCreator<SQLFieldObject>
 {
    static SQLFieldObject createObject()
    {
       SQLFieldObject o;
       o.putFieldNumber(sqlite3_column_int(statement, 0));
       return o;
    }
 };

 // Specialization for constructing SQLNotificationObjects.
 template <> struct ObjectCreator<SQLNotificationObject>
 {
    static SQLNotificationObject createObject()
    {
       SQLNotificationObject o;
       o.putNotificationID(sqlite3_column_int(statement, 0));
       return o;
    }
 };



 template<typename SQLObject>
 std::vector<SQLObject> executeSelectQueryReturnSQLVector(std::string _recordType,
                                                          std::string _sql,
                                                          int _rowCount)
 {
    typename std::vector<SQLObject> v;

    // Not sure whether you need this any more.
    if (_recordType == RecordTypeChooser<SQLObject>::getRecordType())
    {
       for(int r=0; r < _rowCount; r++)
       {
          v.push_back(ObjectCreator<SQLObject>::createObject());
       }
    }

    return v;
 }

更新:完全编译和链接的源代码

 #include <vector>
 #include <string>

 struct SQLFieldObject {};
 struct SQLNotificationObject {};

 // A template class that returns an appropriate string based on the
 // typename used to instantiate.
 template <typename SQLObject> struct RecordTypeChooser;

 // Specialization for returning the record type for SQLFieldObjects.
 template <> struct RecordTypeChooser<SQLFieldObject>
 {
    static std::string getRecordType() { return "SQLFieldObject"; }
 };

 // Specialization for returning the record type for SQLNotificationObjects.
 template <> struct RecordTypeChooser<SQLNotificationObject>
 {
    static std::string getRecordType() { return "SQLNotificationObject"; }
 };

 // A template class that constructs an object and returns it.
 // The object type is based on the typename used to instantiate.
 template <typename SQLObject> struct ObjectCreator;

 // Specialization for constructing SQLFieldObjects.
 template <> struct ObjectCreator<SQLFieldObject>
 {
    static SQLFieldObject createObject()
    {
       SQLFieldObject o;
       // o.putFieldNumber(sqlite3_column_int(statement, 0));
       return o;
    }
 };

 // Specialization for constructing SQLNotificationObjects.
 template <> struct ObjectCreator<SQLNotificationObject>
 {
    static SQLNotificationObject createObject()
    {
       SQLNotificationObject o;
       // o.putNotificationID(sqlite3_column_int(statement, 0));
       return o;
    }
 };



 template<typename SQLObject>
 std::vector<SQLObject> executeSelectQueryReturnSQLVector(std::string _recordType,
                                                          std::string _sql,
                                                          int _rowCount)
 {
    typename std::vector<SQLObject> v;

    // Not sure whether you need this any more.
    if (_recordType == RecordTypeChooser<SQLObject>::getRecordType())
    {
       for(int r=0; r < _rowCount; r++)
       {
          v.push_back(ObjectCreator<SQLObject>::createObject());
       }
    }

    return v;
 }


 void foo()
 {
    std::vector<SQLFieldObject> v1 = executeSelectQueryReturnSQLVector<SQLFieldObject>("SQLFieldObject",
                                                                                       "",
                                                                                       10);

    std::vector<SQLNotificationObject> v2 = executeSelectQueryReturnSQLVector<SQLNotificationObject>("SQLNotificationObject",
                                                                                       "",
                                                                                       10);
 }

 int main() {}