C++ C++；无法添加到模板内的向量
我正在学习模板 我有：C++ C++；无法添加到模板内的向量,c++,templates,vector,compiler-errors,C++,Templates,Vector,Compiler Errors,我正在学习模板 我有： template<typename SQLObject> std::vector<SQLObject> executeSelectQueryReturnSQLVector(std::string _recordType, std::string _sql,
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;
}
和return v上都得到了编译器错误如：
no viable conversion from 'vector<class SQLFieldObject>' to 'vector<class SQLNotificationObject>'
no viable conversion from 'vector<class SQLNotificationObject>' to 'vector<class SQLFieldObject>'
从“vector”到“vector”没有可行的转换
从“vector”到“vector”没有可行的转换
我这样称呼它：
std::vector<SQLFieldObject> _v = executeSelectQueryReturnSQLVector<SQLFieldObject> (AppConstants::sSQLFieldObject, getSQLToSelectFields(), rowCount);
std:：vector _v=executeSelectQueryReturnSQLVector（AppConstants:：sSQLFieldObject，getSQLToSelectFields（），rowCount）；
及
std:：vector\u v=executeSelectQueryReturnSQLVector（AppConstants:：sSQLNotificationObject，getSQLToSelectNotifications（），rowCount）；
v
的类型是std:：vector
。o
的类型是SQLFieldObject
。除非有自动方式将类型为SQLFieldObject
的对象强制转换为SQLObject
，否则
v.push_back(o);
这是不允许的操作
更新
#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() {}
与更新代码相关的错误有：
executeSelectQueryReturnSQLVector的返回类型是std:：vector
。return
语句返回std:：vector
或std:：vector
。嗯，返回的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;
}
//一个模板类，它根据
//用于实例化的typename。
模板结构记录类型选择器；
//用于返回SQLFieldObjects的记录类型的专门化。
模板结构记录类型选择器
{
静态std:：string getRecordType（）{返回AppConstants:：sSQLFieldObject；}
};
//用于返回SQLNotificationObjects的记录类型的专门化。
模板结构记录类型选择器
{
静态std:：string getRecordType（）{返回AppConstants:：sSQLNotificationObject；}
};
//构造并返回对象的模板类。
//对象类型基于用于实例化的类型名。
模板结构ObjectCreator；
//用于构造SQLFieldObjects的专门化。
模板结构ObjectCreator
{
静态SQLFieldObject createObject（）
{
sqlfieldo；
o、 putFieldNumber（sqlite3_column_int（语句，0））；
返回o；
}
};
//用于构造SQLNotificationObjects的专门化。
模板结构ObjectCreator
{
静态SQLNotificationObject createObject（）
{
sqlnotificationobjecto；
o、 putNotificationID（sqlite3_column_int（语句，0））；
返回o；
}
};
模板
std:：vector executeSelectQueryReturnSQLVector（std:：string\u记录类型，
std:：string\u sql，
整数（行计数）
{
std:：vector v；
//不确定你是否还需要这个。
如果（_recordType==RecordTypeChooser:：getRecordType（））
{
对于（int r=0；r<_rowCount；r++）
{
v、 向后推（ObjectCreator:：createObject（））；
}
}
返回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() {}
#包括
#包括
结构SQLFieldObject{}；
结构SQLNotificationObject{}；
//一个模板类，它根据
//用于实例化的typename。
模板结构记录类型选择器；
//用于返回SQLFieldObjects的记录类型的专门化。
模板结构记录类型选择器
{
静态std:：string getRecordType（）{返回“SQLFieldObject”；}
};
//用于返回SQLNotificationObjects的记录类型的专门化。
模板结构记录类型选择器
{
静态std:：string getRecordType（）{返回“SQLNotificationObject”；}
};
//构造并返回对象的模板类。
//对象类型基于用于实例化的类型名。
模板结构ObjectCreator；
//用于构造SQLFieldObjects的专门化。
模板结构ObjectCreator
{
静态SQLFieldObject createObject（）
{
sqlfieldo；
//o.putFieldNumber（sqlite3_column_int（语句，0））；
返回o；
}
};
//用于构造SQLNotificationObjects的专门化。
模板结构ObjectCreator
{
静态SQLNotificationObject createObject（）
{
sqlnotificationobjecto；
//o.putNotificationID（sqlite3_column_int（语句，0））；
返回o；
}
};
模板
std:：vector executeSelectQueryReturnSQLVector（std:：string\u记录类型，
std:：string\u sql，
整数（行计数）
{
std:：vector v；
//不确定你是否还需要这个。
如果（_recordType==RecordTypeChooser:：getRecordType（））
{
对于（int r=0；r<_rowCount；r++）
{
v、 向后推（ObjectCreator:：createObject（））；
}
}
返回v；
}
void foo（）
{
std:：vector v1=executeSelectQueryReturnSQLVector（“SQLFieldObject”，
"",
10);
std:：vector v2=executeSelectQueryReturnSQLVector（“SQLNotificationObject”，
"",
10);
}
int main（）{}
向量和实例中的类型不同。Vector只保存原始类（由于内存分配问题，无法保存子类）。但是，您可以存储指向类的指针。考虑这一点：
typedef boost::shared_ptr<SQLObject> SQLObjectPtr;
typedef std::vector<SQLObjectPtr> SQLObjectPtrVector;
...
for(int r=0; r < _rowCount; r++)
{
SQLFieldObjectPtr o(new SQLFieldObject);
...
v.push_back(o);
}
typedef boost:：sharedptrsqlobjectptr；
typedef std:：vector SQLObjectPtrVector；
...
对于（int r=0；r<_rowCount；r++）
{
SQLFieldObjectPtr o（新的SQLFieldObject）；
...
v、 推回（o）；
}
#包括
#包括
使用名称空间std；
结构A
{
在里面
typedef boost::shared_ptr<SQLObject> SQLObjectPtr;
typedef std::vector<SQLObjectPtr> SQLObjectPtrVector;
...
for(int r=0; r < _rowCount; r++)
{
SQLFieldObjectPtr o(new SQLFieldObject);
...
v.push_back(o);
}
#include <iostream>
#include <vector>
using namespace std;
struct A
{
int a;
};
struct B
{
char b;
};
template<typename T>
vector<T> fun(char type)
{
if (type == 'A')
{
vector<T> v;
// generate objects of A and append to v
return v;
}
else
{
vector<T> v;
// generate objects of B and append to v
return v;
}
}
int main()
{
vector<A> v = fun<A>('A');
return 0;
}