如何定义从虚拟函数继承的静态函数？

Question

我正在为一个小应用程序使用 C++。我有一个 parent class 有一个虚方法。这是头文件。

class personDB:public person
{
public:
    unsigned int id;
public:
    personDB();
    personDB(QString dbName, QString dbSurname);
    personDB(QString dbName, QString dbSurname, unsigned int dbid);
    personDB(std::string dbName, std::string dbSurname, unsigned int dbid);
    virtual std::string getTableName();
    unsigned int getID(void);
    void setID(unsigned int myID);
private:
    static const std::string tableName;
};

这个class被不同的childclass继承。每个 child class 重新定义 tableName 属性，在 *.cpp 文件中分配不同的值。该属性是私有的，由 getter 返回。作为其中一个 child classes 的示例，这里是头文件：

#include "persondb.h"
class ScriptBy : public personDB
{
public:
    ScriptBy();
    ScriptBy(QString dbName, QString dbSurname);
    ScriptBy(QString dbName, QString dbSurname, unsigned int dbid);
    std::string getTableName();
protected:
    static const std::string tableName;
};

我想使函数 getTableName() 成为 class 成员函数，以便它 returns 对于相同 class 的所有实例具有相同的值，甚至可以调用没有 class 的实例。如果我没记错的话，这应该通过在头文件中的函数声明之前放置关键字 static 来完成。但是，当我尝试以这种方式编译时，它会给我一个与 parent class 中的函数被声明为虚拟函数相关的错误。我得到的错误是：

In file included from ../../singlestory.h:4:0,
                 from ../../volume.h:5,
                 from ../../dbinterface.h:8,
                 from ../../dbinterface.cpp:1:
../../scriptby.h:12:24: error: ‘static std::__cxx11::string ScriptBy::getTableName()’ cannot be declared
     static std::string getTableName();
                        ^
In file included from ../../dbinterface.h:7:0,
                 from ../../dbinterface.cpp:1:
../../persondb.h:15:25: error:   since ‘virtual std::__cxx11::string personDB::getTableName()’ declared in base class
     virtual std::string getTableName();

有没有办法让继承的class成员函数静态化？

编辑：如果我不能使函数静态化，如何在没有 class 实例的情况下使私有属性在外部可访问（并保持私有）？

Answer 1

"Is there a way to make the inherited class member function static?" 否

Answer 2

Is there a way to make the inherited class member function static?

你不能。您可以将 static 函数重命名为其他名称并从常规成员函数中使用它，如果这对您的 class.

有意义的话

class ScriptBy : public personDB
{
    virtual std::string getTableName() { return getTableNameStatic(); }
    static std::string getTableNameStatic();
};

Answer 3

virtual 和 static 代表 2 个相互矛盾的要求。

static 函数是一个 class-level 构造，不限于任何特定的 class 实例。

virtual 函数是一个 instance-level 构造，因此它的行为由相关对象定义。

您需要重新考虑您的要求。

Answer 4

我找到了使用 Curiously Recurring template pattern 的解决方案。

对我有用的解决方案如下：

文件persondb.h

#ifndef PERSONDB_H
#define PERSONDB_H

#include "person.h"
#include"common.h"
class personDB:public person
{
public:
    using person::person;
    personDB(QString dbName, QString dbSurname, unsigned int dbid);
    personDB(std::string dbName, std::string dbSurname, unsigned int dbid);
    unsigned int getID(void);
    void setID(unsigned int myID);
protected:
    unsigned int id;
};

/* Curiously recurring template paradigm */
template<class Derived>
class PersonDBX: public virtual personDB
{
    using personDB::personDB;
protected:
    static const std::string tableName;
    static const personRelationTable personIDtable;
public:
    static std::string static_getTableName(){ return tableName; }
    std::string getTableName(){return tableName;}
    static std::string static_getIDTableName(){ return personIDtable.tableName; }
    std::string getIDTableName(){return personIDtable.tableName;}

    static std::string static_getCol1TableID(){ return personIDtable.col1; }
    std::string getCol1TableID(){return personIDtable.col1;}

    static std::string static_getCol2TableID(){ return personIDtable.col2; }
    std::string getCol2TableID(){return personIDtable.col2;}



    personDBX(QString dbName, QString dbSurname, unsigned int dbid):personDB(dbName, dbSurname)
    {
        id = dbid;
    }
    PersonDBX():personDB(){;}

    PersonDBX(QString dbName, QString dbSurname):personDB(dbName, dbSurname){;}
};
template<class Derived> const std::string PersonDBX<Derived>::tableName = "Null";
template<class Derived> const personRelationTable PersonDBX<Derived>::personIDtable = {"Null", "Null", "Null"};

#endif // PERSONDB_H

文件persondb.cpp

#include "persondb.h"

personDB::personDB(QString dbName, QString dbSurname, unsigned int dbid):person(dbName, dbSurname){

    id = dbid;
}

personDB::personDB(std::string dbName, std::string dbSurname,
                   unsigned int dbid):person(dbName, dbSurname){

    id = dbid;
}

unsigned int personDB::getID(void){
    return id;
}

void personDB::setID(unsigned int myID)
{
    id = myID;
}

child class 是 scriptby:

文件scriptby.h

#ifndef SCRIPT_H
#define SCRIPT_H

#include "persondb.h"

class ScriptBy : public PersonDBX<ScriptBy>
{
public:
    using PersonDBX::PersonDBX;
    ScriptBy(QString dbName, QString dbSurname, unsigned int dbid);
protected:
};

#endif // SCRIPT_H

文件scriptby.cpp

#include "scriptby.h"
#include <iostream>
#include <globals.h>

template<> const std::string PersonDBX<ScriptBy>::tableName = "ScriptBy";
template<> const personRelationTable PersonDBX<ScriptBy>::personIDtable = {"Story_ScriptBy","StoryID", "ScriptByID"};

ScriptBy::ScriptBy(QString dbName, QString dbSurname, unsigned int dbid):PersonDBX(dbName, dbSurname)
{
    id = dbid;
}

如何定义从虚拟函数继承的静态函数？

How to define a static function inherited from a virtual one?

c++

inheritance

static

virtual-functions