Question

我想通过继承Db来创建抽象的Db类并实现Pg（postgresql）：

Db.h ：

template <class T>
class Db 
{
public:
    Db(std::string, std::string, std::string, std::string);
    virtual ~Db();

protected:
    T* getConnection();
    std::string getHost();
    std::string getUsername();
    std::string getDatabase();
    std::string getPassword();
    virtual std::string getConnectionString()=0;

private: 
    T* connection;
    std::string host;
    std::string username;
    std::string database;
    std::string password;
};

Db.cpp ：

#include "Db.h"

using namespace std;

template <class T>
Db<T>::Db(string iHost, string iUsername, string iDatabase, string iPassword) 
    : host(iHost), username(iUsername), database(iDatabase), password(iPassword) {
    T* conn(getConnectionString());
    connection = conn;
}

template <class T>
T* Db<T>::getConnection() {
    return connection;
}

... getters

Pg.h ：

#include "../Db.h"

template <class T>
class Pg : public Db<T> {
public:
    virtual std::string getConnectionString();
};

Pg.cpp ：

#include "Pg.h"

template <class T>
std::string Pg<T>::getConnectionString() {
    return "host=" + this->getHost() + " user=" + this->getUsername() 
        + " password=" + this->getPassword() + " dbname=" + this->getDatabase();
}

的main.cpp ：

#include <iostream>
#include <pqxx/connection>
#include <pqxx/transaction>
#include "lib/db/pg/Pg.h"

using namespace std;

int main () {
    string host     = "localhost";
    string username = "username";
    string database = "db";
    string password = "root";

    try {
        Pg<pqxx::connection> *db(host, username, database, password);
    } catch (pqxx::broken_connection) {
        cout << "Failed to establish connection." << endl;
    }

    return 0;
}

很抱歉长期实施。用g ++编译后我得到一个错误：

main.cpp: In function ‘int main()’:
main.cpp:15:62: error: expression list treated as compound expression in initializer [-fpermissive]
   Pg<pqxx::connection> *db(host, username, database, password);
                                                              ^
main.cpp:15:62: error: cannot convert ‘std::string {aka std::basic_string<char>}’ to ‘Pg<pqxx::basic_connection<pqxx::connect_direct> >*’ in initialization

我不知道最后是不是一个好主意，因为我的第一个版本只有main（）函数看起来更紧凑。我只是尝试在某处隐藏连接细节，例如连接字符串。

Answer 1

在main()中，您的db变量被声明为指针。您不能像以前那样将构造函数参数传递给指针。

您需要：

从声明中删除*：

Pg<pqxx::connection> db(host, username, database, password);

保留*并使用new构建对象：

Pg<pqxx::connection> *db = new Pg<pqxx::connection>(host, username, database, password);
...
delete db;

C ++中的抽象数据库实现

1 个答案: