Class templates inheritance

[Gerych]

Hi. I'm new. I try to create abstract database class Db and derived class Pg to handle at least connection to the database. I have private property with database connection type and because it will be unique depending on database I should implement templates.

#include <iostream>
#include <postgresql/libpq-fe.h>

using namespace std;

template <class T>
class Db 
{
public:
    Db(string, string, string, string);
    Db();
    string getHost();
    string getUsername();
    string getDatabase();
    string getPassword();
    virtual string getConnectionString()=0;
    virtual ~Db();

protected: 
    T* m_conn;
    string m_host;
    string m_username;
    string m_database;
    string m_password;
};

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

template <class T>
string Db<T>::getHost() {
    return m_host;
}

// ---------------------------------------------------------------------
// username, database and password getters are implementing the same way
// ---------------------------------------------------------------------

template <class T>
Pg<T>::Pg(string host, string username, string database, string password)
    // I intentionally remove Pg:: for m_host property
    : m_host(host), Pg::m_username(username), Pg::m_database(database), Pg::m_password(password) {
    Pg::m_conn = PQconnectdb(getConnectionString().c_str());

	if (PQstatus(Pg::m_conn) == CONNECTION_BAD) {
	    puts("Unable to connect to database");
	    exit(0);
	}
}

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


int main () {
	Pg<PGconn> conn("localhost", "pg", "pg", "pwd");

	return 0;
}

When I compile this beautiful code I get an errors:

load.cpp: In constructor ‘Pg<T>::Pg(std::string, std::string, std::string, std::string)’:
load.cpp:50:7: error: class ‘Pg<T>’ does not have any field named ‘m_host’
     : m_host(host), Pg::m_username(username), Pg::m_database(database), Pg::m_password(password) {
       ^
load.cpp: In instantiation of ‘Pg<T>::Pg(std::string, std::string, std::string, std::string) [with T = pg_conn; std::string = std::basic_string<char>]’:
load.cpp:73:52:   required from here
load.cpp:50:96: error: no type named ‘m_username’ in ‘class Pg<pg_conn>’
     : m_host(host), Pg::m_username(username), Pg::m_database(database), Pg::m_password(password) {
                                                                                                ^
load.cpp:50:96: error: no type named ‘m_database’ in ‘class Pg<pg_conn>’
load.cpp:50:96: error: no type named ‘m_password’ in ‘class Pg<pg_conn>’

As you can see if I remove Pg:: before m_host property I get the error that class does not have such field. If I place Pg:: before another connection properties I get the error no type named ‘m_database’.

I completely stuck for two days with this issue.

I use linux and g++ compiler:
g++ -o load -g load.cpp -std=c++11 -lcurl -lpq

[2kaud]

To initialise members of the base class, call a base class constructor eg

template <class T>
Pg<T>::Pg(string host, string username, string database, string password)
// I intentionally remove Pg:: for m_host property
	//: m_host(host), Pg::m_username(username), Pg::m_database(database), Pg::m_password(password) {
	: Db<T>(host, username, database, password) {
...

[Gerych]

Thanks a lot, 2kaud! These errors disappear, but now I have another. Seems I don't understand the way inheritance work in C++:

/tmp/cc8AwHUL.o: In function `Pg<pg_conn>::Pg(std::string, std::string, std::string, std::string)':
/home/arra/Projects/mudobot/rates/load.cpp:62: undefined reference to `Db<pg_conn>::Db(std::string, std::string, std::string, std::string)'
collect2: error: ld returned 1 exit status

[2kaud]

Based upon the code in post #1, the below compiles and executes. For test I've just set type PGconn to int

#include <string>
#include <iostream>
using namespace std;

typedef int PGconn;

template <class T>
class Db
{
public:
	Db(string h, string u, string d, string p) : m_host(h), m_username(u), m_database(d), m_password(p) {}
	Db() {}
	string getHost() const { return m_host; }
	string getUsername() const { return m_username; }
	string getDatabase() const { return m_database; }
	string getPassword() const { return m_password; }
	virtual string getConnectionString() const = 0;
	virtual ~Db() {}

protected:
	T* m_conn = nullptr;
	string m_host;
	string m_username;
	string m_database;
	string m_password;
};

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

template <class T>
Pg<T>::Pg(string host, string username, string database, string password)
	: Db<T>(host, username, database, password) {

	/*
	if (PQstatus(Pg::m_conn) == CONNECTION_BAD) {
		puts("Unable to connect to database");
		exit(0);
	}
	*/
}

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

int main() {
	Pg<PGconn> conn("localhost", "pg", "pg", "pwd");
	cout << conn.getConnectionString() << endl;

	return 0;
}

if your code is now different, it would be useful if you could post your revised code.

[Gerych]

Great thanks!! I made it work with help of your better version of my code =)

But one thing I don't understand is why should I add this-> anyway, because your version without this-> crashed with:

rates/load.cpp: In member function ‘virtual std::string Pg<T>::getConnectionString() const’:
rates/load.cpp:55:63: error: there are no arguments to ‘getUsername’ that depend on a template parameter, so a declaration of ‘getUsername’ must be available [-fpermissive]
     return "host=" + this->getHost() + " user=" + getUsername()

May be I forgot something from your version, thanks anyway. Here is the end working version:

#include <iostream>
#include <vector>
#include <string>
#include <postgresql/libpq-fe.h>

using namespace std;

template <class T>
class Db 
{
public:
    Db(string h, string u, string d, string p) 
	    : m_host(h), m_username(u), m_database(d), m_password(p) {}
    Db() {}
    virtual ~Db() {}
    string getHost() const { return m_host; }
    string getUsername() const { return m_username; }
    string getDatabase() const { return m_database; }
    string getPassword() const { return m_password; }
    virtual string getConnectionString() const = 0;

protected: 
    T* m_conn;
    string m_host;
    string m_username;
    string m_database;
    string m_password;
};

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

template <class T>
Pg<T>::Pg(string host, string username, string database, string password)
    : Db<T>(host, username, database, password) {
    Pg::m_conn = PQconnectdb(getConnectionString().c_str());

	if (PQstatus(Pg::m_conn) == CONNECTION_BAD) {
	    puts("Unable to connect to database");
	    exit(0);
	}
}

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


int main () {
	Pg<PGconn> conn("a", "b", "c", "d");

	cout << conn.getConnectionString() << endl;

	return 0;
}

[2kaud]

Without the this-> in getConnectionString(), it works OK with MS VS2015. Sorry. I don't use the g++ compiler so can't comment on its error. Perhaps a guru who does use g++ can elaborate?

[Gerych]

Strange thing. When I separate that code on files Db.h Pg.h and Pg.cpp I again get the error:

/tmp/ccCNyID2.o: In function `main':
rates/load.cpp:71: undefined reference to `Pg<pg_conn>::getConnectionString() const'
/tmp/ccCNyID2.o: In function `Pg<pg_conn>::Pg(std::string, std::string, std::string, std::string)':
rates/../src/db/Pg.h:12: undefined reference to `Pg<pg_conn>::getConnectionString() const'
/tmp/ccCNyID2.o:(.rodata._ZTV2PgI7pg_connE[_ZTV2PgI7pg_connE]+0x20): undefined reference to `Pg<pg_conn>::getConnectionString() const'
collect2: error: ld returned 1 exit status

It's really copy-paste, the difference only in head of the files where I do includes:

Pg.h: #include "Db.h"
Pg.cpp: #include "Pg.h"
main.cpp: #include "../src/db/Pg.h"

[2kaud]

I suspect this is because you are splitting the class definition from its implementation (Pg.g and Pg.cpp). When templates are involved, the implementation is usually in the same unit as its definition (often a .hpp file).

[Gerych]

Thank you!