How to prepare statements and bind parameters in Postgresql for C++

Question

I'm quite new to C++ and know a little bit about pqxx library. What I want to implement is to prepare statements and bind parameters. In PHP I'm used to doing this in such a nice and concise manner:

$s = $db->prepare("SELECT id FROM mytable WHERE id = :id");
$s->bindParam(':id', $id);
$s->execute();

or using tokens:

$data = array();
$data[] = 1;
$data[] = 2;
$s = $db->prepare("SELECT id FROM mytable WHERE id = ? or id = ?");
$s->execute($data);

I tried to fugure out from pqxx documentation how to implement this, but to me documentation looks like a mess and lacks short and simple examples (like I provided above). I hope someone can also provide such simple examples (or of comparable simplicity - without having to write some behemoth code) when dealing with Postgresql in C++.

Answer 1

A simple example. This just prints the number of entries with id value 0.

#include<pqxx/pqxx>
#include<iostream>

int main()
{
    std::string name = "name";
    int id = 0;
    try {
        //established connection to data base
        pqxx::connection c("dbname=mydb user=keutoi");
        pqxx::work w(c);
        //statement template
        c.prepare("example", "SELECT id  FROM mytable WHERE id = $1");
        //invocation as in varible binding
        pqxx::result r = w.prepared("example")(id).exec();
        
        w.commit();
        //result handling for accessing arrays and conversions look at docs
        std::cout << r.size() << std::endl;
    }
    catch(const std::exception &e)
    {
        std::cerr << e.what() << std::endl;
        return 1;
    }
    return 0;
}

The function w.prepared() is a bit convoluted. It's similar to a curried(curry) function in haskell, as in it takes a parameter and returns another function which in turn takes another parameter. That kind of thing.

Documentation says:

How do you pass those parameters? C++ has no good way to let you pass an unlimited, variable number of arguments to a function call, and the compiler does not know how many you are going to pass. There's a trick for that: you can treat the value you get back from prepared as a function, which you call to pass a parameter. What you get back from that call is the same again, so you can call it again to pass another parameter and so on.

Once you've passed all parameters in this way, you invoke the statement with the parameters by calling exec on the invocation

If there are more parameters use $1 $2 and so on in the prepare function.

c.prepare("SELECT id name FROM mytable WHERE id = $1 AND name = $2")

and give the varibles as

w.prepared("example")(dollar1_var)(dollar2_var).exec()

An Example for dynamic preparation

#include<pqxx/pqxx>
#include<iostream>
#include<vector>

//Just give a vector of data you can change the template<int> to any data type
pqxx::prepare::invocation& prep_dynamic(std::vector<int> data, pqxx::prepare::invocation& inv)
{
    for(auto data_val : data)
        inv(data_val);
    return inv;
}

int main()
{
    std::string name = "name";

    //a data array to be used.
    std::vector<int> ids;
    ids.push_back(0);
    ids.push_back(1);

    try {
        pqxx::connection c("dbname=mydb user=keutoi");
        pqxx::work w(c);

        c.prepare("example", "SELECT id  FROM mytable WHERE id = $1 or id = $2");
        pqxx::prepare::invocation w_invocation = w.prepared("example");

        //dynamic array preparation
        prep_dynamic(ids, w_invocation);
        //executing prepared invocation.
        pqxx::result r = w_invocation.exec();

        w.commit();

        std::cout << r.size() << std::endl;
    }
    catch(const std::exception &e)
    {
        std::cerr << e.what() << std::endl;
        return 1;
    }
    return 0;
}

if you want to handle other data types use this function definition

template<class T> pqxx::prepare::invocation& prep_dynamic(std::vector<T> data, pqxx::prepare::invocation& inv)
{
    for(auto data_val : data)
        inv(data_val);
    return inv;
}