OTL 4.0, Example 775 (C++20 range otl_view)

This example demonstrates C++20 range otl_view.

Source Code

#include <iostream>

#include <string>

#include <vector>

#include <ranges>    // C++20 Ranges

#include <algorithm> // for std::ranges::for_each

#define OTL_ODBC_ALTERNATE_RPC
#if !defined(_WIN32) && !defined(_WIN64)
#define OTL_ODBC
#else 
#define OTL_ODBC_POSTGRESQL // required with PG ODBC on Windows
#endif
#define OTL_VIEW 
#define OTL_STREAM_WITH_STD_OPTIONAL_ON 
#include "otlv4.h"

using namespace std;

// Database connection object
otl_connect db; 

void insert()
{ 
    // Basic OTL insert stream
    otl_stream o(10, "insert into test_tab values(:f1<int>, :f2<char[31]>, :f3<double>)", db);
    
    // Normal values are streamed directly; std::optional is used only when entering NULLs
    o << 1 << "James Bond"        << 5500.50;
    o << 2 << "Sherlock Holmes"   << 6200.00;
    o << 3 << "Tony Stark"          << optional<double>(nullopt);   // NULL salary entered
    o << 4 << optional<string>(nullopt) << 9900.00;                 // NULL name entered
    o << 5 << "Peter Parker"        << 1200.25;
}

void select_with_ranges()
{ 
    // Create an OTL stream for a query
    otl_stream s(100, "select id, name, salary from test_tab where id >= :id<int>", db);
    s << 1; // Bind parameter

    cout << "--- Standard C++20 Range Loop ---" << endl;
    
    // Inefficiency removed: using const auto& to prevent copying tuple components & std::string on each iteration
    for (const auto& [id, name, salary] : otl_view<int, optional<string>, optional<double>>(s)) {
        cout << "ID: " << id 
             << " | Name: "   << (name.has_value() ? *name : "NULL") 
             << " | Salary: " << (salary.has_value() ? "$" + to_string(*salary) : "NULL") << endl;
    }

    // Rewind for the next demonstration (if supported by your driver)
    s.rewind();

    cout << "\n--- Range Composition (Pipes) ---" << endl;
    
    auto modern_pipeline = otl_view<int, optional<string>, optional<double>>(s)
        | views::filter([](auto&& row) { 
            const auto& salary = get<2>(row);
            return salary.has_value() && *salary > 6000.0; 
        })
        | views::take(2)
        | views::transform([](auto&& row) { 
            const auto& name = get<1>(row);
            return name.value_or("Unknown Agent"); 
        });

    for (const string& name : modern_pipeline) {
        cout << "High Earner Name: " << name << endl;
    }
}

int main()
{
    // Initialize OTL environment
    otl_connect::otl_initialize(); 
    
    try {
        // Connect to database
        db.rlogon("scott/tiger@postgresql"); 

        // Create table
        otl_cursor::direct_exec(db, "drop table test_tab", otl_exception::disabled);
        otl_cursor::direct_exec(db, "create table test_tab(id int, name varchar(30), salary float)");

        insert();             // Populate table with mixed regular & NULL values
        select_with_ranges(); // Show magic

    } catch(otl_exception& p) {
        // Standard OTL exception handling
        cerr << "Error: " << p.msg << endl;
        cerr << "SQL: " << p.stm_text << endl;
    }

    db.logoff(); // Disconnect
    return 0;
}

Output

--- Standard C++20 Range Loop (Structured Bindings) ---
ID: 1 | Name: James Bond | Salary: $5500.5
ID: 2 | Name: Sherlock Holmes | Salary: $6200
ID: 3 | Name: Tony Stark | Salary: NULL
ID: 4 | Name: NULL | Salary: $9900

--- Range Composition (Pipes) ---
High Earner Name: Sherlock Holmes
High Earner Name: Unknown Agent

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