To enable transactions for your environment, you must initialize the transactional subsystem. Note that doing this also initializes the logging subsystem. In addition, you must initialize the memory pool (in-memory cache). You must also initialize the locking subsystem. For example:

#include "db_cxx.h"

...
                                                                                                                                  
int main(void)
{
    u_int32_t env_flags = DB_CREATE     |  // If the environment does not
                                           // exist, create it.
                          DB_INIT_LOCK  |  // Initialize locking
                          DB_INIT_LOG   |  // Initialize logging
                          DB_INIT_MPOOL |  // Initialize the cache
                          DB_INIT_TXN;     // Initialize transactions
                                                                                                                                  
    std::string envHome("/export1/testEnv");
    DbEnv myEnv(0);

    try {

        myEnv.open(envHome.c_str(), env_flags, 0);

    } catch(DbException &e) {
        std::cerr << "Error opening database environment: "
                  << envHome << std::endl;
        std::cerr << e.what() << std::endl;
        return (EXIT_FAILURE);
    }

    try {
        myEnv.close(0);
    } catch(DbException &e) {
        std::cerr << "Error closing database environment: "
                << envHome << std::endl;
        std::cerr << e.what() << std::endl;
        return (EXIT_FAILURE);
    }

    return (EXIT_SUCCESS);
} 

You then create and open your database(s) as you would for a non-transactional system. The only difference is that you must pass the environment handle to the DbEnv::open() method, and you must open the database within a transaction. Typically auto commit is used for this purpose. To do so, pass DB_AUTO_COMMIT to the database open command. Also, make sure you close all your databases before you close your environment. For example:

#include "db_cxx.h"

...
                                                                                                                                  
int main(void)
{
    u_int32_t env_flags = DB_CREATE     |  // If the environment does not
                                           // exist, create it.
                          DB_INIT_LOCK  |  // Initialize locking
                          DB_INIT_LOG   |  // Initialize logging
                          DB_INIT_MPOOL |  // Initialize the cache
                          DB_INIT_TXN;     // Initialize transactions

    u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
    Db *dbp = NULL;
    const char *file_name = "mydb.db";
                                                                                                                                  
    std::string envHome("/export1/testEnv");
    DbEnv myEnv(0);

    try {

        myEnv.open(envHome.c_str(), env_flags, 0);
        dbp = new Db(&myEnv, 0);
        dbp->open(NULL,       // Txn pointer 
                  file_name,  // File name 
                  NULL,       // Logical db name 
                  DB_BTREE,   // Database type (using btree) 
                  db_flags,   // Open flags 
                  0);         // File mode. Using defaults 

    } catch(DbException &e) {
        std::cerr << "Error opening database and environment: "
                  << file_name << ", "
                  << envHome << std::endl;
        std::cerr << e.what() << std::endl;
    }

    try {
        dbp->close(0);
        myEnv.close(0);
    } catch(DbException &e) {
        std::cerr << "Error closing database and environment: "
                  << file_name << ", "
                  << envHome << std::endl;
        std::cerr << e.what() << std::endl;
        return (EXIT_FAILURE);
    }

    return (EXIT_SUCCESS);
}