C++ examples of leveldb

LevelDB is a kind of key-value C++ database developped by Google engineers.

My installation script is simple on Arch Linux:

sudo pacman -Syu leveldb

Example 1

We create and then delete a leveldb database instance. Another important thing is to learn how to build with leveldb dynamic link library.

#include <iostream>
#include <string>
#include <assert.h>
#include "leveldb/db.h"

using namespace std;

int main(void)
{
    leveldb::DB *db;
    leveldb::Options options;
    options.create_if_missing = true;

    leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
    assert(status.ok());

    delete db;
    
    return 0;
}

compile and link: g++ test-leveldb.cc -Iinclude -l:libleveldb.so

-Iinclude means searching *.h files under /usr/include, so that we can write the relevant referencing path as #include <leveldb/db.h> since the full path of the header file is /usr/include/leveldb/db.h. By default /usr/include is included by gcc. Therefore, omitting -Iinclude is also acceptable.

-l:libleveldb.so means searching for libleveldb.so under link library path, which by default is /usr/lib/. -l:libleveldb.so also has a shorten form: -lleveldb.

Example 2

Create, Delete, Update, Query

#include <iostream>
#include <string>
#include <assert.h>
#include "leveldb/db.h"

using namespace std;

int main(void)
{
    leveldb::DB *db;
    leveldb::Options options;
    options.create_if_missing = true;

    // open
    leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
    assert(status.ok());

    string key = "name";
    string value = "shane";

    // write
    status = db->Put(leveldb::WriteOptions(), key, value);
    assert(status.ok());

    // read
    status = db->Get(leveldb::ReadOptions(), key, &value);
    assert(status.ok());

    cout << value << endl;

    // delete
    status = db->Delete(leveldb::WriteOptions(), key);
    assert(status.ok());

    status = db->Get(leveldb::ReadOptions(), key, &value);
    if (!status.ok()) {
        cerr << key << "    " << status.ToString() << endl;
    } else {
        cout << key << "===" << value << endl;
    }

    //close
    delete db;
    
    return 0;
}

compile and link as example 1 shows.
run ./a.out. /tmp/testdb will appear:

Here we can notice that even though db is deleted, the files created under /tmp/cduq-db are still exist. If you want to delete this file, we can use leveldb::DestroyDB.

LevelDB Structure: