Check if key exists in Berkeley DB C++ API

Question

so this may sound like a stupid question but I have consulted most of the official documentation of the Berkeley DB C++ API and just cannot seem to find an answer. How do you check if a key exists in the Database? Here is some example code.

// Database created.

Dbt key(&some_string, some_size);
Dbt data(&some_struct, some_other_size);

database.put(nullptr, &key, &data, 0);

// Now my &some_string key is definitely in the database, and I can easily get it.
// However if I create an identical string to search for the key, I get DB_NOTFOUND

Dbt search_key(&the_same_string, the_same_size);
int ret = database.exists(nullptr, &search_key, 0);

// This returns DB_NOTFOUND.

If I pass the original key Dbt object, of course ret is 0. But is there no way to check if a key exists, if you do not already have the associated Dbt object for that exact record?

I have also tried using cursors and Dbc::get() works for finding keys if I set the DB_FIRST flag. But that one automatically writes into whatever data Dbt object I pass.

All I want to do is check if a Dbt object with some specific key exists, nothing more, nothing less.

Answer 1

The second argument for instantiating a Dbt is just an integer size, not a pointer. So, for example, your line:

Dbt key(&some_string, &some_size);

Should become:

Dbt key(&some_string, some_size);

In addition, if your some_string is a std::string you'll have to convert it to a plain C character array. Consider including the NUL terminator in the string written to the database. You don't have to, but it often makes debugging easier.