Db::set_h_compare()
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## Db::set_h_compare()

    #include <db_cxx.h>

    extern "C" {
        typedef int (*compare_fcn_type)(DB *db, const DBT *dbt1,
                                        const DBT *dbt2);
    };
    int
    Db::set_h_compare(compare_fcn_type compare_fcn);

Set the Hash key comparison function. The comparison function is called whenever it is necessary to compare a key specified by the application with a key currently stored in the database.

If no comparison function is specified, a byte-by-byte comparison is performed.

The `Db::set_h_compare()` method configures operations performed using the specified Db handle, not all operations performed on the underlying database.

The `Db::set_h_compare()` method may not be called after the Db::open() method is called. If the database already exists when Db::open() is called, the information specified to `Db::set_h_compare()` must be the same as that historically used to create the database or corruption can occur.

The `Db::set_h_compare()` method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success.

### Parameters

#### compare_fcn

The compare_fcn function is the application-specified Hash comparison function. The comparison function takes three parameters:

- `db`

  The db parameter is the enclosing database handle.

- `dbt1`

  The dbt1 parameter is the Dbt representing the application supplied key.

- `dbt2`

  The dbt2 parameter is the Dbt representing the current database's key.

The compare_fcn function must return an integer value less than, equal to, or greater than zero if the first key parameter is considered to be respectively less than, equal to, or greater than the second key parameter. The comparison function must correctly handle any key values used by the application (possibly including zero-length keys). The data and size fields of the Dbt are the only fields that may be used for the purposes of this comparison, and no particular alignment of the memory to which by the data field refers may be assumed.

### Errors

The `Db::set_h_compare()` method may fail and throw a DbException exception, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

#### EINVAL

If the method was called after Db::open() was called; or if an invalid flag value or parameter was specified.

### Class

Db

### See Also

Database and Related Methods

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