This is a comprehensive C++ wrapper for the LMDB embedded database library, offering both an error-checked procedural interface and an object-oriented resource interface with RAII semantics.
Here follows a simple motivating example demonstrating basic use of the object-oriented resource interface:
#include <cstdio>
#include <cstdlib>
#include <lmdb++.h>
int main() {
/* Create and open the LMDB environment: */
auto env = lmdb::env::create();
env.set_mapsize(1UL * 1024UL * 1024UL * 1024UL); /* 1 GiB */
env.open("./example.mdb", 0, 0664);
/* Insert some key/value pairs in a write transaction: */
auto wtxn = lmdb::txn::begin(env);
auto dbi = lmdb::dbi::open(wtxn, nullptr);
dbi.put(wtxn, "username", "jhacker");
dbi.put(wtxn, "email", "jhacker@example.org");
dbi.put(wtxn, "fullname", "J. Random Hacker");
wtxn.commit();
/* Fetch key/value pairs in a read-only transaction: */
auto rtxn = lmdb::txn::begin(env, nullptr, MDB_RDONLY);
auto cursor = lmdb::cursor::open(rtxn, dbi);
std::string key, value;
while (cursor.get(key, value, MDB_NEXT)) {
std::printf("key: '%s', value: '%s'\n", key.c_str(), value.c_str());
}
cursor.close();
rtxn.abort();
/* The enviroment is closed automatically. */
return EXIT_SUCCESS;
}
Should any operation in the above fail, an lmdb::error exception will be
thrown and terminate the program since we don't specify an exception handler.
All resources will regardless get automatically cleaned up due to RAII
semantics.
Note
In order to run this example, you must first manually create the
./example.mdb directory. This is a basic characteristic of LMDB: the
given environment path must already exist, as LMDB will not attempt to
automatically create it.
- Designed to be entirely self-contained as a single
<lmdb++.h>header file that can be dropped into a project. - Implements a straightforward mapping from C to C++, with consistent naming.
- Provides both a procedural interface and an object-oriented RAII interface.
- Simplifies error handling by translating error codes into C++ exceptions.
- Carefully differentiates logic errors, runtime errors, and fatal errors.
- Exception strings include the name of the LMDB function that failed.
- Plays nice with others: all symbols are placed into the
lmdbnamespace. - 100% free and unencumbered public domain software, usable in any context and for any purpose.
The <lmdb++.h> header file requires a C++11 compiler and standard library.
Recent releases of Clang or GCC will work fine.
In addition, for your application to build and run, the underlying
<lmdb.h> header file shipped with LMDB must be available in the
preprocessor's include path, and you must link with the liblmdb native
library. On Ubuntu Linux 14.04 and newer, these prerequisites can be
satisfied by installing the liblmdb-dev package.
This wrapper offers both an error-checked procedural interface and an object-oriented resource interface with RAII semantics. The former will be useful for easily retrofitting existing projects that currently use the raw C interface, but we recommend the latter for all new projects due to the exception safety afforded by RAII semantics.
The high-level resource interface wraps LMDB handles in a loving RAII embrace. This way, you can ensure e.g. that a transaction will get automatically aborted when exiting a lexical scope, regardless of whether the escape happened normally or by throwing an exception.
| C handle | C++ wrapper class |
|---|---|
MDB_env* |
lmdb::env |
MDB_txn* |
lmdb::txn |
MDB_dbi |
lmdb::dbi |
MDB_cursor* |
lmdb::cursor |
MDB_val |
lmdb::val |
The methods available on these C++ classes are named consistently with the procedural interface, below, with the obvious difference of omitting the handle type prefix which is already implied by the class in question.
The low-level procedural interface wraps LMDB functions with error-checking
code that will throw an instance of a corresponding C++ exception class in
case of failure. This interface doesn't offer any convenience overloads as
does the resource interface; the parameter types are exactly the same as for
the raw C interface offered by LMDB itself. The return type is generally
void for these functions since the wrapper eats the error code returned
by the underlying C function, throwing an exception in case of failure and
otherwise returning values in the same output parameters as the C interface.
This interface is implemented entirely using static inline functions, so there are no hidden extra costs to using these wrapper functions so long as you have a decent compiler capable of basic inlining optimization.
| C function | C++ wrapper function |
|---|---|
mdb_version() |
N/A |
mdb_strerror() |
N/A |
mdb_env_create() |
lmdb::env_create() |
mdb_env_open() |
lmdb::env_open() |
mdb_env_copy() |
lmdb::env_copy() [1] |
mdb_env_copyfd() |
lmdb::env_copy_fd() [1] |
mdb_env_copy2() |
lmdb::env_copy() [1] |
mdb_env_copyfd2() |
lmdb::env_copy_fd() [1] |
mdb_env_stat() |
lmdb::env_stat() |
mdb_env_info() |
lmdb::env_info() |
mdb_env_sync() |
lmdb::env_sync() |
mdb_env_close() |
lmdb::env_close() |
mdb_env_set_flags() |
lmdb::env_set_flags() |
mdb_env_get_flags() |
lmdb::env_get_flags() |
mdb_env_get_path() |
lmdb::env_get_path() |
mdb_env_get_fd() |
lmdb::env_get_fd() |
mdb_env_set_mapsize() |
lmdb::env_set_mapsize() |
mdb_env_set_maxreaders() |
lmdb::env_set_max_readers() |
mdb_env_get_maxreaders() |
lmdb::env_get_max_readers() |
mdb_env_set_maxdbs() |
lmdb::env_set_max_dbs() |
mdb_env_get_maxkeysize() |
lmdb::env_get_max_keysize() |
mdb_env_set_userctx() |
lmdb::env_set_userctx() [2] |
mdb_env_get_userctx() |
lmdb::env_get_userctx() [2] |
mdb_env_set_assert() |
N/A |
mdb_txn_begin() |
lmdb::txn_begin() |
mdb_txn_env() |
lmdb::txn_env() |
mdb_txn_id() |
lmdb::txn_id() [3] |
mdb_txn_commit() |
lmdb::txn_commit() |
mdb_txn_abort() |
lmdb::txn_abort() |
mdb_txn_reset() |
lmdb::txn_reset() |
mdb_txn_renew() |
lmdb::txn_renew() |
mdb_dbi_open() |
lmdb::dbi_open() |
mdb_stat() |
lmdb::dbi_stat() [4]_ |
mdb_dbi_flags() |
lmdb::dbi_flags() |
mdb_dbi_close() |
lmdb::dbi_close() |
mdb_drop() |
lmdb::dbi_drop() [4]_ |
mdb_set_compare() |
lmdb::dbi_set_compare() [4]_ |
mdb_set_dupsort() |
lmdb::dbi_set_dupsort() [4]_ |
mdb_set_relfunc() |
lmdb::dbi_set_relfunc() [4]_ |
mdb_set_relctx() |
lmdb::dbi_set_relctx() [4]_ |
mdb_get() |
lmdb::dbi_get() [4]_ |
mdb_put() |
lmdb::dbi_put() [4]_ |
mdb_del() |
lmdb::dbi_del() [4]_ |
mdb_cursor_open() |
lmdb::cursor_open() |
mdb_cursor_close() |
lmdb::cursor_close() |
mdb_cursor_renew() |
lmdb::cursor_renew() |
mdb_cursor_txn() |
lmdb::cursor_txn() |
mdb_cursor_dbi() |
lmdb::cursor_dbi() |
mdb_cursor_get() |
lmdb::cursor_get() |
mdb_cursor_put() |
lmdb::cursor_put() |
mdb_cursor_del() |
lmdb::cursor_del() |
mdb_cursor_count() |
lmdb::cursor_count() |
mdb_cmp() |
N/A |
mdb_dcmp() |
N/A |
mdb_reader_list() |
TODO |
mdb_reader_check() |
TODO |
Footnotes
| [1] | (1, 2, 3, 4) Three-parameter signature available since LMDB 0.9.14 (2014/09/20). |
| [2] | (1, 2) Only available since LMDB 0.9.11 (2014/01/15). |
| [3] | Only available in LMDB HEAD, not yet in any 0.9.x release (as of 0.9.16).
Define the LMDBXX_TXN_ID preprocessor symbol to unhide this. |
| [4] | Note the difference in naming. (See below.) |
- The C++ procedural interface is more strictly and consistently grouped by
handle type than is the LMDB native interface. For instance,
mdb_put()is wrapped as the C++ functionlmdb::dbi_put(), notlmdb::put(). These differences--a handful in number--all concern operations on database handles. - The C++ interface takes some care to be const-correct for input-only
parameters, something the original C interface largely ignores.
Hence occasional use of
const_castin the wrapper code base. lmdb::dbi_put()does not throw an exception if LMDB returns theMDB_KEYEXISTerror code; it instead just returnsfalse. This is intended to simplify common usage patterns.lmdb::dbi_get(),lmdb::dbi_del(), andlmdb::cursor_get()do not throw an exception if LMDB returns theMDB_NOTFOUNDerror code; they instead just returnfalse. This is intended to simplify common usage patterns.lmdb::env_get_max_keysize()returns an unsigned integer, instead of a signed integer as the underlyingmdb_env_get_maxkeysize()function does. This conversion is done since the return value cannot in fact be negative.
This wrapper draws a careful distinction between three different classes of possible LMDB error conditions:
- Logic errors, represented by
lmdb::logic_error. Errors of this class are thrown due to programming errors where the function interfaces are used in violation of documented preconditions. A common strategy for handling this class of error conditions is to abort the program with a core dump, facilitating introspection to locate and remedy the bug. - Fatal errors, represented by
lmdb::fatal_error. Errors of this class are thrown due to the exhaustion of critical system resources, in particular available memory (ENOMEM), or due to attempts to exceed applicable system resource limits. A typical strategy for handling this class of error conditions is to terminate the program with a descriptive error message. More robust programs and shared libraries may wish to implement another strategy, such as retrying the operation after first letting most of the call stack unwind in order to free up scarce resources. - Runtime errors, represented by
lmdb::runtime_error. Errors of this class are thrown as a matter of course to indicate various exceptional conditions. These conditions are generally recoverable, and robust programs will take care to correctly handle them.
Note
The distinction between logic errors and runtime errors mirrors that
found in the C++11 standard library, where the <stdexcept> header
defines the standard exception base classes std::logic_error and
std::runtime_error. The standard exception class std::bad_alloc,
on the other hand, is a representative example of a fatal error.
| Error code | Exception class | Exception type |
|---|---|---|
MDB_KEYEXIST |
lmdb::key_exist_error |
runtime |
MDB_NOTFOUND |
lmdb::not_found_error |
runtime |
MDB_CORRUPTED |
lmdb::corrupted_error |
fatal |
MDB_PANIC |
lmdb::panic_error |
fatal |
MDB_VERSION_MISMATCH |
lmdb::version_mismatch_error |
fatal |
MDB_MAP_FULL |
lmdb::map_full_error |
runtime |
MDB_BAD_DBI |
lmdb::bad_dbi_error |
runtime [4]_ |
| (others) | lmdb::runtime_error |
runtime |
Footnotes
| [4] | Available since LMDB 0.9.14 (2014/09/20). |
Note
MDB_KEYEXIST and MDB_NOTFOUND are handled specially by some functions.
The lmdb++ version tracks the upstream LMDB release (x.y.z) that it is compatible with, and appends a sub-patch-level version (x.y.z.N) to indicate changes to the wrapper itself.
For example, an lmdb++ release of 0.9.14.2 would indicate that it is designed for compatibility with LMDB 0.9.14, and is the third wrapper release (the first being .0, and the second .1) for that upstream target.
Note
To the extent that LMDB will preserve API and ABI compatibility going
forward, older versions of the wrapper should work with newer versions of
LMDB; and newer versions of the wrapper will generally work with older
versions of LMDB by using the preprocessor to conditionalize the
visibility of newer symbols--see, for example, the preprocessor guards
around the definition of lmdb::env_set_userctx().
lmdb++ is currently available as a package/port in the following operating system distributions and package management systems:
| Distribution | Package Name | Installation Hint |
|---|---|---|
| Arch Linux AUR | liblmdb++ | yaourt -Sa liblmdb++ |
| Fink [5] | lmdb++ | sudo fink install lmdb++ |
| MacPorts | lmdbxx | sudo port install lmdbxx |
| Portage [6] | lmdb++ | sudo emerge --ask lmdb++ |
Footnotes
| [5] | Still pending review. |
| [6] | Compatible with Gentoo Linux, Funtoo Linux, and Sabayon Linux. |
To report a bug or submit a patch for lmdb++, please file an issue in the issue tracker on GitHub.
Questions and discussions about LMDB itself should be directed to the OpenLDAP mailing lists.
Find this project at: GitHub, Bitbucket, Open Hub, SourceForge, Travis CI, and Coverity Scan.
The API documentation is published at: http://lmdbxx.sourceforge.net/
This is free and unencumbered public domain software. For more information,
see http://unlicense.org/ or the accompanying UNLICENSE file.