Allow passing std::string_view to dbi::open

- Useful in case the DBI names are themselves stored in the DB.
- Fixes hoytech#5
- Requested by @deepbluev7

README.md

@@ -379,6 +379,8 @@ This C++17 version is a fork of Arto Bendiken's C++11 version with the following
 
 * If an exception was throw by `txn.commit()` (ie `MDB_MAP_FULL`), and this transaction was later aborted (because it went out of scope while unwinding the stack), then a double-free would occur. This was [fixed](https://github.com/hoytech/lmdbxx/pull/3) by Niklas Salmoukas.
 
+* `dbi::open()` now optionally accepts the DBI name as a `string_view`. This is useful when the DBI names themselves are stored in the DB. [Requested](https://github.com/hoytech/lmdbxx/issues/5) by deepbluev7.
+
 
 
 ## Author

check.cc

@@ -16,7 +16,7 @@ int main() {
     lmdb::dbi mydb;
     lmdb::dbi mydbdups;
 
-    // Put some values in and read them back out
+    // Put some values in
 
     {
         auto txn = lmdb::txn::begin(env);
@@ -28,6 +28,8 @@ int main() {
         txn.commit();
     }
 
+    // Read them back out
+
     {
         auto txn = lmdb::txn::begin(env, nullptr, MDB_RDONLY);
 
@@ -38,6 +40,36 @@ int main() {
         longLivedValue = v;
     }
 
+    // Open DBI with a string_view
+
+    {
+        std::string mydbStr = "mydbJUNK";
+
+        auto txn = lmdb::txn::begin(env, nullptr, MDB_RDONLY);
+        auto mydb2 = lmdb::dbi::open(txn, std::string_view(mydbStr).substr(0, 4));
+
+        std::string_view v;
+        mydb2.get(txn, "hello", v);
+        if (v != "world") throw std::runtime_error("bad read 1.5");
+    }
+
+    // Loop over null DB, to make sure it contains the table we just created
+
+    {
+        auto txn = lmdb::txn::begin(env, nullptr, MDB_RDONLY);
+
+        auto emptyDb = lmdb::dbi::open(txn, nullptr);
+
+        auto cursor = lmdb::cursor::open(txn, emptyDb);
+        std::string_view key, val;
+
+        if (!cursor.get(key, val, MDB_FIRST)) throw std::runtime_error("emptyDb cursor err 1");
+        if (key != "mydb") throw std::runtime_error("emptyDb cursor err 2");
+
+        if (cursor.get(key, val, MDB_NEXT)) throw std::runtime_error("emptyDb cursor err 3");
+
+        cursor.close();
+    }
 
     // Update one of the values
 
@@ -49,6 +81,8 @@ int main() {
         txn.commit();
     }
 
+    // Make sure updated value was stored
+
     {
         auto txn = lmdb::txn::begin(env, nullptr, MDB_RDONLY);
 

lmdb++.h

@@ -1265,8 +1265,8 @@ class lmdb::dbi {
    * Opens a database handle.
    *
    * @param txn the transaction handle
-   * @param name
-   * @param flags
+   * @param name the database name, or nullptr
+   * @param flags dbi flags, ie MDB_CREATE
    * @throws lmdb::error on failure
    */
   static dbi
@@ -1278,6 +1278,23 @@ class lmdb::dbi {
     return dbi{handle};
   }
 
+  /*
+   * This overload is so that DBI names can be passed in as string_views, which can be
+   * useful when storing the names of DBIs in the database itself. We need to convert it
+   * to a std::string to ensure that it is NUL-byte terminated. This overload comes after
+   * the const char* one above so that in that case there is no std::string overhead.
+   */
+
+  static dbi
+  open(MDB_txn* const txn,
+       std::string_view name,
+       const unsigned int flags = default_flags) {
+    MDB_dbi handle{};
+    std::string nameStr(name);
+    lmdb::dbi_open(txn, nameStr.c_str(), flags, &handle);
+    return dbi{handle};
+  }
+
   /**
    * Constructor.
    *