feat: add support for reading free floating std::vector

src/uproot/reading.py

@@ -1106,6 +1106,16 @@ def class_named(self, classname, version=None):
             if len(streamers) == 0 and self._custom_classes is not None:
                 cls = uproot.classes.get(classname)
 
+        if (
+            cls is None
+            and re.match(r"(std\s*::\s*)?(vector|map|set|bitset)\s*<", classname)
+            is not None
+        ):
+            cls = uproot.interpretation.identify.parse_typename(classname)
+            cls._header = False
+
+            return cls
+
         if cls is None:
             if len(streamers) == 0:
                 unknown_cls = uproot.unknown_classes.get(classname)
@@ -2494,7 +2504,7 @@ def get(self):
             start_cursor = cursor.copy()
             context = {"breadcrumbs": (), "TKey": self}
 
-            if self._fClassName == "string":
+            if re.match(r"(std\s*::\s*)?string", self._fClassName):
                 return cursor.string(chunk, context)
 
             cls = self._file.class_named(self._fClassName)

tests/test_1180_read_free_floating_vector_issue_1179.py

@@ -0,0 +1,25 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/uproot5/blob/main/LICENSE
+
+import pytest
+import uproot
+import os
+import numpy as np
+
+ROOT = pytest.importorskip("ROOT")
+
+
+def test_read_free_floating_vector(tmp_path):
+    newfile = os.path.join(tmp_path, "test_freevec.root")
+    f = ROOT.TFile(newfile, "recreate")
+    a = np.array([1.0, 2.0, 3.0, 4.0], dtype=np.float64)
+    c = np.array([0, 1, 4, 5, 6, 7, 8, 9], dtype=np.uint32)
+    avec = ROOT.std.vector("double")(a)
+    bvec = ROOT.std.vector("unsigned int")(c)
+    f.WriteObject(avec, "avec")
+    f.WriteObject(bvec, "bvec")
+    f.Write()
+    f.Close()
+
+    with uproot.open(newfile) as f:
+        assert f["avec"].tolist() == [1.0, 2.0, 3.0, 4.0]
+        assert f["bvec"].tolist() == [0, 1, 4, 5, 6, 7, 8, 9]