diff --git a/cxxheaderparser/options.py b/cxxheaderparser/options.py
index 6e0930c..82a59f6 100644
--- a/cxxheaderparser/options.py
+++ b/cxxheaderparser/options.py
@@ -2,7 +2,7 @@
 from typing import Callable, Optional
 
 #: arguments are (filename, content)
-PreprocessorFunction = Callable[[str, str], str]
+PreprocessorFunction = Callable[[str, Optional[str]], str]
 
 
 @dataclass
diff --git a/cxxheaderparser/parser.py b/cxxheaderparser/parser.py
index a2c031a..18614c5 100644
--- a/cxxheaderparser/parser.py
+++ b/cxxheaderparser/parser.py
@@ -74,9 +74,10 @@ class CxxParser:
     def __init__(
         self,
         filename: str,
-        content: str,
+        content: typing.Optional[str],
         visitor: CxxVisitor,
         options: typing.Optional[ParserOptions] = None,
+        encoding: typing.Optional[str] = None,
     ) -> None:
         self.visitor = visitor
         self.filename = filename
@@ -85,6 +86,13 @@ def __init__(
         if options and options.preprocessor is not None:
             content = options.preprocessor(filename, content)
 
+        if content is None:
+            if encoding is None:
+                encoding = "utf-8-sig"
+
+            with open(filename, "r", encoding=encoding) as fp:
+                content = fp.read()
+
         self.lex: lexer.TokenStream = lexer.LexerTokenStream(filename, content)
 
         global_ns = NamespaceDecl([], False)
diff --git a/cxxheaderparser/preprocessor.py b/cxxheaderparser/preprocessor.py
index 1e5719b..99a19ba 100644
--- a/cxxheaderparser/preprocessor.py
+++ b/cxxheaderparser/preprocessor.py
@@ -74,7 +74,7 @@ def make_gcc_preprocessor(
     if not encoding:
         encoding = "utf-8"
 
-    def _preprocess_file(filename: str, content: str) -> str:
+    def _preprocess_file(filename: str, content: typing.Optional[str]) -> str:
         cmd = gcc_args + ["-w", "-E", "-C"]
 
         for p in include_paths:
@@ -86,6 +86,8 @@ def _preprocess_file(filename: str, content: str) -> str:
         if filename == "<str>":
             cmd.append("-")
             filename = "<stdin>"
+            if content is None:
+                raise PreprocessorError("no content specified for stdin")
             kwargs["input"] = content
         else:
             cmd.append(filename)
@@ -191,7 +193,7 @@ def make_pcpp_preprocessor(
     if pcpp is None:
         raise PreprocessorError("pcpp is not installed")
 
-    def _preprocess_file(filename: str, content: str) -> str:
+    def _preprocess_file(filename: str, content: typing.Optional[str]) -> str:
         pp = _CustomPreprocessor(encoding, passthru_includes)
         if include_paths:
             for p in include_paths:
@@ -203,6 +205,10 @@ def _preprocess_file(filename: str, content: str) -> str:
         if not retain_all_content:
             pp.line_directive = "#line"
 
+        if content is None:
+            with open(filename, "r", encoding=encoding) as fp:
+                content = fp.read()
+
         pp.parse(content, filename)
 
         if pp.errors:
diff --git a/cxxheaderparser/simple.py b/cxxheaderparser/simple.py
index 421666b..3efb048 100644
--- a/cxxheaderparser/simple.py
+++ b/cxxheaderparser/simple.py
@@ -348,7 +348,10 @@ def parse_file(
     if filename == "-":
         content = sys.stdin.read()
     else:
-        with open(filename, encoding=encoding) as fp:
-            content = fp.read()
+        content = None
 
-    return parse_string(content, filename=filename, options=options)
+    visitor = SimpleCxxVisitor()
+    parser = CxxParser(filename, content, visitor, options)
+    parser.parse()
+
+    return visitor.data