diff --git a/m2cgen/interpreters/c/interpreter.py b/m2cgen/interpreters/c/interpreter.py
index 8c5a2ee6..51c89cb8 100644
--- a/m2cgen/interpreters/c/interpreter.py
+++ b/m2cgen/interpreters/c/interpreter.py
@@ -63,7 +63,7 @@ def interpret(self, expr):
         if self.with_math_module:
             self._cg.add_dependency("<math.h>")
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
 
     # Both methods supporting linear algebra do several things:
     #
diff --git a/m2cgen/interpreters/c_sharp/interpreter.py b/m2cgen/interpreters/c_sharp/interpreter.py
index 6e32b763..d72d599e 100644
--- a/m2cgen/interpreters/c_sharp/interpreter.py
+++ b/m2cgen/interpreters/c_sharp/interpreter.py
@@ -58,4 +58,4 @@ def interpret(self, expr):
         if self.with_math_module:
             self._cg.add_dependency("System.Math")
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
diff --git a/m2cgen/interpreters/code_generator.py b/m2cgen/interpreters/code_generator.py
index 36401c55..dc5dc523 100644
--- a/m2cgen/interpreters/code_generator.py
+++ b/m2cgen/interpreters/code_generator.py
@@ -1,4 +1,6 @@
+from io import StringIO
 from string import Template
+from weakref import finalize
 
 
 class CodeTemplate:
@@ -29,11 +31,38 @@ class BaseCodeGenerator:
 
     def __init__(self, indent=4):
         self._indent = indent
+        self._code_buf = None
         self.reset_state()
 
     def reset_state(self):
         self._current_indent = 0
-        self.code = ""
+        self._finalize_buffer()
+        self._code_buf = StringIO()
+        self._code = None
+        self._finalizer = finalize(self, self._finalize_buffer)
+
+    def _finalize_buffer(self):
+        if self._code_buf is not None and not self._code_buf.closed:
+            self._code_buf.close()
+
+    def _write_to_code_buffer(self, text, prepend=False):
+        if self._code_buf.closed:
+            raise BufferError(
+                "Cannot modify code after getting generated code and "
+                "closing the underlying buffer!\n"
+                "Call reset_state() to allocate new buffer.")
+        if prepend:
+            self._code_buf.seek(0)
+            old_content = self._code_buf.read()
+            self._code_buf.seek(0)
+            text += old_content
+        self._code_buf.write(text)
+
+    def finalize_and_get_generated_code(self):
+        if not self._code_buf.closed:
+            self._code = self._code_buf.getvalue()
+            self._finalize_buffer()
+        return self._code if self._code is not None else ""
 
     def increase_indent(self):
         self._current_indent += self._indent
@@ -48,22 +77,24 @@ def decrease_indent(self):
     def add_code_line(self, line):
         if not line:
             return
-        indent = " " * self._current_indent
-        self.code += indent + line + "\n"
+        self.add_code_lines([line.strip()])
 
     def add_code_lines(self, lines):
         if isinstance(lines, str):
             lines = lines.strip().split("\n")
         indent = " " * self._current_indent
-        self.code += indent + "\n{}".format(indent).join(lines) + "\n"
+        self._write_to_code_buffer(
+            indent + "\n{}".format(indent).join(lines) + "\n")
 
     def prepend_code_line(self, line):
-        self.code = line + "\n" + self.code
+        if not line:
+            return
+        self.prepend_code_lines([line.strip()])
 
     def prepend_code_lines(self, lines):
         if isinstance(lines, str):
             lines = lines.strip().split("\n")
-        self.code = "\n".join(lines) + "\n" + self.code
+        self._write_to_code_buffer("\n".join(lines) + "\n", prepend=True)
 
     # Following methods simply compute expressions using templates without
     # changing result.
diff --git a/m2cgen/interpreters/dart/interpreter.py b/m2cgen/interpreters/dart/interpreter.py
index 34d658e3..967aa296 100644
--- a/m2cgen/interpreters/dart/interpreter.py
+++ b/m2cgen/interpreters/dart/interpreter.py
@@ -62,7 +62,7 @@ def interpret(self, expr):
         if self.with_math_module:
             self._cg.add_dependency("dart:math")
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
 
     def interpret_tanh_expr(self, expr, **kwargs):
         self.with_tanh_expr = True
diff --git a/m2cgen/interpreters/go/interpreter.py b/m2cgen/interpreters/go/interpreter.py
index 95ca0ee2..74826f1b 100644
--- a/m2cgen/interpreters/go/interpreter.py
+++ b/m2cgen/interpreters/go/interpreter.py
@@ -50,4 +50,4 @@ def interpret(self, expr):
         if self.with_math_module:
             self._cg.add_dependency("math")
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
diff --git a/m2cgen/interpreters/haskell/interpreter.py b/m2cgen/interpreters/haskell/interpreter.py
index 577aa8af..3c37010e 100644
--- a/m2cgen/interpreters/haskell/interpreter.py
+++ b/m2cgen/interpreters/haskell/interpreter.py
@@ -52,7 +52,7 @@ def interpret(self, expr):
         self._cg.prepend_code_line(self._cg.tpl_module_definition(
             module_name=self.module_name))
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
 
     def interpret_if_expr(self, expr, if_code_gen=None, **kwargs):
         if if_code_gen is None:
@@ -72,7 +72,8 @@ def interpret_if_expr(self, expr, if_code_gen=None, **kwargs):
         code_gen.add_if_termination()
 
         if not nested:
-            return self._cache_reused_expr(expr, code_gen.code)
+            return self._cache_reused_expr(
+                expr, code_gen.finalize_and_get_generated_code())
 
     def interpret_pow_expr(self, expr, **kwargs):
         base_result = self._do_interpret(expr.base_expr, **kwargs)
diff --git a/m2cgen/interpreters/java/interpreter.py b/m2cgen/interpreters/java/interpreter.py
index 01b4d41a..92d03007 100644
--- a/m2cgen/interpreters/java/interpreter.py
+++ b/m2cgen/interpreters/java/interpreter.py
@@ -60,7 +60,7 @@ def interpret(self, expr):
                     os.path.dirname(__file__), "linear_algebra.java")
                 top_cg.add_code_lines(utils.get_file_content(filename))
 
-        return top_cg.code
+        return top_cg.finalize_and_get_generated_code()
 
     # Required by SubroutinesMixin to create new code generator for
     # each subroutine.
diff --git a/m2cgen/interpreters/javascript/interpreter.py b/m2cgen/interpreters/javascript/interpreter.py
index 09fb05e7..366b894b 100644
--- a/m2cgen/interpreters/javascript/interpreter.py
+++ b/m2cgen/interpreters/javascript/interpreter.py
@@ -49,4 +49,4 @@ def interpret(self, expr):
                 os.path.dirname(__file__), "linear_algebra.js")
             self._cg.add_code_lines(utils.get_file_content(filename))
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
diff --git a/m2cgen/interpreters/mixins.py b/m2cgen/interpreters/mixins.py
index 199633bb..fd4bc454 100644
--- a/m2cgen/interpreters/mixins.py
+++ b/m2cgen/interpreters/mixins.py
@@ -187,7 +187,7 @@ def _process_subroutine(self, subroutine):
             last_result = self._do_interpret(subroutine.expr)
             self._cg.add_return_statement(last_result)
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
 
     def _get_subroutine_name(self):
         subroutine_name = "subroutine" + str(self._subroutine_idx)
diff --git a/m2cgen/interpreters/php/interpreter.py b/m2cgen/interpreters/php/interpreter.py
index 06d0416b..3f288c1d 100644
--- a/m2cgen/interpreters/php/interpreter.py
+++ b/m2cgen/interpreters/php/interpreter.py
@@ -45,4 +45,4 @@ def interpret(self, expr):
 
         self._cg.prepend_code_line("<?php")
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
diff --git a/m2cgen/interpreters/powershell/interpreter.py b/m2cgen/interpreters/powershell/interpreter.py
index 3b75ab8c..b88fdd43 100644
--- a/m2cgen/interpreters/powershell/interpreter.py
+++ b/m2cgen/interpreters/powershell/interpreter.py
@@ -46,7 +46,7 @@ def interpret(self, expr):
                 os.path.dirname(__file__), "linear_algebra.ps1")
             self._cg.prepend_code_lines(utils.get_file_content(filename))
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
 
     def interpret_exp_expr(self, expr, **kwargs):
         nested_result = self._do_interpret(expr.expr, **kwargs)
diff --git a/m2cgen/interpreters/python/interpreter.py b/m2cgen/interpreters/python/interpreter.py
index e884ad75..d1b1b421 100644
--- a/m2cgen/interpreters/python/interpreter.py
+++ b/m2cgen/interpreters/python/interpreter.py
@@ -37,7 +37,7 @@ def interpret(self, expr):
         if self.with_linear_algebra:
             self._cg.add_dependency("numpy", alias="np")
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
 
     def interpret_bin_vector_expr(self, expr, **kwargs):
         self.with_linear_algebra = True
diff --git a/m2cgen/interpreters/r/interpreter.py b/m2cgen/interpreters/r/interpreter.py
index 67d959d1..d77c20f2 100644
--- a/m2cgen/interpreters/r/interpreter.py
+++ b/m2cgen/interpreters/r/interpreter.py
@@ -38,7 +38,7 @@ def interpret(self, expr):
         self.enqueue_subroutine(self.function_name, expr)
         self.process_subroutine_queue(top_cg)
 
-        return top_cg.code
+        return top_cg.finalize_and_get_generated_code()
 
     def create_code_generator(self):
         return RCodeGenerator(indent=self.indent)
diff --git a/m2cgen/interpreters/visual_basic/interpreter.py b/m2cgen/interpreters/visual_basic/interpreter.py
index 01ab2c6e..e4baf390 100644
--- a/m2cgen/interpreters/visual_basic/interpreter.py
+++ b/m2cgen/interpreters/visual_basic/interpreter.py
@@ -60,7 +60,7 @@ def interpret(self, expr):
         self._cg.add_code_line(self._cg.tpl_block_termination(
             block_name="Module"))
 
-        return self._cg.code
+        return self._cg.finalize_and_get_generated_code()
 
     def interpret_pow_expr(self, expr, **kwargs):
         base_result = self._do_interpret(expr.base_expr, **kwargs)