Cleanup base/stl_util

base/stl_util.h

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Derived from google3/util/gtl/stl_util.h
+
+#ifndef BASE_STL_UTIL_H_
+#define BASE_STL_UTIL_H_
+#pragma once
+
+#include <assert.h>
+#include <string.h>  // for memcpy

[joth, 2011/07/20 11:38:48]

we don't actually call memcpy anymore.

[Denis Lagno, 2011/07/20 12:01:13]

thanks for noticing.

And BTW we do not call assert anymore as well.

+
+#include <string>
+#include <vector>
+
+// Clear internal memory of an STL object.
+// STL clear()/reserve(0) does not always free internal memory allocated
+// This function uses swap/destructor to ensure the internal memory is freed.
+template<class T> void STLClearObject(T* obj) {
+  T tmp;
+  tmp.swap(*obj);
+  // Sometimes "T tmp" allocates objects with memory (arena implementation?).
+  // Hence using additional reserve(0) even if it doesn't always work.
+  obj->reserve(0);
+}
+
+// STLDeleteContainerPointers()
+//  For a range within a container of pointers, calls delete
+//  (non-array version) on these pointers.
+// NOTE: for these three functions, we could just implement a DeleteObject
+// functor and then call for_each() on the range and functor, but this
+// requires us to pull in all of algorithm.h, which seems expensive.
+// For hash_[multi]set, it is important that this deletes behind the iterator
+// because the hash_set may call the hash function on the iterator when it is
+// advanced, which could result in the hash function trying to deference a
+// stale pointer.
+template <class ForwardIterator>
+void STLDeleteContainerPointers(ForwardIterator begin, ForwardIterator end) {
+  while (begin != end) {
+    ForwardIterator temp = begin;
+    ++begin;
+    delete *temp;
+  }
+}
+
+// STLDeleteContainerPairPointers()
+//  For a range within a container of pairs, calls delete
+//  (non-array version) on BOTH items in the pairs.
+// NOTE: Like STLDeleteContainerPointers, it is important that this deletes
+// behind the iterator because if both the key and value are deleted, the
+// container may call the hash function on the iterator when it is advanced,
+// which could result in the hash function trying to dereference a stale
+// pointer.
+template <class ForwardIterator>
+void STLDeleteContainerPairPointers(ForwardIterator begin,
+                                    ForwardIterator end) {
+  while (begin != end) {
+    ForwardIterator temp = begin;
+    ++begin;
+    delete temp->first;
+    delete temp->second;
+  }
+}
+
+// STLDeleteContainerPairFirstPointers()
+//  For a range within a container of pairs, calls delete (non-array version)
+//  on the FIRST item in the pairs.
+// NOTE: Like STLDeleteContainerPointers, deleting behind the iterator.
+template <class ForwardIterator>
+void STLDeleteContainerPairFirstPointers(ForwardIterator begin,
+                                         ForwardIterator end) {
+  while (begin != end) {
+    ForwardIterator temp = begin;
+    ++begin;
+    delete temp->first;
+  }
+}
+
+// STLDeleteContainerPairSecondPointers()
+//  For a range within a container of pairs, calls delete
+//  (non-array version) on the SECOND item in the pairs.
+template <class ForwardIterator>
+void STLDeleteContainerPairSecondPointers(ForwardIterator begin,
+                                          ForwardIterator end) {
+  while (begin != end) {
+    delete begin->second;
+    ++begin;
+  }
+}

[joth, 2011/07/20 11:38:48]

fwiw server now has this as
  while (begin != end) {
    ForwardIterator temp = begin;
    ++begin;
    delete temp->second;
  }
}
(with comment to explain why - 'first' maybe a pointer into 'second' which could
invalidate the iterator)

[Denis Lagno, 2011/07/20 12:01:13]

yes, it looks better to sync with server.

+
+// To treat a possibly-empty vector as an array, use these functions.
+// If you know the array will never be empty, you can use &*v.begin()
+// directly, but that is undefined behaviour if v is empty.
+
+template<typename T>
+inline T* vector_as_array(std::vector<T>* v) {
+# ifdef NDEBUG
+  return &*v->begin();

[joth, 2011/07/20 11:38:48]

this always seems to me to be a quite obscure non-portable optimization of
minimal benefit: only having the #else version would make this method a whole
load easier to understand.

Thoughts on removing this, or at least adding a test case for this (passing in
empty vector) to ensure all versions of STL we use allow this optimization?

[Denis Lagno, 2011/07/20 12:54:36]

agree, chromium (client) and server differs in that we target much larger number
of compilers/environments so it's much more important for us to be standards
compliant.

+# else
+  return v->empty() ? NULL : &*v->begin();
+# endif
+}
+
+template<typename T>
+inline const T* vector_as_array(const std::vector<T>* v) {
+# ifdef NDEBUG
+  return &*v->begin();
+# else
+  return v->empty() ? NULL : &*v->begin();
+# endif
+}
+
+// Return a mutable char* pointing to a string's internal buffer,
+// which may not be null-terminated. Writing through this pointer will
+// modify the string.
+//
+// string_as_array(&str)[i] is valid for 0 <= i < str.size() until the
+// next call to a string method that invalidates iterators.
+//
+// As of 2006-04, there is no standard-blessed way of getting a
+// mutable reference to a string's internal buffer. However, issue 530
+// (http://www.open-std.org/JTC1/SC22/WG21/docs/lwg-active.html#530)
+// proposes this as the method. According to Matt Austern, this should
+// already work on all current implementations.

[joth, 2011/07/20 11:38:48]

FWIW, C++ 2008 does now clarify this:

21.3.1 basic_string general requirements
5 The char-like objects in a basic_string object shall be stored contiguously.
That is, for any basic_string
object s, the identity &*(s.begin() + n) == &*s.begin() + n shall hold for all
values of n such that 0 <= n < s.size()

+inline char* string_as_array(std::string* str) {
+  // DO NOT USE const_cast<char*>(str->data())
+  return str->empty() ? NULL : &*str->begin();
+}
+
+// The following functions are useful for cleaning up STL containers
+// whose elements point to allocated memory.
+
+// STLDeleteElements() deletes all the elements in an STL container and clears
+// the container.  This function is suitable for use with a vector, set,
+// hash_set, or any other STL container which defines sensible begin(), end(),
+// and clear() methods.
+//
+// If container is NULL, this function is a no-op.
+//
+// As an alternative to calling STLDeleteElements() directly, consider
+// STLElementDeleter (defined below), which ensures that your container's
+// elements are deleted when the STLElementDeleter goes out of scope.
+template <class T>
+void STLDeleteElements(T *container) {
+  if (!container) return;
+  STLDeleteContainerPointers(container->begin(), container->end());
+  container->clear();
+}
+
+// Given an STL container consisting of (key, value) pairs, STLDeleteValues
+// deletes all the "value" components and clears the container.  Does nothing
+// in the case it's given a NULL pointer.
+
+template <class T>
+void STLDeleteValues(T *v) {
+  if (!v) return;
+  for (typename T::iterator i = v->begin(); i != v->end(); ++i) {
+    delete i->second;
+  }
+  v->clear();
+}
+
+
+// The following classes provide a convenient way to delete all elements or
+// values from STL containers when they goes out of scope.  This greatly
+// simplifies code that creates temporary objects and has multiple return
+// statements.  Example:
+//
+// vector<MyProto *> tmp_proto;
+// STLElementDeleter<vector<MyProto *> > d(&tmp_proto);
+// if (...) return false;
+// ...
+// return success;
+
+// Given a pointer to an STL container this class will delete all the element
+// pointers when it goes out of scope.
+
+template<class STLContainer> class STLElementDeleter {
+ public:
+  STLElementDeleter<STLContainer>(STLContainer *ptr) : container_ptr_(ptr) {}
+  ~STLElementDeleter<STLContainer>() { STLDeleteElements(container_ptr_); }
+ private:
+  STLContainer *container_ptr_;
+};
+
+// Given a pointer to an STL container this class will delete all the value
+// pointers when it goes out of scope.
+
+template<class STLContainer> class STLValueDeleter {
+ public:
+  STLValueDeleter<STLContainer>(STLContainer *ptr) : container_ptr_(ptr) {}
+  ~STLValueDeleter<STLContainer>() { STLDeleteValues(container_ptr_); }
+ private:
+  STLContainer *container_ptr_;
+};
+
+// Test to see if a set, map, hash_set or hash_map contains a particular key.
+// Returns true if the key is in the collection.
+template <typename Collection, typename Key>
+bool ContainsKey(const Collection& collection, const Key& key) {
+  return collection.find(key) != collection.end();
+}
+
+#endif  // BASE_STL_UTIL_H_