Avoid heap allocations in IPAddress

net/base/ip_address.h

Index: net/base/ip_address.h
diff --git a/net/base/ip_address.h b/net/base/ip_address.h
index e326e78da6e08a75f97fc4647c2c7252d7bd321a..aeb53821d9134cf23694f291f16976eba847329f 100644
--- a/net/base/ip_address.h
+++ b/net/base/ip_address.h
@@ -8,10 +8,12 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <array>
 #include <string>
 #include <vector>
 
 #include "base/compiler_specific.h"
+#include "base/containers/stack_container.h"
 #include "base/strings/string_piece.h"
 #include "net/base/net_export.h"
 
@@ -21,6 +23,78 @@ class NET_EXPORT IPAddress {
  public:
   enum : size_t { kIPv4AddressSize = 4, kIPv6AddressSize = 16 };
 
+  // Helper class to represent the sequence of bytes in an IP address.
+  // A vector<uint8_t> would be simpler but incurs heap allocation, so
+  // IPAddressBytes uses a fixed size array.
+  class IPAddressBytes {
+   public:
+    IPAddressBytes();
+    IPAddressBytes(const uint8_t* data, size_t data_len);
+    IPAddressBytes(const IPAddressBytes& other);
+    ~IPAddressBytes();
+
+    // Returns the number of elements in the underlying array.
+    size_t size() const { return size_; }
+
+    // Sets the size to be |size|. Does not actually change the size
+    // of the underlying array or zero-initialze the bytes.

[eroman, 2017/05/19 17:37:54]

typo: initialize

[Ryan Hamilton, 2017/05/19 21:30:46]

Done.

+    void Resize(size_t size) {
+      DCHECK_LE(size, 16u);
+      size_ = static_cast<uint8_t>(size);
+    }
+
+    // Returns true if the underlying array is empty.
+    bool empty() const { return size_ == 0; }
+
+    // Returns a pointer to the underlying array of bytes.
+    const uint8_t* data() const { return bytes_.data(); }
+    uint8_t* data() { return bytes_.data(); }
+
+    // Returns a pointer to the first element.
+    const uint8_t* begin() const { return data(); }
+
+    // Returns a pointer past the last element.
+    const uint8_t* end() const { return data() + size_; }
+
+    // Returns a reference to the last element.
+    uint8_t& back() {
+      DCHECK(!empty());
+      return bytes_[size_ - 1];
+    }
+    const uint8_t& back() const {
+      DCHECK(!empty());
+      return bytes_[size_ - 1];
+    }
+
+    // Appends |val| to the end and increments the size.
+    void push_back(uint8_t val) {
+      DCHECK_GT(16, size_);
+      bytes_[size_++] = val;
+    }
+
+    // Returns a reference to the byte at index |pos|.
+    uint8_t& operator[](size_t pos) {
+      DCHECK_LT(pos, size_);
+      return bytes_[pos];
+    }
+    const uint8_t& operator[](size_t pos) const {
+      DCHECK_LT(pos, size_);
+      return bytes_[pos];
+    }
+
+    bool operator<(const IPAddressBytes& other) const;
+    bool operator!=(const IPAddressBytes& other) const;
+    bool operator==(const IPAddressBytes& other) const;
+
+   private:
+    // Underlying sequence of bytes
+    std::array<uint8_t, kIPv6AddressSize> bytes_;
+
+    // Number of elements in |bytes_|. Should be either kIPv4AddressSize
+    // or kIPv6AddressSize or 0.
+    uint8_t size_;
+  };
+
   // Creates a zero-sized, invalid address.
   IPAddress();
 
@@ -28,6 +102,10 @@ class NET_EXPORT IPAddress {
   // network byte order.
   explicit IPAddress(const std::vector<uint8_t>& address);
 
+  // Copies the input address to |ip_address_|. The input is expected to be in
+  // network byte order.
+  explicit IPAddress(const base::StackVector<uint8_t, 16>& address);
+
   IPAddress(const IPAddress& other);
 
   // Copies the input address to |ip_address_|. The input is expected to be in
@@ -102,8 +180,12 @@ class NET_EXPORT IPAddress {
   bool AssignFromIPLiteral(const base::StringPiece& ip_literal)
       WARN_UNUSED_RESULT;
 
-  // Returns the underlying byte vector.
-  const std::vector<uint8_t>& bytes() const { return ip_address_; };
+  // Returns the underlying bytes.
+  const IPAddressBytes& bytes() const { return ip_address_; };
+
+  // Returns the underlying bytes as a vector. Since this allocates a new
+  // vector, this method should be avoided unless absolutely necessary.

[eroman, 2017/05/19 17:37:54]

optional: "Copies the bytes to a new vector. Generally callers should be using
|bytes()| and the IPAddressBytes abstraction. This method is provided as a
convenience for callsites that existed prior to the introduction of
IPAddressBytes.".

If you wanted to make this point more strongly, we can rename the function to: 
CopyBytesToVector(), then there is no doubt in callers minds that they are
copying.

[Ryan Hamilton, 2017/05/19 21:30:46]

Done.
Done.

+  std::vector<uint8_t> BytesAsVector() const;
 
   // Returns an IPAddress instance representing the 127.0.0.1 address.
   static IPAddress IPv4Localhost();
@@ -125,9 +207,7 @@ class NET_EXPORT IPAddress {
   bool operator<(const IPAddress& that) const;
 
  private:
-  // IPv4 addresses will have length kIPv4AddressSize, whereas IPv6 address
-  // will have length kIPv6AddressSize.
-  std::vector<uint8_t> ip_address_;
+  IPAddressBytes ip_address_;
 
   // This class is copyable and assignable.
 };