Separate the key setting code in the constructor of HMAC class into the Init...

base/hmac.h

 // Copyright (c) 2006-2008 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.
 
 // Utility class for calculating the HMAC for a given message. We currently
 // only support SHA1 for the hash algorithm, but this can be extended easily.
 
 #ifndef BASE_HMAC_H_
 #define BASE_HMAC_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/scoped_ptr.h"
 
 namespace base {
 
 // Simplify the interface and reduce includes by abstracting out the internals.
 struct HMACPlatformData;
 
 class HMAC {
  public:
   // The set of supported hash functions. Extend as required.
   enum HashAlgorithm {
     SHA1
   };
 
-  HMAC(HashAlgorithm hash_alg, const unsigned char* key, int key_length);
+  explicit HMAC(HashAlgorithm hash_alg);
   ~HMAC();
 
-  // Calculates the HMAC for the message in |data| using the algorithm and key
-  // supplied to the constructor.  The HMAC is returned in |digest|, which
-  // has |digest_length| bytes of storage available.
+  // Initializes this instance using |key| of the length |key_length|. Call Init
+  // only once. It returns false on the second or later calls.
+  bool Init(const unsigned char* key, int key_length);
+
+  // Initializes this instance using |key|. Call Init only once. It returns
+  // false on the second or later calls.
+  bool Init(const std::string& key) {
+    return Init(reinterpret_cast<const unsigned char*>(key.data()),
+                static_cast<int>(key.size()));
+  }
+
+  // Calculates the HMAC for the message in |data| using the algorithm supplied
+  // to the constructor and the key supplied to the Init method. The HMAC is
+  // returned in |digest|, which has |digest_length| bytes of storage available.
   bool Sign(const std::string& data, unsigned char* digest, int digest_length);
 
  private:
   HashAlgorithm hash_alg_;
   scoped_ptr<HMACPlatformData> plat_;
 
   DISALLOW_COPY_AND_ASSIGN(HMAC);
 };
 
 }  // namespace base
 
 #endif  // BASE_HMAC_H_