diff --git a/include/exiv2/basicio.hpp b/include/exiv2/basicio.hpp
index df3707af..e3f21508 100644
--- a/include/exiv2/basicio.hpp
+++ b/include/exiv2/basicio.hpp
@@ -270,7 +270,7 @@ namespace Exiv2 {
         //! @name Creators
         //@{
         //! Constructor, takes a BasicIo reference
-        IoCloser(BasicIo& bio) : bio_(bio) {}
+        explicit IoCloser(BasicIo& bio) : bio_(bio) {}
         //! Destructor, closes the BasicIo reference
         virtual ~IoCloser() { close(); }
         //@}
@@ -307,7 +307,7 @@ namespace Exiv2 {
               therefore never failes.
           @param path The full path of a file
          */
-        FileIo(const std::string& path);
+        explicit FileIo(const std::string& path);
 #ifdef EXV_UNICODE_PATH
         /*!
           @brief Like FileIo(const std::string& path) but accepts a
@@ -778,7 +778,7 @@ namespace Exiv2 {
         //! @name Creators
         //@{
         //! Default constructor that reads data from stdin/data uri path and writes them to the temp file.
-        XPathIo(const std::string& orgPath);
+        explicit XPathIo(const std::string& orgPath);
 #ifdef EXV_UNICODE_PATH
         /*!
           @brief Like XPathIo(const std::string& orgPath) but accepts a
diff --git a/include/exiv2/bmpimage.hpp b/include/exiv2/bmpimage.hpp
index b598f2a5..07b0f3a3 100644
--- a/include/exiv2/bmpimage.hpp
+++ b/include/exiv2/bmpimage.hpp
@@ -77,7 +77,7 @@ namespace Exiv2 {
               instance after it is passed to this method.  Use the Image::io()
               method to get a temporary reference.
          */
-        BmpImage(BasicIo::AutoPtr io);
+        explicit BmpImage(BasicIo::AutoPtr io);
         //@}
 
         //! @name Manipulators
diff --git a/include/exiv2/exif.hpp b/include/exiv2/exif.hpp
index 5523650a..a0f15d97 100644
--- a/include/exiv2/exif.hpp
+++ b/include/exiv2/exif.hpp
@@ -236,7 +236,7 @@ namespace Exiv2 {
         //! @name Creators
         //@{
         //! Constructor.
-        ExifThumbC(const ExifData& exifData);
+        explicit ExifThumbC(const ExifData& exifData);
         //@}
 
         //! @name Accessors
@@ -305,7 +305,7 @@ namespace Exiv2 {
         //! @name Creators
         //@{
         //! Constructor.
-        ExifThumb(ExifData& exifData);
+        explicit ExifThumb(ExifData& exifData);
         //@}
 
         //! @name Manipulators
diff --git a/include/exiv2/gifimage.hpp b/include/exiv2/gifimage.hpp
index 0649a756..479edc74 100644
--- a/include/exiv2/gifimage.hpp
+++ b/include/exiv2/gifimage.hpp
@@ -78,7 +78,7 @@ namespace Exiv2 {
               instance after it is passed to this method.  Use the Image::io()
               method to get a temporary reference.
          */
-        GifImage(BasicIo::AutoPtr io);
+        explicit GifImage(BasicIo::AutoPtr io);
         //@}
 
         //! @name Manipulators
diff --git a/include/exiv2/ini.hpp b/include/exiv2/ini.hpp
index d08bf8aa..17d544b5 100755
--- a/include/exiv2/ini.hpp
+++ b/include/exiv2/ini.hpp
@@ -140,7 +140,7 @@ public:
     /*! @brief Construct INIReader and parse given filename. See ini.h for more info
        about the parsing.
     */
-    INIReader(std::string filename);
+    explicit INIReader(const std::string& filename);
 
     /*! @brief Return the result of ini_parse(), i.e., 0 on success, line number of
         first error on parse error, or -1 on file open error.
diff --git a/include/exiv2/preview.hpp b/include/exiv2/preview.hpp
index 14d4814a..b18d0a49 100644
--- a/include/exiv2/preview.hpp
+++ b/include/exiv2/preview.hpp
@@ -174,7 +174,7 @@ namespace Exiv2 {
         //! @name Constructors
         //@{
         //! Constructor.
-        PreviewManager(const Image& image);
+        explicit PreviewManager(const Image& image);
         //@}
 
         //! @name Accessors
diff --git a/include/exiv2/properties.hpp b/include/exiv2/properties.hpp
index ee227c5f..ab6d01eb 100644
--- a/include/exiv2/properties.hpp
+++ b/include/exiv2/properties.hpp
@@ -76,14 +76,14 @@ namespace Exiv2 {
         //! For comparison with prefix
         struct Prefix {
             //! Constructor.
-            Prefix(const std::string& prefix);
+            explicit Prefix(const std::string& prefix);
             //! The prefix string.
             std::string prefix_;
         };
         //! For comparison with namespace
         struct Ns {
             //! Constructor.
-            Ns(const std::string& ns);
+            explicit Ns(const std::string& ns);
             //! The namespace string
             std::string ns_;
         };
diff --git a/include/exiv2/psdimage.hpp b/include/exiv2/psdimage.hpp
index 205be811..cbe1ff28 100644
--- a/include/exiv2/psdimage.hpp
+++ b/include/exiv2/psdimage.hpp
@@ -79,7 +79,7 @@ namespace Exiv2 {
               instance after it is passed to this method.  Use the Image::io()
               method to get a temporary reference.
          */
-        PsdImage(BasicIo::AutoPtr io);
+        explicit PsdImage(BasicIo::AutoPtr io);
         //@}
 
         //! @name Manipulators
diff --git a/include/exiv2/rw2image.hpp b/include/exiv2/rw2image.hpp
index adea5a76..21f49d5b 100644
--- a/include/exiv2/rw2image.hpp
+++ b/include/exiv2/rw2image.hpp
@@ -69,7 +69,7 @@ namespace Exiv2 {
               instance after it is passed to this method.  Use the Image::io()
               method to get a temporary reference.
          */
-        Rw2Image(BasicIo::AutoPtr io);
+        explicit Rw2Image(BasicIo::AutoPtr io);
         //@}
 
         //! @name Manipulators
diff --git a/include/exiv2/rwlock.hpp b/include/exiv2/rwlock.hpp
index 36608278..1ebbdb90 100644
--- a/include/exiv2/rwlock.hpp
+++ b/include/exiv2/rwlock.hpp
@@ -90,7 +90,7 @@ namespace Exiv2 {
         {
         public:
             //! constructor (acquires the lock)
-            RWLock(const pthread_rwlockattr_t *attr = 0)
+            explicit RWLock(const pthread_rwlockattr_t *attr = 0)
             {
                 pthread_rwlock_init(&rwlock_, attr);
             }
@@ -151,7 +151,7 @@ namespace Exiv2 {
         {
         public:
             //! constructor - locks the object
-            ScopedReadLock(RWLock &rwlock):
+            explicit ScopedReadLock(RWLock &rwlock):
                 rwlock_(rwlock)
             {
                 rwlock_.rdlock();
@@ -173,7 +173,7 @@ namespace Exiv2 {
         {
         public:
             //! constructor - locks the object
-            ScopedWriteLock(RWLock &rwlock):
+            explicit ScopedWriteLock(RWLock &rwlock):
                 rwlock_(rwlock)
             {
                 rwlock_.wrlock();
diff --git a/include/exiv2/tags.hpp b/include/exiv2/tags.hpp
index 90e92568..8ca8d61a 100644
--- a/include/exiv2/tags.hpp
+++ b/include/exiv2/tags.hpp
@@ -73,7 +73,7 @@ namespace Exiv2 {
 
     //! Search key to find a GroupInfo by its group name.
     struct GroupInfo::GroupName {
-        GroupName(const std::string& groupName); //!< Constructor
+        explicit GroupName(const std::string& groupName);
         std::string g_;                          //!< Group name
     };
 
@@ -176,7 +176,8 @@ namespace Exiv2 {
           @throw Error if the key cannot be constructed from the tag number
                  and group name.
          */
-        ExifKey(const TagInfo& ti);
+        explicit ExifKey(const TagInfo& ti);
+
         //! Copy constructor
         ExifKey(const ExifKey& rhs);
         //! Destructor
diff --git a/include/exiv2/tgaimage.hpp b/include/exiv2/tgaimage.hpp
index 11941355..2c40f0bf 100644
--- a/include/exiv2/tgaimage.hpp
+++ b/include/exiv2/tgaimage.hpp
@@ -79,7 +79,7 @@ namespace Exiv2 {
               instance after it is passed to this method.  Use the Image::io()
               method to get a temporary reference.
          */
-        TgaImage(BasicIo::AutoPtr io);
+        explicit TgaImage(BasicIo::AutoPtr io);
         //@}
 
         //! @name Manipulators
diff --git a/include/exiv2/types.hpp b/include/exiv2/types.hpp
index ece9ef62..73280c09 100644
--- a/include/exiv2/types.hpp
+++ b/include/exiv2/types.hpp
@@ -195,7 +195,7 @@ namespace Exiv2 {
      */
     struct EXIV2API DataBufRef {
         //! Constructor
-        DataBufRef(std::pair<byte*, long> rhs) : p(rhs) {}
+        explicit DataBufRef(std::pair<byte*, long> rhs) : p(rhs) {}
         //! Pointer to a byte array and its size
         std::pair<byte*, long> p;
     };
diff --git a/samples/geotag.cpp b/samples/geotag.cpp
index 217c5131..4d8b47f0 100644
--- a/samples/geotag.cpp
+++ b/samples/geotag.cpp
@@ -282,7 +282,7 @@ time_t Position::deltaMax_ = 60 ;
 class UserData
 {
 public:
-    UserData(Options& options):
+    explicit UserData(Options& options):
         indent(0)
       , count(0)
       , nTrkpt(0)
diff --git a/src/cr2header_int.hpp b/src/cr2header_int.hpp
index dd6bd949..8e31e34e 100644
--- a/src/cr2header_int.hpp
+++ b/src/cr2header_int.hpp
@@ -45,7 +45,7 @@ namespace Exiv2 {
         //! @name Creators
         //@{
         //! Default constructor
-        Cr2Header(ByteOrder byteOrder =littleEndian);
+        explicit Cr2Header(ByteOrder byteOrder =littleEndian);
         //! Destructor.
         ~Cr2Header();
         //@}
diff --git a/src/ini.cpp b/src/ini.cpp
index 43336e81..f35f18d0 100755
--- a/src/ini.cpp
+++ b/src/ini.cpp
@@ -208,7 +208,7 @@ int Exiv2::ini_parse(const char* filename, ini_handler handler, void* user)
     return error;
 }
 
-INIReader::INIReader(string filename)
+INIReader::INIReader(const std::string &filename)
 {
     _error = ini_parse(filename.c_str(), ValueHandler, this);
 }
diff --git a/src/orfimage_int.hpp b/src/orfimage_int.hpp
index b9001b8f..4803885a 100644
--- a/src/orfimage_int.hpp
+++ b/src/orfimage_int.hpp
@@ -49,7 +49,7 @@ namespace Exiv2 {
         //! @name Creators
         //@{
         //! Default constructor
-        OrfHeader(ByteOrder byteOrder =littleEndian);
+        explicit OrfHeader(ByteOrder byteOrder =littleEndian);
         //! Destructor.
         ~OrfHeader();
         //@}
diff --git a/src/tiffimage_int.hpp b/src/tiffimage_int.hpp
index 5fc5c0e7..c3fa5eee 100644
--- a/src/tiffimage_int.hpp
+++ b/src/tiffimage_int.hpp
@@ -476,7 +476,7 @@ namespace Exiv2 {
     class FindExifdatum {
     public:
         //! Constructor, initializes the object with the IfdId to look for.
-        FindExifdatum(Exiv2::Internal::IfdId ifdId) : ifdId_(ifdId) {}
+        explicit FindExifdatum(Exiv2::Internal::IfdId ifdId) : ifdId_(ifdId) {}
         //! Returns true if IFD id matches.
         bool operator()(const Exiv2::Exifdatum& md) const { return ifdId_ == md.ifdId(); }