Fix unnecessary copies in auto iter (fixes #426)

Also switch to colon-style iteration in some cases. Thanks to Dean
Scarff for drawing this to my attention after detecting some
unnecessary copies with
https://clang.llvm.org/extra/clang-tidy/checks/performance-for-range-copy.html

libqpdf/JSON.cc

@@ -346,7 +346,7 @@ JSON::checkSchemaInternal(JSON_value* this_v, JSON_value* sch_v,
     if (sch_dict && (! pattern_key.empty()))
     {
         auto pattern_schema = sch_dict->members[pattern_key].getPointer();
-        for (auto iter: this_dict->members)
+        for (auto const& iter: this_dict->members)
         {
             std::string const& key = iter.first;
             checkSchemaInternal(

libqpdf/QPDFCryptoProvider.cc

@@ -110,10 +110,9 @@ QPDFCryptoProvider::getRegisteredImpls()
 {
     std::set<std::string> result;
     QPDFCryptoProvider& p = getInstance();
-    for (auto iter = p.m->providers.begin(); iter != p.m->providers.end();
-         ++iter)
+    for (auto const& iter: p.m->providers)
     {
-        result.insert((*iter).first);
+        result.insert(iter.first);
     }
     return result;
 }

libqpdf/QPDF_Array.cc

@@ -146,9 +146,8 @@ QPDF_Array::getElementsForShallowCopy() const
 void
 QPDF_Array::addExplicitElementsToList(std::list<QPDFObjectHandle>& l) const
 {
-    for (auto iter = this->elements.begin();
-         iter != this->elements.end(); ++iter)
+    for (auto const& iter: this->elements)
     {
-        l.push_back((*iter).second);
+        l.push_back(iter.second);
     }
 }

libqpdf/SparseOHArray.cc

@@ -30,7 +30,7 @@ SparseOHArray::at(size_t idx) const
 	throw std::logic_error(
 	    "INTERNAL ERROR: bounds error accessing SparseOHArray element");
     }
-    auto iter = this->elements.find(idx);
+    auto const& iter = this->elements.find(idx);
     if (iter == this->elements.end())
     {
         return QPDFObjectHandle::newNull();
@@ -57,10 +57,9 @@ SparseOHArray::remove_last()
 void
 SparseOHArray::releaseResolved()
 {
-    for (auto iter = this->elements.begin();
-	 iter != this->elements.end(); ++iter)
+    for (auto iter: this->elements)
     {
-	QPDFObjectHandle::ReleaseResolver::releaseResolved((*iter).second);
+	QPDFObjectHandle::ReleaseResolver::releaseResolved(iter.second);
     }
 }
 
@@ -89,16 +88,15 @@ SparseOHArray::erase(size_t idx)
 	throw std::logic_error("bounds error erasing item from SparseOHArray");
     }
     decltype(this->elements) dest;
-    for (auto iter = this->elements.begin();
-	 iter != this->elements.end(); ++iter)
+    for (auto const& iter: this->elements)
     {
-        if ((*iter).first < idx)
+        if (iter.first < idx)
         {
-            dest.insert(*iter);
+            dest.insert(iter);
         }
-        else if ((*iter).first > idx)
+        else if (iter.first > idx)
         {
-            dest[(*iter).first - 1] = (*iter).second;
+            dest[iter.first - 1] = iter.second;
         }
     }
     this->elements = dest;
@@ -120,16 +118,15 @@ SparseOHArray::insert(size_t idx, QPDFObjectHandle oh)
     else
     {
         decltype(this->elements) dest;
-        for (auto iter = this->elements.begin();
-             iter != this->elements.end(); ++iter)
+        for (auto const& iter: this->elements)
         {
-            if ((*iter).first < idx)
+            if (iter.first < idx)
             {
-                dest.insert(*iter);
+                dest.insert(iter);
             }
             else
             {
-                dest[(*iter).first + 1] = (*iter).second;
+                dest[iter.first + 1] = iter.second;
             }
         }
         this->elements = dest;

qpdf/fix-qdf.cc

@@ -387,13 +387,12 @@ QdfFixer::writeOstream()
     auto onum = ostream_id;
     std::string offsets;
     auto n = ostream_offsets.size();
-    for (auto iter = ostream_offsets.begin();
-         iter != ostream_offsets.end(); ++iter)
+    for (auto iter: ostream_offsets)
     {
-	(*iter) -= QIntC::to_offset(first);
+	iter -= QIntC::to_offset(first);
 	++onum;
 	offsets += QUtil::int_to_string(onum) + " " +
-            QUtil::int_to_string(*iter) + "\n";
+            QUtil::int_to_string(iter) + "\n";
     }
     auto offset_adjust = QIntC::to_offset(offsets.size());
     first += offset_adjust;

qpdf/qpdf.cc

@@ -1694,11 +1694,11 @@ ArgParser::argShowCrypto()
     auto crypto = QPDFCryptoProvider::getRegisteredImpls();
     std::string default_crypto = QPDFCryptoProvider::getDefaultProvider();
     std::cout << default_crypto << std::endl;
-    for (auto iter = crypto.begin(); iter != crypto.end(); ++iter)
+    for (auto const& iter: crypto)
     {
-        if (*iter != default_crypto)
+        if (iter != default_crypto)
         {
-            std::cout << *iter << std::endl;
+            std::cout << iter << std::endl;
         }
     }
 }
@@ -3646,7 +3646,7 @@ static std::set<QPDFObjGen>
 get_wanted_json_objects(Options& o)
 {
     std::set<QPDFObjGen> wanted_og;
-    for (auto iter: o.json_objects)
+    for (auto const& iter: o.json_objects)
     {
         bool trailer;
         int obj = 0;