add std::string overload for cv::read()

modules/core/include/opencv2/core/cvstd.inl.hpp

@@ -156,9 +156,7 @@ FileNode::operator std::string() const
 template<> inline
 void operator >> (const FileNode& n, std::string& value)
 {
-    String val;
-    read(n, val, val);
-    value = val;
+    read(n, value, std::string());
 }
 
 template<> inline

modules/core/include/opencv2/core/persistence.hpp

@@ -714,6 +714,7 @@ CV_EXPORTS void read(const FileNode& node, int& value, int default_value);
 CV_EXPORTS void read(const FileNode& node, float& value, float default_value);
 CV_EXPORTS void read(const FileNode& node, double& value, double default_value);
 CV_EXPORTS void read(const FileNode& node, String& value, const String& default_value);
+CV_EXPORTS void read(const FileNode& node, std::string& value, const std::string& default_value);
 CV_EXPORTS void read(const FileNode& node, Mat& mat, const Mat& default_mat = Mat() );
 CV_EXPORTS void read(const FileNode& node, SparseMat& mat, const SparseMat& default_mat = SparseMat() );
 CV_EXPORTS void read(const FileNode& node, std::vector<KeyPoint>& keypoints);

modules/core/src/persistence.cpp

@@ -7396,6 +7396,11 @@ void read(const FileNode& node, String& value, const String& default_value)
     value = !node.node ? default_value : CV_NODE_IS_STRING(node.node->tag) ? String(node.node->data.str.ptr) : String();
 }
 
+void read(const FileNode& node, std::string& value, const std::string& default_value)
+{
+    value = !node.node ? default_value : CV_NODE_IS_STRING(node.node->tag) ? std::string(node.node->data.str.ptr) : default_value;
+}
+
 }