cv::FileStorage does not work for std::vector of user-defined struct (Bug #3599)

cv::FileStorage supports writing std::vector<T> and allows extending operator<< to accept user-defined struct like:

// excerpted from opencv/samples/cpp/filestorage.cpp
struct MyData {
  int A;
  double X;
  string id;
  void write(FileStorage& fs) const //Write serialization for this class
  {
    fs << "{" << "A" << A << "X" << X << "id" << id << "}";
  }
};
static void write(FileStorage& fs, const std::string&, const MyData& x){
  x.write(fs);
}

So I expected vector<MyData> can be written by adding overloading for MyData, but it does not work. More precisely, when we use operator<< to implement write(FileStorage&, const std::string&, const MyData&), it cannot be an element of vector.

I'm using following workaround. With this modification, attached code will work expectedly. But this workaround changes the behaviour of modified function. I think it is safe if this function is always called via operator<< and not directly from user code.

// in file opencv/modules/core/include/opencv2/core/persistence.hpp
// at line 697
 template<typename _Tp> static inline
 void write( FileStorage& fs, const String& name, const std::vector<_Tp>& vec )
 {
-    internal::WriteStructContext ws(fs, name, FileNode::SEQ+(DataType<_Tp>::fmt != 0 ? FileNode::FLOW : 0));
+    fs << (DataType<_Tp>::fmt != 0 ? "[:" : "[");
     write(fs, vec);
+    fs << "]";
 }