My .h file generated by OSS ASN.1 compiler for C++ contains definitions like these (double underscore): class __seq1 {...} class A {... typedef __seq1 b; ...}. Should I use __seq1 in my C++ program to create an instance of class A?

No, you shouldn't use names starting with a double underscore (i.e. __seq1, __seq2, __choice1, __shared1, etc.) explicitly. They are for internal use.

These generated names can be different in different versions of the ASN.1 compiler for C++. Use the scope resolution operator (::) to reference the anonymous SEQUENCE and CHOICE types.

Thus, for the definition above use A::b instead of __seq1.

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