Design pattern for multiple output formats

Question

I have a class structure which represents (internally) the data I wish to output to a file.

Some of the member variables are private to the data class so that it ca

Answer 1

If you need to implement file formatting and save/load from outside of the class, then you can only do it with data that is publicly available. If saving/loading needs to deal with non-public data, if reloading the class cannot reconstruct the original non-public data from public data, then either the class itself or friends of that class must be involved. There's not really a way around that.

The most you might be able to do is to make it easier to write new types, with a friend template. For example:

class DataType
{
...
private:
    template friend void SaveFile(const DataType *, ofstream&);
};

The format template type would be empty types. So if you have formatA and formatB, you would have empty structs:

struct FormatA {};
struct FormatB {};

Then, all you need to do is write specialized versions of SaveFile for those formats:

template<> void SaveFile(const DataType *, ofstream&);
template<> void SaveFile(const DataType *, ofstream&);

They will automatically be friends of DataType.