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You can rewrite your constructors to not have any default parameters by adding some constructors

Your text and statementText1 variables are declared all wrong.

You are dereferencing a pointer to a narrow string literal, thus accessing its 1st character, which you then assign to text, which is declared as a single wchar_t.

You are then assigning that wchar_t to statementText1, which is declared as a single unsigned char.

You are then passing that single character to SQLPrepare(), but it expects a pointer to a (non-const) null-terminated string instead, thus the compile fails.

Try this instead:

(i.e. std::string

No, string literals are not related to the class std::string. Technically this could happen if vector had a special deduction guide for this, but it doesn't.

... char*

No, string literals are const.

The contained type will be deduced as const char*.

Dynamic arrays are dynamic i.e. they have dynamic lifetime / dynamic storage (i.e. they are stored in free store aka "heap").

Dynamic arrays are also dynamic in the sense that unlike array variables, their size can be determined at runtime i.e. it doesn't need to be compile time constant. Example:

I tested your code, and the only thing I changed was the return type of readResults() from int to double. Once I did that, I got the results I expected!

Is function template specialization really allowed?

Yes, but not partial specialisation.

It seems that function partcial template specialization is not allowed.Is it really correct?

If you mean partial specialisation, that is indeed not allowed for function templates.

Your long_number operator+ can easily be improved by eliminating multiple copies of the vector in get_numero().

You don't really need to make a copy of your vector to then ask for its size. Just provide a size() method for your class.

Similarly, instead of (a.get_numero())[i] (that, again, makes a copy of the entire vector before indexing into it), implement operator [] to directly access individual digits.

1,000 times longer that addition - it's a lot! however, something like 100 times would be expected - just count how many steps you have in place of a simple add instruction

They are binary bits (1s and 0s) but whatever piece of software you are using to view the file's contents is trying to read them as human readable characters, not as machine code.

If you think about it, everything that you open in a text editor is comprised of binary bits stored on bare metal. Those 1s and 0s can be interpreted in many many different ways, and most text editors will attempt to read them in as characters. Take the character 'A' for example. It's ASCII code is 65 which is 01000001 in binary. When a text editor reads through the file on your computer it is processing those bits as characters rather than machine instructions, and therefore it reads in 8 bits (byte) in the pattern 01000001 it knows that it has just read an 'A'.

This process results in that jumble of symbols you see in the executable file. While some of the content happens to be in the right pattern to make human readable characters, the majority of them will likely be outside of what either the character encoding considers valid or knows how to print, resulting in the '�' that you see.

I won't go into the intricacies of how character encodings work here, but read Character Encodings for Beginners for a bit more info.