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I tried doing a custom class that has an extension method, but I am getting compile time errors. I put this in my .h file:

class Byte
{
    public:
        Byte();
        ~Byte();
        std::string  DataType;
        int  ColumnWidth;
        std::vector<unsigned char> data;
        int ToLittleEndianInt() {
            long int Int = 0;
            int arraySize = this->ColumnWidth;
            if (arraySize == 2)
            {
                Int = this->data[0] | ((int)this->data[1] << 8);
            }
            else if (arraySize == 1)
            {
                Int = this->data[0];
            }
            return Int;
        };
};

That throws the error:

Error   LNK2019 unresolved external symbol "public: __thiscall Byte::Byte(void)" (??0Byte@@QAE@XZ) referenced in function "public: static void __cdecl DataParser::Parse(class nlohmann::basic_json<class std::map,class std::vector,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,bool,__int64,unsigned __int64,double,class std::allocator,struct nlohmann::adl_serializer>)" (?ParseToDataTable@Parse@@SAXV?$basic_json@Vmap@std@@Vvector@2@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@_N_J_KNVallocator@2@Uadl_serializer@nlohmann@@@nlohmann@@@Z)

I also tried declaring it as:

const Byte& ToLittleEndianInt(const Byte& s)const {}

That was the way I saw done in Stephen Prata's C++ Primer, but then I still get errors, and I would have to use it like this in my cpp:

std::vector<unsigned char> test(3);
for (int i = 0; i < 2; i++)
        test[i] = i;
    Byte CurrentBytes;
    CurrentBytes.data = test;
    CurrentBytes.ColumnWidth=2;
    int results = CurrentBytes.ToLEInt(CurrentBytes);
    //instead of this way which is better
    int results = CurrentBytes.ToLittleEndianInt();

I then tried putting the declaration outside of the class block, but then I got an error that LEInt wasn't defined,

int Byte::ToLittleEndianInt(){}

When I tried adding int ToLittleEndianInt(); to the public class block, then it complained that it was already defined. What is the right way to add this?

Update:

When I do it as a struct, it works:

struct Byte
{
    std::string  DataType;
    int  ColumnWidth;
    int StartingPosition;
    std::string Column;
    std::vector<unsigned char> data;
    int ToLEInt() {
        long int Int = 0;
        int arraySize = this->ColumnWidth;
        if (arraySize == 2)
        {
            Int = this->data[0] | ((int)this->data[1] << 8);
        }
        else if (arraySize == 1)
        {
            Int = this->data[0];
        }
        return Int;
    };
    std::string ToString()
    {
        int i;
        std::string s = "";
        int arraySize = this->ColumnWidth;
        for (i = 0; i < arraySize; i++) {
            std::string n(1, this->data[i]);
            s = s + n;
        }
        return s;
    }
};
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Alan
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    The issue is that you didn't define your constructor or destructor. You declared them, but never gave them a function body... – ChrisMM Oct 17 '19 at 17:03
  • Wow, that was the fastest response ever! I am looking in the book for what that should look like. – Alan Oct 17 '19 at 17:05
  • Changing it from a `class` to a `struct` seems to have solved all the issues – Alan Oct 17 '19 at 17:32
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    @Alan changing `class` to `struct` in the code you've shown will make precisely zero difference. The only difference is the default access visibility (`public` for `struct` and `private` for `class`), which you've overridden with `public:`. Perhaps you changed something else too? – alter_igel Oct 17 '19 at 17:34
  • I removed the `public:`, otherwise it is the same. Posting the working version as an update. – Alan Oct 17 '19 at 17:48

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