templated parameter packs

J.Hilk

So I turned to templated programming to expand my c++ horizon.

I know, I already regret it :D

I have this very simple class, using templated parameter pack:

//Assumption/Usage: 1 Byte == 1 entry in m_versionParts
    template<typename... TArgs>
    explicit ApplicationVersion(TArgs... input){
        //Check all arguments are an integer
        static_assert((std::conjunction_v<std::is_integral<TArgs>...>), "Only integer types are allowed.");

        QVector <uchar>  versionParts;

        // Iterate through all inputs
        //Fold Expression, requieres C++17, calls processInteger for all arguments in the pack
        //Right Fold -> last argument given to the function is the least importan version part
        (..., versionParts.append(integerToByteVector(input)));
        //Left Fold
        //(versionParts.append(integerToByteVector(input)), ...);

        m_versionParts = versionParts;
    }

the helper function is here, if needed:

private:
    //Helper functions:
    template<typename T>
    [[nodiscard]] static QVector<uchar> integerToByteVector(T value) noexcept {
        static_assert(sizeof(T) > 0 && sizeof(T) <= sizeof(std::intmax_t), "Type size is not supported");

        const auto typeSize = sizeof(T);
        QVector<uchar> bytes(typeSize, uchar{0});

        for (uchar i = 0; i < typeSize; ++i) {
            bytes[i] = static_cast<uchar>((value >> (i * 8)) & 0xFF);
        }
        return bytes;
    }

the idea was to unravel the arguments list from back to front left and right fold do not seem to matter much/at all: see this modified example:

//Assumption/Usage: 1 Byte == 1 entry in m_versionParts
    template<typename... TArgs>
    explicit ApplicationVersion(TArgs... input){
        qDebug() << Q_FUNC_INFO << "template<typename... TArgs> -> TArgs... input";
        //Check all arguments are an integer
        static_assert((std::conjunction_v<std::is_integral<TArgs>...>), "Only integer types are allowed.");

        QVector <uchar>  versionParts;
        QVector <uchar>  versionParts2;

        // Iterthrough all inputs
        //Fold Expression, requieres C++17, calls processInteger for all arguments in the pack
        //Right Fold -> last argument given to the function is the least importan version part
        (versionParts.append(integerToByteVector(input)) , ...);
        (..., versionParts2.append(integerToByteVector(input)));
        qDebug() << Q_FUNC_INFO << versionParts << versionParts2;

        m_versionParts = versionParts;
    }

results in this console output:
ApplicationVersion::ApplicationVersion(TArgs...) [TArgs = <int, int, int, int>] QVector(1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0) QVector(1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0)

with the same vector generated.

So I'm doing something wrong, or I have a wrong understanding. Any insight appreciated!