编译错误：

无法确定条件表达式的类型,因为”和’MyEnum’之间没有隐式转换

源代码

public enum MyEnum
{
    Value1,Value2,Value3
}

public class MyClass
{
    public MyClass() {}
    public MyEnum? MyClassEnum { get; set; }
}

public class Main()
{
   object x = new object();

   MyClass mc = new MyClass()
   {
        MyClassEnum = Convert.Isdbnull(x) : null ? 
            (MyEnum) Enum.Parse(typeof(MyEnum),x.ToString(),true)
   };
}

如何解决这个错误？

int? i;
i = shouldBeNull ? null : (int?) 123;

所以：

MyClassEnum = Convert.Isdbnull(x)
    ? null
    : (MyEnum?) Enum.Parse(typeof(MyEnum),true)

>在调用f前执行一些代码.
>调用f().
>调用f后执行一些代码.
>如果f不返回void返回f的结果值.

由于上述第四点,小代码实施步骤困扰了我,

template <typename TF>auto step(TF&& f){       // Execute some actions before `f`.     do_something();    using f_return_type = decltype(f());    return static_if(std::is_same<f_return_type,void>{})        .then([](auto&& xf) mutable            {                // Do not return anything if `xf` returns void.                xf();                // Execute some actions after `f`.                do_something_after_f();            })        .else_([](auto&& xf) mutable            {                auto result = xf();                // Execute some actions after `f`.                do_something_after_f();                return result;            })(std::forward<TF>(f));}

(注意重复调用f和do_something_after_f.)

我必须使用某种条件编译时(模板专门化或者static_if,如示例所示)根据f的返回类型进行分支.

理想情况下,我想这样编译：

template <typename TF>auto step(TF&& f){       // Execute some actions before `f`.     do_something();    decltype(auto) eventual_result = f();    // Execute some actions after `f`.    do_something_after_f();    return result;}

但是它不是,因为finalual_result可以是void,这是一个不完整的类型.

有没有任何重构这个代码的方式,以避免重复调用f()和do_something_after_f()？

struct Cleanup {    ~Cleanup() { do_something_after_f(); }} cleanup;return f(); // it's legal to return a void expression in C++

private void setData() {

        RealmResults<RealmDemoData> result = mRealm.where(RealmDemoData.class).findAll();

        //RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,"stackValues","xIndex"); // normal entries
        RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,"xValue","floatValue"); // stacked entries
        set.setColors(new int[]{ColorTemplate.rgb("#8BC34A"),ColorTemplate.rgb("#FFC107"),ColorTemplate.rgb("#9E9E9E")});
        set.setLabel("Mobile OS distribution");
        set.setStackLabels(new String[]{"iOS","Android","Other"});

        ArrayList<IBarDataSet> dataSets = new ArrayList<IBarDataSet>();
        dataSets.add(set); // add the dataset

        // create a data object with the dataset list
        BarData data = new BarData(dataSets);
        styleData(data);
        data.setValueTextColor(Color.WHITE);

        // set data
        mChart.setData(data);
        mChart.animateY(1400,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        RealmResults<RealmDemoData> result = mRealm.where(RealmDemoData.class).findAll();

        //RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,"yValue"); // stacked entries
        set.setColors(new int[] {ColorTemplate.rgb("#FF5722"),ColorTemplate.rgb("#03A9F4")});
        set.setLabel("Realm BarDataSet");

        ArrayList<IBarDataSet> dataSets = new ArrayList<IBarDataSet>();
        dataSets.add(set); // add the dataset

        // create a data object with the dataset list
        BarData data = new BarData(dataSets);
        styleData(data);

        // set data
        mChart.setData(data);
        mChart.setFitBars(true);
        mChart.animateY(1400,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        RealmResults<RealmDemoData> result = mRealm.allObjects(RealmDemoData.class);

        //RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,"xIndex","Other"});

        ArrayList<IBarDataSet> dataSets = new ArrayList<IBarDataSet>();
        dataSets.add(set); // add the dataset

        // create a data object with the dataset list
        RealmBarData data = new RealmBarData(result,dataSets);
        styleData(data);
        data.setValueTextColor(Color.WHITE);

        // set data
        mChart.setData(data);
        mChart.animateY(1400,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        RealmResults<RealmDemoData> result = mRealm.allObjects(RealmDemoData.class);

        //RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,"value","xIndex"); // stacked entries
        set.setColors(new int[] {ColorTemplate.rgb("#FF5722"),ColorTemplate.rgb("#03A9F4")});
        set.setLabel("Realm BarDataSet");

        ArrayList<IBarDataSet> dataSets = new ArrayList<IBarDataSet>();
        dataSets.add(set); // add the dataset

        // create a data object with the dataset list
        RealmBarData data = new RealmBarData(result,dataSets);
        styleData(data);

        // set data
        mChart.setData(data);
        mChart.animateY(1400,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        RealmResults<RealmDemoData> result = mRealm.where(RealmDemoData.class).findAll();

        //RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        RealmResults<RealmDemoData> result = mRealm.where(RealmDemoData.class).findAll();

        //RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        RealmResults<RealmDemoData> result = mRealm.where(RealmDemoData.class).findAll();

        //RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        RealmResults<RealmDemoData> result = mRealm.where(RealmDemoData.class).findAll();

        //RealmBarDataSet<RealmDemoData> set = new RealmBarDataSet<RealmDemoData>(result,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        // LINE-CHART
        RealmResults<score> results = mRealm.allObjects(score.class);

        RealmLineDataSet<score> lineDataSet = new RealmLineDataSet<score>(results,"totalscore","scoreNr");
        lineDataSet.setDrawCubic(false);
        lineDataSet.setLabel("Realm LineDataSet");
        lineDataSet.setDrawCircleHole(false);
        lineDataSet.setColor(ColorTemplate.rgb("#FF5722"));
        lineDataSet.setCircleColor(ColorTemplate.rgb("#FF5722"));
        lineDataSet.setlinewidth(1.8f);
        lineDataSet.setCircleSize(3.6f);

        ArrayList<ILineDataSet> dataSets = new ArrayList<ILineDataSet>();
        dataSets.add(lineDataSet);

        RealmLineData lineData = new RealmLineData(results,"playerName",dataSets);
        styleData(lineData);

        // set data
        lineChart.setData(lineData);
        lineChart.animateY(1400,Easing.EasingOption.EaseInOutQuart);


        // BAR-CHART
        RealmBarDataSet<score> barDataSet = new RealmBarDataSet<score>(results,"scoreNr");
        barDataSet.setColors(new int[]{ColorTemplate.rgb("#FF5722"),ColorTemplate.rgb("#03A9F4")});
        barDataSet.setLabel("Realm BarDataSet");

        ArrayList<IBarDataSet> barDataSets = new ArrayList<IBarDataSet>();
        barDataSets.add(barDataSet);

        RealmBarData barData = new RealmBarData(results,barDataSets);
        styleData(barData);

        barChart.setData(barData);
        barChart.animateY(1400,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        // LINE-CHART
        final RealmResults<score> results = mRealm.where(score.class).findAll();


        IAxisValueFormatter formatter = new IAxisValueFormatter() {
            @Override
            public String getFormattedValue(float value,AxisBase axis) {
                return results.get((int) value).getPlayerName();
            }
        };

        lineChart.getXAxis().setValueFormatter(formatter);
        barChart.getXAxis().setValueFormatter(formatter);

        RealmLineDataSet<score> lineDataSet = new RealmLineDataSet<score>(results,"scoreNr","totalscore");
        lineDataSet.setMode(LineDataSet.Mode.CUBIC_BEZIER);
        lineDataSet.setLabel("Result scores");
        lineDataSet.setDrawCircleHole(false);
        lineDataSet.setColor(ColorTemplate.rgb("#FF5722"));
        lineDataSet.setCircleColor(ColorTemplate.rgb("#FF5722"));
        lineDataSet.setlinewidth(1.8f);
        lineDataSet.setCircleRadius(3.6f);

        ArrayList<ILineDataSet> dataSets = new ArrayList<ILineDataSet>();
        dataSets.add(lineDataSet);

        LineData lineData = new LineData(dataSets);
        styleData(lineData);

        // set data
        lineChart.setData(lineData);
        lineChart.animateY(1400,"totalscore");
        barDataSet.setColors(new int[]{ColorTemplate.rgb("#FF5722"),ColorTemplate.rgb("#03A9F4")});
        barDataSet.setLabel("Realm BarDataSet");

        ArrayList<IBarDataSet> barDataSets = new ArrayList<IBarDataSet>();
        barDataSets.add(barDataSet);

        BarData barData = new BarData(barDataSets);
        styleData(barData);

        barChart.setData(barData);
        barChart.setFitBars(true);
        barChart.animateY(1400,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        // LINE-CHART
        final RealmResults<score> results = mRealm.where(score.class).findAll();


        IAxisValueFormatter formatter = new IAxisValueFormatter() {
            @Override
            public String getFormattedValue(float value,Easing.EasingOption.EaseInOutQuart);
    }

private void setData() {

        // LINE-CHART
        final RealmResults<score> results = mRealm.where(score.class).findAll();


        IAxisValueFormatter formatter = new IAxisValueFormatter() {
            @Override
            public String getFormattedValue(float value,Easing.EasingOption.EaseInOutQuart);
    }