GCC vs VS2013中{st} :: setprecision（0）的正确行为

时间：2016-01-05 22:50:35

标签： c++ gcc visual-studio-2013

根据我使用的编译器，我在n = 0时得到一个不同的输出。 

std::string ToStrWPrec(double a_value, const int n)
{
    std::ostringstream out; 
    out << std::setprecision(n) << a_value; 
    return out.str();
}

（GCC）4.8.3 20140911（Red Hat 4.8.3-9）为ToStrWPrec（1.2345678,0）返回1。对于相同的代码，VS2013返回1.2346。

我的问题是：

  1. setprecision的正确/标准行为是什么？
  2. 使用setprecision有什么好办法？

    3. 这是基于以下评论的更新代码

    std::string ToStrWPrec(double a_value, const int n)
{
    std::ostringstream out; 
    out << std::setprecision(n) << std::fixed<< a_value; 
    return out.str();
}

1 个答案:

答案 0 :(得分：2)

根据22.4.2.2.2 [facet.num.put.virtuals]第5段，第1阶段，这是关于精度的说法：

  

对于从浮点类型的转换，如果在转换规范中将01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: FATAL EXCEPTION: main 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: Process: com.example.ukasz.carrom, PID: 2303 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ukasz.carrom/com.example.ukasz.carrom.ManagePlayers}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread.-wrap11(ActivityThread.java) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.os.Looper.loop(Looper.java:148) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5417) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.widget.SimpleAdapter.getCount(SimpleAdapter.java:100) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.widget.ListView.setAdapter(ListView.java:491) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at com.example.ukasz.carrom.ManagePlayers.playersListView(ManagePlayers.java:62) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at com.example.ukasz.carrom.ManagePlayers.onCreate(ManagePlayers.java:75) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.Activity.performCreate(Activity.java:6237) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369) 01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)  01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread.-wrap11(ActivityThread.java)  01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)  01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)  01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.os.Looper.loop(Looper.java:148)  01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5417)  01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)  01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)  01-06 00:00:44.078 2303-2303/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 指定为精度。否则，不指定精度。

同一段落在其他地方指定定义结果的格式说明符为 <?xml version="1.0" encoding="utf-8"?> <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:fitsSystemWindows="true"> <include layout="@layout/content_manage_players" /> </android.support.design.widget.CoordinatorLayout> 。

floatfield != (ios_base::fixed | ios_base::scientific), str.precision()的默认值没有根据27.5.5.2 [basic.ios.cons]第3段，表128设置：

  

%g floatfield

因此，它归结为格式字符串flags()如何格式化值。 7.21.6.1第8段中的C标准指出：

  

如果非零，则 P 等于精度，如果省略精度则等于6，如果精度为零，则为1。

看来，正确的结果是skipws | dec。

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