Question

我有一个10位的签名值。实际上，这是我要转换为int的10位颜色值。

在较大的int中，这10位将被解释为有符号值，而其他位为零。

我可以想到3种方式，如下所示：

#include <stdio.h>

void p(int v)
{
    printf("\t%d", v);
}

int main()
{
    for (int i = -2; i < 3; ++i)
    {
        unsigned int u = i & 0x3ff;

        int v;

        // method 1: test the bit
        v = u;
        if (u & 0x200) v = v ^ -0x400;
        p(v);

        // method 2: shift
        v = u<<(sizeof(int)*8-10);
        v >>= (sizeof(int)*8-10);
        p(v);

        // method 3: use sign extend
        v = (signed char)(u >> 2);
        v <<= 2;
        v |= u;
        p(v);


        printf("\n");

    }

    return 0;
}

有没有更好的方法？经常会有一些纠结的问题。

感谢您的任何输入。

更新

再添加两个方法。 @ cruz-jean建议的第一个是位域，第二个由gcc建议的是编译后的位域。

      // method 4: bit fields
        struct { int v:10; } f;
        f.v = u;
        v = f.v;

        // method 5: sign extend short
        v = (signed short)(u << 6);
        v >>= 6;

出于兴趣，似乎MSVC将＃4编译为＃2，而gcc将＃4编译为＃5。

方法5看起来不错。

Answer 1

您可以声明一个10位带符号的位域助手类型，并以Starting local Bazel server and connecting to it... INFO: Analysed target //:onos (759 packages loaded, 12923 targets configured). INFO: Found 1 target... . . . enconfig-native; [2,128 / 2,367] //models/openconfig:onos-models-openconfig-native; ERROR: /home/mohamedzidan/onos/models/openconfig/BUILD:11:1: Building models/openconfig/libonos-models-openconfig-native-class.jar (2 source jars) failed (Exit 1) [2,128 / 2,367] //models/openconfig:onos-models-openconfig-native; An exception has occurred in the compiler (10.0.1). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program and the following diagnostic in your report. Thank you. java.lang.OutOfMemoryError: Java heap space at jdk.compiler/com.sun.tools.javac.util.ArrayUtils.ensureCapacity(ArrayUtils.java:60) at jdk.compiler/com.sun.tools.javac.util.SharedNameTable.fromUtf(SharedNameTable.java:132) at jdk.compiler/com.sun.tools.javac.util.Names.fromUtf(Names.java:392) at jdk.compiler/com.sun.tools.javac.util.ByteBuffer.toName(ByteBuffer.java:159) at jdk.compiler/com.sun.tools.javac.jvm.ClassWriter$CWSignatureGenerator.toName(ClassWriter.java:320) at jdk.compiler/com.sun.tools.javac.jvm.ClassWriter$CWSignatureGenerator.access$300(ClassWriter.java:266) at jdk.compiler/com.sun.tools.javac.jvm.ClassWriter.typeSig(ClassWriter.java:335) at jdk.compiler/com.sun.tools.javac.jvm.ClassWriter.writeMethod(ClassWriter.java:1153) at jdk.compiler/com.sun.tools.javac.jvm.ClassWriter.writeMethods(ClassWriter.java:1653) at jdk.compiler/com.sun.tools.javac.jvm.ClassWriter.writeClassFile(ClassWriter.java:1761) at jdk.compiler/com.sun.tools.javac.jvm.ClassWriter.writeClass(ClassWriter.java:1679) at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.genCode(JavaCompiler.java:743) at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1641) at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.generate(JavaCompiler.java:1609) at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:959) at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.lambda$doCall$0(JavacTaskImpl.java:100) at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl$$Lambda$97/1225568095.call(Unknown Source) at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.handleExceptions(JavacTaskImpl.java:142) at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:96) at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:90) at com.google.devtools.build.buildjar.javac.BlazeJavacMain.compile(BlazeJavacMain.java:113) at com.google.devtools.build.buildjar.SimpleJavaLibraryBuilder$$Lambda$70/778731861.invokeJavac(Unknown Source) at com.google.devtools.build.buildjar.ReducedClasspathJavaLibraryBuilder.compileSources(ReducedClasspathJavaLibraryBuilder.java:57) at com.google.devtools.build.buildjar.SimpleJavaLibraryBuilder.compileJavaLibrary(SimpleJavaLibraryBuilder.java:116) at com.google.devtools.build.buildjar.SimpleJavaLibraryBuilder.run(SimpleJavaLibraryBuilder.java:123) at com.google.devtools.build.buildjar.BazelJavaBuilder.processRequest(BazelJavaBuilder.java:105) at com.google.devtools.build.buildjar.BazelJavaBuilder.runPersistentWorker(BazelJavaBuilder.java:67) at com.google.devtools.build.buildjar.BazelJavaBuilder.main(BazelJavaBuilder.java:45) [2,128 / 2,367] //models/openconfig:onos-models-openconfig-native; Target //:onos failed to build Use --verbose_failures to see the command lines of failed build steps. INFO: Elapsed time: 1386.685s, Critical Path: 117.31s INFO: 379 processes: 125 linux-sandbox, 254 worker. **FAILED: Build did NOT complete successfully**的形式访问该值：

int

Answer 2

另一种尝试：

v = u | (0 - (u&0x200));

对移位较慢的cpus有益。

在C / C ++中扩展数字的最佳方法是什么

更新

2 个答案: