c++ - 得到一堆十字架初始化错误

Question

我从下载的示例中提取了以下代码片段:

bool ChatServer::event(QEvent * event)
{
    if(event->type() == QEvent::User)
    {
        UserEvent * ue = static_cast<UserEvent *>(event);
        switch(ue->userType)
        {
            case CR::ErrorEvent:
            case CR::LogEvent:
            {   
                TraceEvent * te = static_cast<TraceEvent *>(ue);
                if(te->userType == CR::ErrorEvent)
                {
                    error(te->msg);
                }
                else
                {
                    log(te->msg);
                }
            }
                break;
            default:
                return false;
        }
    }
    else
    {
        return QTcpServer::event(event);
    }
    return true;
}

现在，如果编译程序，则会出现以下错误:

g++ -c -pipe -O2 -Wall -W -DQT_NO_DEBUG -DQT_NETWORK_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/local/share/qt4/mkspecs/freebsd-g++ -I. -I/usr/local/include/qt4/QtCore -I/usr/local/include/qt4/QtNetwork -I/usr/local/include/qt4 -I. -I/usr/local/include/qt4 -I/usr/local/include -o chatserver.o chatserver.cpp
g++ -c -pipe -O2 -Wall -W -DQT_NO_DEBUG -DQT_NETWORK_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/local/share/qt4/mkspecs/freebsd-g++ -I. -I/usr/local/include/qt4/QtCore -I/usr/local/include/qt4/QtNetwork -I/usr/local/include/qt4 -I. -I/usr/local/include/qt4 -I/usr/local/include -o clientservice.o clientservice.cpp
clientservice.cpp: In member function 'virtual bool ClientService::event(QEvent*)':
clientservice.cpp:37: error: jump to case label
clientservice.cpp:34: error:   crosses initialization of 'MessageEvent* me'
clientservice.cpp:41: error: jump to case label
clientservice.cpp:38: error:   crosses initialization of 'UserInfoEvent* uie'
clientservice.cpp:34: error:   crosses initialization of 'MessageEvent* me'
clientservice.cpp:44: error: jump to case label
clientservice.cpp:38: error:   crosses initialization of 'UserInfoEvent* uie'
clientservice.cpp:34: error:   crosses initialization of 'MessageEvent* me'
clientservice.cpp:31: warning: enumeration value 'EventTypeBegin' not handled in switch
clientservice.cpp:31: warning: enumeration value 'LogEvent' not handled in switch
clientservice.cpp:31: warning: enumeration value 'ErrorEvent' not handled in switch
clientservice.cpp:31: warning: enumeration value 'ClientConnected' not handled in switch
clientservice.cpp:31: warning: enumeration value 'ClientReg' not handled in switch
clientservice.cpp:31: warning: enumeration value 'ClientDisconnect' not handled in switch
clientservice.cpp:31: warning: enumeration value 'ServerConnected' not handled in switch
clientservice.cpp:31: warning: enumeration value 'ServerDisconnect' not handled in switch
clientservice.cpp:31: warning: enumeration value 'DoConnect' not handled in switch
clientservice.cpp:31: warning: enumeration value 'DoDisconnect' not handled in switch
clientservice.cpp:31: warning: enumeration value 'GotMessage' not handled in switch
clientservice.cpp:31: warning: enumeration value 'GotUserInfo' not handled in switch
clientservice.cpp:31: warning: enumeration value 'SendUserInfo' not handled in switch
clientservice.cpp:31: warning: enumeration value 'GotClientInfo' not handled in switch
clientservice.cpp:31: warning: enumeration value 'SendClientInfo' not handled in switch
clientservice.cpp:31: warning: enumeration value 'EventTypeEnd' not handled in switch
clientservice.cpp: In member function 'void ClientService::readClient()':
clientservice.cpp:63: warning: comparison between signed and unsigned integer expressions
clientservice.cpp:87: error: jump to case label
clientservice.cpp:83: error:   crosses initialization of 'Message* msg'
clientservice.cpp:92: error: jump to case label
clientservice.cpp:88: error:   crosses initialization of 'UserInfo* ui'
clientservice.cpp:83: error:   crosses initialization of 'Message* msg'
clientservice.cpp:97: error: jump to case label
clientservice.cpp:93: error:   crosses initialization of 'ClientInfo* ci'
clientservice.cpp:88: error:   crosses initialization of 'UserInfo* ui'
clientservice.cpp:83: error:   crosses initialization of 'Message* msg'
clientservice.cpp: In member function 'bool ClientService::sendToClient(CR::MsgType::MsgType, SendAble*)':
clientservice.cpp:124: warning: comparison of unsigned expression < 0 is always false
clientservice.cpp: In member function 'void ClientService::gotUserInfo(UserInfo*)':
clientservice.cpp:176: error: cast from 'ClientService*' to 'quint32' loses precision
*** Error code 1

Stop in ~/SimpleChatRoomServer

现在，如果删除了 case声明的括号，则会出现此错误:

g++ -c -pipe -O2 -Wall -W -DQT_NO_DEBUG -DQT_NETWORK_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/local/share/qt4/mkspecs/freebsd-g++ -I. -I/usr/local/include/qt4/QtCore -I/usr/local/include/qt4/QtNetwork -I/usr/local/include/qt4 -I. -I/usr/local/include/qt4 -I/usr/local/include -o chatserver.o chatserver.cpp
chatserver.cpp: In member function 'virtual bool ChatServer::event(QEvent*)':
chatserver.cpp:108: error: jump to case label
chatserver.cpp:98: error:   crosses initialization of 'TraceEvent* te'
chatserver.cpp:94: warning: enumeration value 'EventTypeBegin' not handled in switch
chatserver.cpp:94: warning: enumeration value 'ClientConnected' not handled in switch
chatserver.cpp:94: warning: enumeration value 'ClientReg' not handled in switch
chatserver.cpp:94: warning: enumeration value 'ClientDisconnect' not handled in switch
chatserver.cpp:94: warning: enumeration value 'ServerConnected' not handled in switch
chatserver.cpp:94: warning: enumeration value 'ServerDisconnect' not handled in switch
chatserver.cpp:94: warning: enumeration value 'DoConnect' not handled in switch
chatserver.cpp:94: warning: enumeration value 'DoDisconnect' not handled in switch
chatserver.cpp:94: warning: enumeration value 'GotMessage' not handled in switch
chatserver.cpp:94: warning: enumeration value 'SendMessage' not handled in switch
chatserver.cpp:94: warning: enumeration value 'GotUserInfo' not handled in switch
chatserver.cpp:94: warning: enumeration value 'SendUserInfo' not handled in switch
chatserver.cpp:94: warning: enumeration value 'GotClientInfo' not handled in switch
chatserver.cpp:94: warning: enumeration value 'SendClientInfo' not handled in switch
chatserver.cpp:94: warning: enumeration value 'EventTypeEnd' not handled in switch
*** Error code 1

Stop in ~/SimpleChatRoomServer.

SimpleChatRoomServer.cpp的完整源代码:

#include <QTextStream>
#include <QDateTime>
#include "traceevent.h"
#include "chatserver.h"
#include "chatcenterthread.h"
#include "clientservicethread.h"
#include "message.h"
#include "server.h"

ChatServer::ChatServer(QObject * parent)
        :QTcpServer(parent)
{
    QFile * inFile = new QFile(this);
    QFile * outFile = new QFile(this);
    QFile * errFile = new QFile(this);
    inFile->open(stdin,QIODevice::ReadOnly);
    outFile->open(stdout,QIODevice::WriteOnly);
    errFile->open(stderr,QIODevice::WriteOnly);
    setIO(inFile,outFile,errFile);

    qobject_cast<Server *>(qApp)->csrId = this;
    csrId = (QObject *)(0+CR::ServerId);
}

ChatServer::~ChatServer()
{
    for(int i = 0; i < ccs.size(); i++)
    {
        ccs.at(i)->deleteLater();
    }
}

void ChatServer::setIO(QFile * inFile,QFile * outFile,QFile * errFile)
{
    in.setDevice(inFile);
    out.setDevice(outFile);
    err.setDevice(errFile);
}

void ChatServer::incomingConnection(int socketId)
{
    ClientServiceThread *cst = new ClientServiceThread(socketId,this);
    cst->start();
    connect(cst,SIGNAL(finished()),cst,SLOT(deleteLater()));
}

bool ChatServer::init(quint16 & port)
{
    if(!port)
    {
        bool ok = false;
        while(!ok)
        {
            out<<tr("Port[5555]: ");
            out.flush();
            QString p;
            p = in.read(1);
            if(p!=QString("\n"))
            {
                QString strport;
                strport = in.readLine(10);
                strport = p + strport;
                port = strport.toUInt(&ok);
            }
            else
            {
                port = 5555;
                ok = true;
            }
        }
    }

    //ADD ONE ChatCenter
    ChatCenterThread * cct = new ChatCenterThread(this);
    cct->start();
    connect(cct,SIGNAL(finished()),cct,SLOT(deleteLater()));

    if(!listen(QHostAddress::Any,port))
    {
        error(tr("Listen at [%1] fail!").arg(port));
        deleteLater();
        exit(1);
        return false;
    }
    log(tr("Listen at [%1]").arg(port));
    return true;
}

bool ChatServer::event(QEvent * event)
{
    if(event->type() == QEvent::User)
    {
        UserEvent * ue = static_cast<UserEvent *>(event);
        switch(ue->userType)
        {
            case CR::ErrorEvent:
            case CR::LogEvent:
                TraceEvent * te = static_cast<TraceEvent *>(ue);
                if(te->userType == CR::ErrorEvent)
                {
                    error(te->msg);
                }
                else
                {
                    log(te->msg);
                }
                break;
            default:
                return false;
        }
    }
    else
    {
        return QTcpServer::event(event);
    }
    return true;
}

void ChatServer::error(QString msg)
{
    err<<QDateTime::currentDateTime().toString(Qt::ISODate)
            <<" !ERROR! "<<msg<<endl;
}

void ChatServer::log(QString msg)
{
    out<<QDateTime::currentDateTime().toString(Qt::ISODate)
            <<" "<<msg<<endl;
}

1)我是否使用了不正确的括号？

2)为什么在 case语句中使用方括号将 crosses initialization显示为其他代码行。

Answer 1

C++标准说:

It is possible to transfer into a block, but not in a way that bypasses declarations with initialization. A program that jumps from a point where a local variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD type (3.9) and is declared without an initializer.

switch中的案例被视为“跳转”。

只需将所有对象和变量初始化放在切换之前，一切都会好起来的。

考虑以下代码:

switch(k)
{
    case 1:
        int t = 4;
    break;
    default:
    break;
}

这将导致“交叉初始化”错误，因为可以跳过t的初始化，但是此后它仍然在作用域内，即使它从来没有创建过也是如此。

现在考虑一下:

switch(k)
{
    case 1:
    {
        int t = 4;
    }
    break;
    default:
    break;
}

在这里，您不会有错误，因为该变量位于一个块内，并且将在该块的末尾(在结束 {处)死亡，因此在任何情况下，它都不会在范围内。

要解决第一种情况，您只需要执行以下操作:

int t = 0;
switch(k)
{
    case 1:
        t = 4;
    break;
    default:
    break;
}