Question

我在Qt5开发并想要一些建议。

我想在一个类中声明const变量，并在其他类中使用此变量。最好的方法是什么？

现在我的代码出错：

protocolcommands.cpp

const QString CNT_FIRMWARE_ACK = "ACK";
const QString CNT_FIRMWARE_NAK = "NAK";
const QString CNT_FIRMWARE_NYET = "NYET";
const QString CNT_FIRMWARE_STALL = "STALL";
const QString CNT_FIRMWARE_HEARTBEAT = "HEARTBEAT";

QString ProtocolCommands::parseCommand(QByteArray ba)
{

    QString s;
    if (ba[2] == 0x02)
    {
        s = "flow control";
        if (ba[3] == 0x01 && ba[4] == 0x01)
          return CNT_FIRMWARE_ACK;
        if (ba[3] == 0x01 && ba[4] == 0x02)
          return CNT_FIRMWARE_NAK;
        if (ba[3] == 0x01 && ba[4] == 0x03)
          return CNT_FIRMWARE_STALL;
        if (ba[3] == 0x01 && ba[4] == 0x04)
          return CNT_FIRMWARE_NYET;
    }
    else if (ba[2] == 0x03)
    {
        s = "data packet";
        qDebug() << ba.toHex();
        if (ba[3] == 0x1a && ba[4] == 0x02)
         return "getJsonStringFor_GetStimulatorOperaionType";

    }
    else if (ba[2] == 0x04)
    {
          return CNT_FIRMWARE_HEARTBEAT;
    }
   // return "";
}

engine.cpp：

void Engine::onSerialArrivedMsg(QByteArray msg)
{
ProtocolCommands pc;
qDebug() << "somthing has arrived : " << msg.toHex();
//if we here - this is new message from the firmware
//this function refer just to one message at a time

//first check if we are in the middle of flow:
if (mode)
{
     qDebug() << "mode = true";
     switch (curr_step_in_flow) {
     case 1:  //last command send to FW was [Set SinglePulse Parameters]
         if (pc.parseCommand(msg) == ProtocolCommands::CNT_FIRMWARE_ACK)    // compile eror
         {
            qDebug() << "ack recieved";
            curr_step_in_flow = 2;
         }
         else if (pc.parseCommand(msg) == CNT_FIRMWARE_HEARTBEAT ) // compile error
         {
             //heartbeat msg
             //send signal to the heartbeat thread
             emit fwHeartBeatArrived();
         }
         break;
     default:
         break;
     }

}
else
{
   if (pc.parseCommand(msg) == CNT_FIRMWARE_HEARTBEAT)  // compile eror
   {
       //heartbeat msg
       //send signal to the heartbeat thread
       emit fwHeartBeatArrived();
   }

}

}

Answer 1

我想在一个类中声明const变量，并在其他类中使用此变量。

这些字符串不属于您的ProtocolCommands类;他们恰巧在那个竞争单位宣布。

如果你想让它们成为类的（静态）成员，那么在protocolcommands.h中ProtocolCommands类的声明中声明它们：

class ProtocolCommands {
public:
   static const QString CNT_FIRMWARE_ACK;
   // ...
};

然后在protocolcommands.cpp中初始化它们：

const QString ProtocolCommands::CNT_FIRMWARE_ACK = "ACK";

然后你可以在名为ProtocolCommands::CNT_FIRMWARE_ACK的包含protocolcommands.h的任何代码中引用它。

Answer 2

我会使用enum类型而不是将常量声明为字符串来执行此操作。因此，ProtocolCommands类看起来像：

class ProtocolCommands
{
public:
    enum Type {
        CNT_FIRMWARE_ACK,
        CNT_FIRMWARE_NAK,
        CNT_FIRMWARE_NYET,
        CNT_FIRMWARE_STALL,
        CNT_FIRMWARE_HEARTBEAT    
    };

    ProtocolCommands::Type parseCommand(const QByteArray &ba);
    [..]
};

因此，您的ProtocolCommands::parseCommand()应该返回一个整数值而不是字符串。因此，您的Engine::onSerialArrivedMsg(QByteArray msg)函数将如下所示：

void Engine::onSerialArrivedMsg(QByteArray msg)
{
    ProtocolCommands pc;
    [..]
    switch (pc.parseCommand(msg)) {    
    case ProtocolCommands::CNT_FIRMWARE_ACK:
    [..]
    case ProtocolCommands::CNT_FIRMWARE_NYET:
    [..]
    }
 }

使用这种方法的好处是：

它更快，因为你需要比较数字而不是字符串，
设计简单明了，因为您不需要处理全局或静态变量。

Answer 3

在.cpp文件中定义您的常量，或将pc.parseCommand(msg) == CNT_FIRMWARE_HEARTBEAT个文件包含为附加（不良气味样式）。然后你可以将它们称为常量，如

<li id="contact|NoRelationship|Form|Mscrm.Form.contact.MainTab.ExportData" class="ms-cui-group" unselectable="on">
<span class="ms-cui-groupContainer" unselectable="on">
<span class="ms-cui-groupBody" unselectable="on">
<span id="contact|NoRelationship|Form|Mscrm.Form.contact.MainTab.ExportData-LargeMedium" class="ms-cui-layout" unselectable="on">
<span id="contact|NoRelationship|Form|Mscrm.Form.contact.MainTab.ExportData-LargeMedium-0" class="ms-cui-section" unselectable="on">
<span id="contact|NoRelationship|Form|Mscrm.Form.contact.MainTab.ExportData-LargeMedium-1" class="ms-cui-section" unselectable="on">
<span id="contact|NoRelationship|Form|Mscrm.Form.contact.MainTab.ExportData-LargeMedium-1-0" class="ms-cui-row" unselectable="on">
<a id="contact|NoRelationship|Form|fmc.contact.form.Button.Customer-Medium" class="ms-cui-ctl-medium " unselectable="on" href="javascript:;" onclick="return false;" aria-describedby="contact|NoRelationship|Form|fmc.contact.form.Button.Customer_ToolTip" mscui:controltype="Button" role="button">
<span class="ms-cui-ctl-iconContainer" unselectable="on">
<span class=" ms-cui-img-16by16 ms-cui-img-cont-float" unselectable="on">
<img unselectable="on" alt="" src="/%7B635732422790004376%7D/WebResources/fmc_/Images/HP.FMC.MSS360.CRM.Image.UpdateCustomer16X16">
</span>
</span>
<span class="ms-cui-ctl-mediumlabel" unselectable="on">Update Customer</span>
</a>
</span>

这是因为您只包含头文件，但标题不知道您要使用的常量。

在Qt中声明const并从2个不同的类访问

3 个答案: