Qt compile error: C2228: '.key' must have class/struct/union on the left

目录

一、产生原因

二、解决方案

方案1：Static functions access static variables

方案2：Objects are defined inside static functions,Access the variable again

---

一、产生原因

原因：A static function accesses a non-static variable, staticfunction and notstatic变量混用,示例代码如下

class FLOWEDITORSHARED_EXPORT NETranslator
{
public:
    NETranslator();
    static QString ch2En(const QString &ch);
    static QString en2Ch(const QString &en);

private:
    QHash<QString, QString> m_hash;
};
//
NETranslator::NETranslator()
{
    m_hash.insert("image", QStringLiteral("图像数据"));
    m_hash.insert("region", QStringLiteral("区域数据"));
    m_hash.insert("contours", QStringLiteral("轮廓数据"));
}

QString NETranslator::ch2En(const QString &ch)
{
    QString en =  m_hash.key(ch, ch);
    return en;
}

QString NETranslator::en2Ch(const QString &en)
{
    QString ch = m_hash.value(en, en);
    return ch;
}

Compilation of the above code will give the following error：

NETranslator.cpp:12: error: C2228: “.key”的左边必须有类/结构/联合

NETranslator.cpp:18: error: C2228: “.value”的左边必须有类/结构/联合

二、解决方案

方案1：Static functions access static variables

Then declare the class variable as staticIs it useful,修改代码如下：

class FLOWEDITORSHARED_EXPORT NETranslator
{
public:
    NETranslator();
    static QString ch2En(const QString &ch);
    static QString en2Ch(const QString &en);

private:
    static  QHash<QString, QString> m_hash;    //修改了此处,静态声明
};

Compiling again gave the following error

NETranslator.obj error: LNK2019: 无法解析的外部符号 "public: static class QHash<class QString,class QString> NETranslator::m_hash" ([email protected]@@[email protected]@@[email protected]@@A),该符号在函数 "public: __cdecl NETranslator::NETranslator(void)" ([email protected]@[email protected]) 中被引用

Unresolved external symbols are mostly just declared,没有定义.Static members of a class must be declared within the class,在类外初始化.修改代码如下：

class FLOWEDITORSHARED_EXPORT NETranslator
{
public:
    NETranslator();
    static QString ch2En(const QString &ch);
    static QString en2Ch(const QString &en);

private:
    static  QHash<QString, QString> m_hash;    //修改了此处,静态声明
};
//
QHash<QString, QString> NETranslator::m_hash;  //修改了此处,定义

NETranslator::NETranslator()
{
    m_hash.insert("image", QStringLiteral("图像数据"));
    m_hash.insert("region", QStringLiteral("区域数据"));
    m_hash.insert("contours", QStringLiteral("轮廓数据"));
}

QString NETranslator::ch2En(const QString &ch)
{
    QString en =  m_hash.key(ch, ch);
    return en;
}

QString NETranslator::en2Ch(const QString &en)
{
    QString ch = m_hash.value(en, en);
    return ch;
}

There is also a statement,There are also definitions,编译通过.上述方案m_hashAlthough the value is inserted,But the function is accessedm_hash数据为空.

原因：使用NETranslator::ch2En() 方式去调用,并没有调用NETranslator构造函数,So there is no initialization data

那怎么办了,It can only be assigned when a static variable is initialized,修改代码如下：

QHash<QString, QString> LinkTranslator::m_hash =
{
    {"image", QStringLiteral("图像数据")},
    {"region", QStringLiteral("区域数据")},
    {"contours", QStringLiteral("轮廓数据")},
};

方案2：Objects are defined inside static functions,Access the variable again

If you initialize the data in the constructor,Just call the constructor and that's it

class FLOWEDITORSHARED_EXPORT NETranslator
{
public:
    NETranslator();
    static QString ch2En(const QString &ch);
    static QString en2Ch(const QString &en);

private:
    QHash<QString, QString> m_hash;
};


NETranslator::NETranslator()
{
    m_hash.insert("image", QStringLiteral("图像数据"));
    m_hash.insert("region", QStringLiteral("区域数据"));
    m_hash.insert("contours", QStringLiteral("轮廓数据"));
}
QString NETranslator::ch2En(const QString &ch)
{
    NETranslator tran;   //修改了此处
    QString en =  tran.m_hash.key(ch, ch);
    return en;
}

QString NETranslator::en2Ch(const QString &en)
{
    NETranslator tran;   //修改了此处
    QString ch = tran.m_hash.value(en, en);
    return ch;
}