C++11 in Qt

àì夳堔傛蜴生んèń 2022-08-22 13:24 108阅读 0赞

欧冠决赛结束了：所有有权有势的人都不希望马竞赢球，这样诞生了一个最水的欧洲冠军。
    
    创造历史真不是一件容易的事儿。

言归正传，之前写了很多关于C++11的文章，因为C++11使得C++看上去像是一门全新的语言。

Qt可以说是对C++的类用到了极致，所以接下来跟大家分享的就是C++11在Qt中的使用。

为了能够使用C++11，需要在.pro文件中加入：

CONFIG += c++11

**Slots中使用Lambda表达式**

lambda表达式：[c++11特性之Lambda表达式][c_11_Lambda]

slots：[QT中的信号－槽比我们常用的callback到底牛在哪里？][QT_callback]

connect(sender, &Sender::valueChanged, [=](const QString &newValue) {
     receiver->updateValue("senderValue", newValue);
       });

**Unicode literal**  
在C++11中，可以使用u”MyString”来表示一个UTF-16。  
这样在Qt中有相应的QStringLiteral。

QString s1 = "foo";        // Implementation character set string
    QString s2 = R"(foo\bar)"; // Raw string, no need to escape backslashes
    QString s3 = u8"foo";      // UTF-8 string
    QString s4 = u8R"(foo)";   // UTF-8 raw string
    QString s5 = QString::fromUtf16(reinterpret_cast<const ushort *>(u"foo"));    // UTF-16 string
    QString s6 = QString::fromUtf16(reinterpret_cast<const ushort *>(uR"(foo)")); // UTF-16 raw string
    QString s7 = QString::fromUcs4(reinterpret_cast<const uint *>(U"foo"));       // UTF-32 string
    QString s8 = QString::fromUcs4(reinterpret_cast<const uint *>(UR"(foo)"));    // UTF-32 raw string

**常量表达式：constexpr**  
constexpr变量

在一个复杂系统中，很难（几乎肯定不能）分辨一个初始值到底是不是常量表达式。当然可以定义一个const变量并把它的初始值设为我们认为的某个常量表达式，但在实际使用时，尽管要求如此却常常发现初始值并非常量表达式的情况。可以这么说，在此种情况下，对象的定义和使用根本就是两回事儿。

C++11新标准规定，允许将变量声明为constexpr类型以便由编译器来验证变量的值是否是一个常量表达式。声明为constexpr的变量一定是一个常量，而且必须用常量表达式初始化：

constexpr int mf = 20;          // 20是常量表达式 
    constexpr int limit = mf + 1;   // mf + 1是常量表达式 
    constexpr int sz = size();      // 只有当size是一个constexpr函数时

z在Qt 5, 有Q\_DECL\_CONSTEXPR:

enum SomeEnum { Value1, Value2, Value3 };
    Q_DECLARE_OPERATORS_FOR_FLAGS(QFlags<SomeEnum>)
    // The previous line declares
    // Q_DECL_CONSTEXPR QFlags<SomeValue> operator|(SomeValue,SomeValue) {...}
    
    int someFunction(QFlags<SomeEnum> value) {
        switch (value) {
            case SomeEnum::Value1:
                return 1;
            case SomeEnum::Value2:
                return 2;
            case SomeEnum::Value1 | SomeEnum::Value3:
            // Only possible with C++11 and because QFlags operators are constexpr
            // Previously this line would call
            // QFlags<SomeValue> operator|(SomeValue,SomeValue)
            // that would have thrown an error because only compiler constants
            // are allowed as case satement
    
                return 3;
            default:
                return 0;
        }
    }

**static\_assert**  
C++11 helps producing better error messages when something wrong can be detected at compile time using static\_assert. In Qt5 we introduced the macros Q\_STATIC\_ASSERT, and Q\_STATIC\_ASSERT\_X That will use static\_assert if available, or some other template trick if not.

**Override and final**  
C++11中增加了override和final关键字，之前也说过：  
[c++11特性之override和final关键字][c_11_override_final]

qt中 Q\_DECL\_OVERRIDE和Q\_DECL\_FINAL

class MyModel : public QStringListModel {
        //...
    protected:
         Qt::ItemFlags flags (const QModelIndex & index)  Q_DECL_OVERRIDE;
    };

**nullptr常量**

char *pc = nullptr;   // OK
        int  *pi = nullptr;   // OK
        bool  bp = nullptr;   // OK, bp will be set to false.
        int i    = nullptr;   // Compile error: can't convert a nullptr to int.

**初始化列表Initializer Lists**  
之前：

QStringList fruit;
        fruit << "apple" << "pear" << "peach" << "tomato";

现在：

QStringList fruit{
       "apple", "pear", "peach", "tomato"};

**基于范围的for循环**  
使用qt中的foreach:

QStringList options{
       "a", "b", "c", "d"};
    
        foreach(const QString &option, options) {
            std::cout << qPrintable(option) << std::endl;
        }

C++11:

QStringList options{
       "a", "b", "c", "d"};
    
        for (const QString & option : options) {
            std::cout << qPrintable(option) << std::endl;
        }

**总结**  
MSVC does not require any special flags and enables the C++11 features by default, but GCC or Clang require -std=c++0x.

By default, Qt5 itself will be compiled with the C++11 flags on compilers that need it.

If you use qmake, you can add that line to your .pro file (Qt5):

CONFIG += c++11  
(In Qt4, it should be something like: gcc:CXXFLAGS += -std=c++0x)

And now you can enjoy all the nice features of C++11. (It is already worth doing it only for being able to use auto)

[c_11_Lambda]: http://blog.csdn.net/wangshubo1989/article/details/49256795
[QT_callback]: http://blog.csdn.net/wangshubo1989/article/details/51289160
[c_11_override_final]: http://blog.csdn.net/wangshubo1989/article/details/49539251