C++ 使用 for 迭代 QMap

Iterating over a QMap with for

提问by user336635

I've a QMapobject and I am trying to write its content to a file.

我有一个QMap对象，我正在尝试将其内容写入文件。

QMap<QString, QString> extensions;
//.. 

for(auto e : extensions)
{
  fout << e.first << "," << e.second << '\n';
}  

Why do I get: error: 'class QString' has no member named 'first' nor 'second'

为什么我得到： error: 'class QString' has no member named 'first' nor 'second'

Is enot of type QPair?

是e不是类型QPair？

采纳答案by Arlen

If you want the STL style with firstand second, do this:

如果您想要带有firstand的 STL 样式second，请执行以下操作：

for(auto e : extensions.toStdMap())
{
  fout << e.first << "," << e.second << '\n';
}

If you want to use what Qt offers, do this:

如果您想使用 Qt 提供的功能，请执行以下操作：

for(auto e : extensions.keys())
{
  fout << e << "," << extensions.value(e) << '\n';
}

回答by alexisdm

C++11 range-based-for uses the type of the dereferenced iterator as the automatically deduced "cursor" type. Here, it is the type of the expression *map.begin().
And since QMap::iterator::operator*()returns a reference to the value (of type QString &), the key isn't accessible using that method.

C++11 range-based-for 使用解引用迭代器的类型作为自动推导的“游标”类型。在这里，它是表达式的类型*map.begin()。
并且由于QMap::iterator::operator*()返回对值（类型QString &）的引用，因此无法使用该方法访问该键。

You should use one of the iterator methods described in the documentation but you should avoid using

您应该使用文档中描述的迭代器方法之一，但应避免使用

• keys()because it involves creating a list of keys and then searching the value for each key, or,
• toStdMap()because it copies all the map elements to another one,

• keys()因为它涉及创建一个键列表，然后搜索每个键的值，或者，
• toStdMap()因为它将所有地图元素复制到另一个元素，

and that wouldn't be very optimal.

这不会是非常理想的。

---

---

你也可以使用一个包装来获得QMap::iteratorQMap::iterator的autoauto类型：

template<class Map>
struct RangeWrapper {
    typedef typename Map::iterator MapIterator;
    Map &map;

    RangeWrapper(Map & map_) : map(map_) {}

    struct iterator {
        MapIterator mapIterator;
        iterator(const MapIterator &mapIterator_): mapIterator(mapIterator_) {}
        MapIterator operator*() {
            return mapIterator;
        }
        iterator & operator++() {
            ++mapIterator;
            return *this;
        }
        bool operator!=(const iterator & other) {
            return this->mapIterator != other.mapIterator;
        }
    };
    iterator begin() {
        return map.begin();
    }
    iterator end() {
        return map.end();
    }
};

// Function to be able to use automatic template type deduction
template<class Map>
RangeWrapper<Map> toRange(Map & map)
{
    return RangeWrapper<Map>(map);
}

// Usage code
QMap<QString, QString> extensions;
...
for(auto e : toRange(extensions)) {
    fout << e.key() << "," << e.value() << '\n';
}

There is another wrapper here.

还有另一种包装这里。

回答by Fezvez

For people interested in optimizations, I have tried several approaches, did some micro benchmarks, and I can conclude that STL style approach is significantly faster.

对于对优化感兴趣的人，我尝试了几种方法，做了一些微基准测试，我可以得出结论，STL 风格的方法明显更快。

I have tried adding integers with these methods :

我试过用这些方法添加整数：

• QMap::values()
• Java style iterator (as advised in the documentation)
• STL style iterator (as advised in the documentation too)

• QMap::values()
• Java 风格的迭代器（如文档中所建议的）
• STL 样式迭代器（也如文档中所述）

And I have compared it with summing integers of a QList/QVector

我将它与 QList/QVector 的整数相加进行了比较

Results :

结果 ：

Reference vector :   244  ms
Reference list :     1239  ms

QMap::values() :     6504  ms
Java style iterator :    6199  ms
STL style iterator :     2343  ms

Code for those interested :

感兴趣的人的代码：

#include <QDateTime>
#include <QMap>
#include <QVector>
#include <QList>
#include <QDebug>

void testQMap(){
    QMap<int, int> map;
    QVector<int> vec;
    QList<int> list;

    int nbIterations = 100;
    int size = 1000000;
    volatile int sum = 0;

    for(int i = 0; i<size; ++i){
        int randomInt = qrand()%128;
        map[i] = randomInt;
        vec.append(randomInt);
        list.append(randomInt);
    }


    // Rererence vector/list
    qint64 start = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        for(int j : vec){
            sum += j;
        }
    }
    qint64 end = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "Reference vector : \t" << (end-start) << " ms";

    qint64 startList = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        for(int j : list){
            sum += j;
        }
    }
    qint64 endList = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "Reference list : \t" << (endList-startList) << " ms";

    // QMap::values()
    qint64 start0 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        QList<int> values = map.values();
        for(int k : values){
            sum += k;
        }
    }
    qint64 end0 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "QMap::values() : \t" << (end0-start0) << " ms";


    // Java style iterator
    qint64 start1 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        QMapIterator<int, int> it(map);
        while (it.hasNext()) {
            it.next();
            sum += it.value();
        }
    }
    qint64 end1 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "Java style iterator : \t" << (end1-start1) << " ms";


    // STL style iterator
    qint64 start2 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        QMap<int, int>::const_iterator it = map.constBegin();
        auto end = map.constEnd();
        while (it != end) {
            sum += it.value();
            ++it;
        }
    }
    qint64 end2 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "STL style iterator : \t" << (end2-start2) << " ms";


    qint64 start3 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        auto end = map.cend();
        for (auto it = map.cbegin(); it != end; ++it)
        {
            sum += it.value();
        }
    }
    qint64 end3 = QDateTime::currentMSecsSinceEpoch();

    qDebug() << "STL style iterator v2 : \t" << (end3-start3) << " ms";
}

Edit July 2017 : I ran this code again on my new laptop (Qt 5.9, i7-7560U) and got some interesting changes

2017 年 7 月编辑：我在我的新笔记本电脑（Qt 5.9、i7-7560U）上再次运行此代码并进行了一些有趣的更改

Reference vector :   155  ms 
Reference list :     157  ms
QMap::values():      1874  ms 
Java style iterator: 1156  ms 
STL style iterator:  1143  ms

STL style and Java style have very similar performances in this benchmark

STL 风格和 Java 风格在这个基准测试中的表现非常相似

回答by hmuelner

QMap::iterator uses key() and value() - which can be found easily in the documentation for Qt 4.8or the documentation for Qt-5.

QMap::iterator 使用 key() 和 value() - 这可以在Qt 4.8的文档或Qt-5的文档中轻松找到。

Edit:

编辑：

A range-based for loop generates codes similar to this (see CPP reference):

基于范围的 for 循环生成与此类似的代码（请参阅CPP 参考）：

{
    for (auto __begin = extensions.begin(), __end = extensions.end();
            __begin != __end; ++__begin) {
        auto e = *__begin; // <--- this is QMap::iterator::operator*()
        fout << e.first << "," << e.second << '\n';
    }
} 

QMap::iterator::iterator*() is equivalent to QMap::iterator::value(), and does not give a pair.

QMap::iterator::iterator*() 等价于 QMap::iterator::value()，并且不给出对。

The best way to write this is without range-based for loop:

最好的写法是不使用基于范围的 for 循环：

auto end = extensions.cend();
for (auto it = extensions.cbegin(); it != end; ++it)
{
    std::cout << qPrintable(it.key()) << "," << qPrintable(it.value());
}

回答by Tim Meyer

In "old" C++, using Qt, you would do it like this:

在“旧”C++ 中，使用 Qt，你会这样做：

QMap< QString, whatever > extensions;
//...
foreach( QString key, extensions.keys() )
{
    fout << key << "," << extensions.value( key ) << '\n';
}

I don't have a C++11 compiler here but maybe the following will work:

我这里没有 C++11 编译器，但也许以下内容会起作用：

for( auto key: extensions.keys() )
{
    fout << key << "," << extensions.value( key ) << '\n';
}

You can also use iterators instead, check out hmuelners link if you prefer using them

您也可以使用迭代器，如果您更喜欢使用它们，请查看 hmuelners 链接

回答by bcmpinc

Since Qt 5.10 you can use a simple wrapper class to use a range based for loop, but still be able to access both the key and value of the map entries.

从 Qt 5.10 开始，您可以使用简单的包装类来使用基于范围的 for 循环，但仍然能够访问映射条目的键和值。

Put the following code somewhere at the top of your source file or in a header that you include:

将以下代码放在源文件顶部或包含的标题中：

template<class K,class V>
struct QMapWrapper {
    const QMap<K,V>& map;
    QMapWrapper(const QMap<K,V>& map) : map(map) {}
    auto begin() { return map.keyValueBegin(); }
    auto end()   { return map.keyValueEnd();   }
};

To iterate over all entries you can simply write:

要遍历所有条目，您可以简单地编写：

QMap<QString, QString> extensions;
//.. 

for(auto e : QMapWrapper(extensions))
{
  fout << e.first << "," << e.second << '\n';
}

The type of ewill be std::pair<const QString&, const QString&>as is partially specified in the QKeyValueIterator documentation.

的类型的e将被std::pair<const QString&, const QString&>作为在被指定的部分QKeyValueIterator文档。

Considering that the pair uses references, removing the const qualifier for map might allow you to modify values in the map by assigning to e.second. I haven't tested this though.

考虑到该对使用引用，删除 map 的 const 限定符可能允许您通过分配给 来修改映射中的值e.second。不过我还没有测试过这个。

I'm using a reference for map as this avoids calling the copy constructor and hopefully allows the compiler to optimize the QMapWrapper class away.

我正在使用 map 的引用，因为这避免了调用复制构造函数，并希望允许编译器优化 QMapWrapper 类。

---

---

The above example uses class template argument deduction, which was introduced in C++17. If you're using an older standard, the template parameters for QMapWrapper must be specified when calling the constructor. In this case a factory method might be useful:

上面的例子使用了 C++17 中引入的类模板参数推导。如果您使用的是旧标准，则在调用构造函数时必须指定 QMapWrapper 的模板参数。在这种情况下，工厂方法可能很有用：

template<class K,class V>
QMapWrapper<K,V> wrapQMap(const QMap<K,V>& map) {
    return QMapWrapper<K,V>(map);
}

This is used as:

这用作：

for(auto e : wrapQMap(extensions))
{
  fout << e.first << "," << e.second << '\n';
}

回答by Thierry Joel

I used something like this, to achieve my own result. Just in case someone needed the keys and values separately.

我用这样的东西来达到我自己的结果。以防万一有人需要单独的键和值。

{
   QMap<int,string> map; 

   map.insert(1,"One");
   map.insert(2,"Two");
   map.insert(3,"Three");
   map.insert(4,"Four");   

   fout<<"Values in QMap 'map' are:"<<endl;
   foreach(string str,map)
   {
     cout<<str<<endl;
   };


   fout<<"Keys in QMap 'map' are:"<<endl;
   foreach(int key,map.keys())
   {
     cout<<key<<endl;
   }; 
}  

回答by DomTomCat

Another convenient method, from the QMap Docs. It allows explicit access to key and value (Java-Style iterator):

另一种方便的方法，来自QMap Docs。它允许显式访问键和值（Java 风格的迭代器）：

QMap<QString, QString> extensions;
// ... fill extensions
QMapIterator<QString, QString> i(extensions);
while (i.hasNext()) {
    i.next();
    qDebug() << i.key() << ": " << i.value();
}

In case you want to be able to overwrite, use QMutableMapIteratorinstead.

如果您希望能够覆盖，请QMutableMapIterator改用。

There's another convenient Qtmethod, if you're only interested in reading the values, without the keys (using Qts foreachand c++11):

还有另一种方便的Qt方法，如果您只对读取值感兴趣，而没有键（使用Qtsforeach和 c++11）：

QMap<QString, QString> extensions;
// ... fill extensions
foreach (const auto& value, extensions)
{
    // to stuff with value
}