c++11 : range-based for loop

c++ range-based for loop sample

//z 2014-06-12 13:26:11 L.202'38029 BG57IV3@XCL T2508411853.K.F636940351 [T11,L175,R6,V152]

0. 形式

for ( declaration : expression ) statement

0.1 根据标准将会扩展成这样的形式：

1   {
2     auto&& __range = expression;
3     for (auto __begin = begin-expression,
4               __end = end-expression;
5          __begin != __end;
6          ++__begin)
7     {
8       declaration = *__begin;
9       statement
10    }
11  }

0.1.1  行3,4 ，begin 和 end 的判断规则：

The begin-expression and end-expression (lines 3 and 4) are determined as follows:

• A. If expression is an array, then begin-expression and end-expressionare __range and __range + __bound, respectively, where __bound is the array bound.
• B. If expression is of a class type that declares begin() and end()member functions, then begin-expression and end-expression are__range.begin() and __range.end(), respectively.
• C. Otherwise, begin-expression and end-expression are begin(__range)and end(__range), respectively, where the begin() and end()functions are looked up using the argument-dependent lookup (ADL) which also includes the std namespace.

With arrays taken care of by the first rule, the second rule makes sure that all the standard containers as well as all the user-defined ones that follow the standard sequence interface will work with range-based for out of the box. For example, in ODB (an ORM for C++), we have the container-like result class template which allows iteration over the query result. Because it has the standard sequence interface with a forward iterator, we didn’t have to do anything extra to make it work with range-based for.

The last rule (the fallback to the free-standing begin()and end()functions) allows us to non-invasively adapt an existing container to the range-based for loop interface.

0.2 类型推断

std::vector<int> v = {1, 2, 3, 5, 7, 11};
const std::vector<int> cv = {1, 2, 3, 5, 7, 11};
 
for (auto x: v) // x is int
  ...;
 
for (auto x: cv) // x is int
  ...;
 
for (auto& x: v) // x is int&
  ...;
 
for (auto& x: cv) // x is const int&

1.  例子

#include <iostream>
#include <vector>


int main ()
{
    std::vector<int> data = { 1, 2, 3, 4 };


    for ( int datum : data )
    {
        std::cout << datum << std::endl;
    }
}

/*output
1
2
3
4
*/

2.  性能上的考虑
2.1 每次循环会创建一份 a 的拷贝

for(autoa : a_vec)

{

}

2.2 避免拷贝

for(constauto&a : a_vec)

{

}

3. 一个实现了 container semantics 的例子：

3.1 simple iterator

#include <iostream>

using namespace std;

// forward-declaration to allow use in Iter
class IntVector;

class Iter
{
public:
	Iter(const IntVector* p_vec, int pos)
		: _pos(pos)
		, _p_vec(p_vec)
	{ }

	// these three methods form the basis of an iterator for use with
	// a range-based for loop
	bool
		operator!= (const Iter& other) const
	{
		return _pos != other._pos;
	}

	// this method must be defined after the definition of IntVector
	// since it needs to use it
	int operator* () const;

	const Iter& operator++ ()
	{
		++_pos;
		// although not strictly necessary for a range-based for loop
		// following the normal convention of returning a value from
		// operator++ is a good idea.
		return *this;
	}

private:
	int _pos;
	const IntVector *_p_vec;
};

class IntVector
{
public:
	IntVector()
	{
	}

	int get(int col) const
	{
		return _data[col];
	}
	Iter begin() const
	{
		return Iter(this, 0);
	}

	Iter end() const
	{
		return Iter(this, 100);
	}

	void set(int index, int val)
	{
		_data[index] = val;
	}

private:
	int _data[100];
};

int
Iter::operator* () const
{
	return _p_vec->get(_pos);
}

// sample usage of the range-based for loop on IntVector
int main()
{
	IntVector v;
	for (int i = 0; i < 100; i++)
	{
		v.set(i, i);
	}
	for (int i : v) { cout << i << endl; }
}

3.2 reverse iterator

template <typename T>
struct reverse_range
{
private:
  T& x_;
 
public:
  reverse_range (T& x): x_ (x) {}
 
  auto begin () const -> decltype (this->x_.rbegin ())
  {
    return x_.rbegin ();
  }
 
  auto end () const -> decltype (this->x_.rend ())
  {
    return x_.rend ();
  }
};
 
template <typename T>
reverse_range<T> reverse_iterate (T& x)
{
  return reverse_range<T> (x);
}
 
std::vector<int> v = {1, 2, 3, 5, 7, 11};
 
for (auto x: reverse_iterate (v))

4. 一个完整的例子 (编译出错，说找不到容器 begin end 实现)

#include <iostream>
#include <fstream>
#include <string>
#include <iterator>
#include <algorithm>
#include <unordered_map>
 
template<classITERATOR>
ITERATOR begin( std::pair<ITERATOR,ITERATOR> &range )
{
    returnrange.first;
}
 
template<classITERATOR>
ITERATOR end( std::pair<ITERATOR,ITERATOR> &range )
{
    returnrange.second;
}
 
template<classT>
std::istream_iterator<T> begin(std::istream_iterator<T> &ii_stream)
{
    returnii_stream;
}
 
template<classT>
std::istream_iterator<T> end(std::istream_iterator<T> &ii_stream)
{
    returnstd::istream_iterator<T>();
}
 
intmain(intargc, char* argv[])
{
    std::ifstream data( "sowpods.txt");
    std::unordered_map<std::string,int> counts;
    std::unordered_multimap<std::string,std::string> words;
 
    for( conststd::string &s : std::istream_iterator<std::string>( data ) )
    {
        std::string temp = s;
        std::sort(temp.begin(), temp.end() );
        counts[temp]++;
        words.insert( std::make_pair(temp,s) );
    }
    auto ii = std::max_element( counts.begin(), 
                                counts.end(),
                                [](conststd::pair<std::string,int> &v1, 
                                   conststd::pair<std::string,int> &v2)
                                {
                                    returnv1.second < v2.second; 
                                }
                              );
    std::cout << "The maximum anagram family has " << ii->second << " members:
";
    for( constauto &map_entry : words.equal_range( ii->first ) )
        std::cout << map_entry.second << " ";
    std::cout << std::endl;
    return0;
}

//z 2014-06-12 13:26:11 L.202'38029 BG57IV3@XCL T2508411853.K.F636940351 [T11,L175,R6,V152]
5. 一些 wrapper 或 iterator 例子

#include <memory>
#include <iterator>

/*  Only provides the bare minimum to support range-based for loops.
    Since the internal iterator of a range-based for is inaccessible,
    there is no point in more functionality here. */
template< typename iter >
struct range_iterator_reference_wrapper
    : std::reference_wrapper< iter > {
    iter &operator++() { return ++ this->get(); }
    decltype( * std::declval< iter >() ) operator*() { return * this->get(); }
    range_iterator_reference_wrapper( iter &in )
        : std::reference_wrapper< iter >( in ) {}
    friend bool operator!= ( range_iterator_reference_wrapper const &l,
                             range_iterator_reference_wrapper const &r )
        { return l.get() != r.get(); }
};

namespace unpolluted {
    /*  Cannot call unqualified free functions begin() and end() from 
        within a class with members begin() and end() without this hack. */
    template< typename u >
    auto b( u &c ) -> decltype( begin( c ) ) { return begin( c ); }
    template< typename u >
    auto e( u &c ) -> decltype( end( c ) ) { return end( c ); }
}

template< typename iter >
struct range_proxy {
    range_proxy( iter &in_first, iter in_last )
        : first( in_first ), last( in_last ) {}

    template< typename T >
    range_proxy( iter &out_first, T &in_container )
        : first( out_first ),
        last( unpolluted::e( in_container ) ) {
        out_first = unpolluted::b( in_container );
    }

    range_iterator_reference_wrapper< iter > begin() const
        { return first; }
    range_iterator_reference_wrapper< iter > end()
        { return last; }

    iter &first;
    iter last;
};

template< typename iter >
range_proxy< iter > visible_range( iter &in_first, iter in_last )
    { return range_proxy< iter >( in_first, in_last ); }

template< typename iter, typename container >
range_proxy< iter > visible_range( iter &first, container &in_container )
    { return range_proxy< iter >( first, in_container ); }

Usage:

#include <vector>
#include <iostream>
std::vector< int > values{ 1, 3, 9 };

int main() {
    // Either provide one iterator to see it through the whole container...
    std::vector< int >::iterator i;
    for ( auto &value : visible_range( i, values ) )
        std::cout << "# " << i - values.begin() << " = " << ++ value << '
';

    // ... or two iterators to see the first incremented up to the second.
    auto j = values.begin(), end = values.end();
    for ( auto &value : visible_range( j, end ) )
        std::cout << "# " << j - values.begin() << " = " << ++ value << '
';
}

for(auto i : ForIterator(some_list)) {
    // i is the iterator, which was returned by some_list.begin()
    // might be useful for whatever reason
}

The implementation was not that difficult:

template <typename T> struct Iterator {
    T& list;
    typedef decltype(list.begin()) I;

    struct InnerIterator {
        I i;
        InnerIterator(I i) : i(i) {}
        I operator * () { return i; }
        I operator ++ () { return ++i; }
        bool operator != (const InnerIterator& o) { return i != o.i; }
    };

    Iterator(T& list) : list(list) {}
    InnerIterator begin() { return InnerIterator(list.begin()); }
    InnerIterator end() { return InnerIterator(list.end()); }
};
template <typename T> Iterator<T> ForIterator(T& list) {
    return Iterator<T>(list);
}

//z 2014-06-12 13:26:11 L.202'38029 BG57IV3@XCL T2508411853.K.F636940351 [T11,L175,R6,V152]
6. auto 部分的简单指导原则：
auto x : 使用拷贝
auto &x : 使用引用，指向原item，并且可能变更其值
const auto&x ：指向原item，并且保证不改变其值

7. MAP 例子

Each element of the container is a map<K, V>::value_type, which is a typedef for std::pair<const K, V>. Consequently, you'd write this as

for (auto& kv : myMap) {
    std::cout << kv.first << " has value " << kv.second << std::endl;
}

如前所述，为效率考虑，使用reference，如果不改变其值（如这里），还应该加上 const 。

8. 来自 ms 的例子

Executes statement repeatedly and sequentially for each element in expression.

for ( for-range-declaration : expression ) 
   statement

Remarks

Use the range-based for statement to construct loops that must execute through a "range", which is defined as anything that you can iterate through—for example, std::vector, or any other STL sequence whose range is defined by a begin() and end(). The name that is declared in the for-range-declaration portion is local to the for statement and cannot be re-declared in expression or statement. Note that the auto keyword is preferred in the for-range-declaration portion of the statement.

This code shows how to use ranged for loops to iterate through an array and a vector:

C++

// range-based-for.cpp
// compile by using: cl /EHsc /nologo /W4
#include <iostream>
#include <vector>
using namespace std;

int main() 
{
    // Basic 10-element integer array.
    int x[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

    // Range-based for loop to iterate through the array.
    for( int y : x ) { // Access by value using a copy declared as a specific type. 
                       // Not preferred.
        cout << y << " ";
    }
    cout << endl;

    // The auto keyword causes type inference to be used. Preferred.

    for( auto y : x ) { // Copy of 'x', almost always undesirable
        cout << y << " ";
    }
    cout << endl;

    for( auto &y : x ) { // Type inference by reference.
        // Observes and/or modifies in-place. Preferred when modify is needed.
        cout << y << " ";
    }
    cout << endl;

    for( const auto &y : x ) { // Type inference by reference.
        // Observes in-place. Preferred when no modify is needed.
        cout << y << " ";
    }
    cout << endl;
    cout << "end of integer array test" << endl;
    cout << endl;

    // Create a vector object that contains 10 elements.
    vector<double> v;
    for (int i = 0; i < 10; ++i) {
        v.push_back(i + 0.14159);
    }

    // Range-based for loop to iterate through the vector, observing in-place.
    for( const auto &j : v ) {
        cout << j << " ";
    }
    cout << endl;
    cout << "end of vector test" << endl;
}

Here is the output:

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

end of integer array test

0.14159 1.14159 2.14159 3.14159 4.14159 5.14159 6.14159 7.14159 8.14159 9.14159

end of vector test

A range-based for loop terminates when one of these in statement is executed: a break, return, or goto to a labeled statement outside the range-based for loop. A continue statement in a range-based for loop terminates only the current iteration.

Keep in mind these facts about range-based for:

• Automatically recognizes arrays.

• Recognizes containers that have .begin() and .end().

• Uses argument-dependent lookup begin() and end() for anything else.