用BOOST_FOREACH std::地图

I'd like to iterate over a std::map using BOOST_FOREACH and edit the values. I can't quite get it.

我想使用BOOST_FOREACH迭代std::map并编辑值。我不太明白。

typedef std::pair<int, int> IdSizePair_t;
std::map<int,int> mmap;    
mmap[1] = 1;
mmap[2] = 2;
mmap[3] = 3;
BOOST_FOREACH( IdSizePair_t i, mmap )
    i.second++;
// mmap should contain {2,3,4} here

Of course this doesn't change anything because I'm not iterating by reference. So I substitute this line instead (as per the example in the Boost docs):

当然，这不会改变任何东西，因为我没有按引用迭代。所以我替换了这一行(如Boost文档中的示例):

BOOST_FOREACH( IdSizePair_t &i, mmap )

and I get the compiler error:

我得到了编译错误:

error C2440: 'initializing' : 
cannot convert from 'std::pair<_Ty1,_Ty2>' to 'IdSizePair_t &'
    with
    [
        _Ty1=const int,
        _Ty2=int
    ]

Any suggestions?

有什么建议吗?

4 个解决方案

#1

The problem is with the first member of the pair, which should be const. Try this:

问题是这一对中的第一个元素，应该是const。试试这个:

typedef std::map<int, int> map_t;
map_t mmap;  
BOOST_FOREACH( map_t::value_type &i, mmap )
    i.second++;

#2

This is an old thread, but there is a more convenient solution.

这是一个旧的线程，但是有一个更方便的解决方案。

boost has the notion of 'range adapters' that perform a transformation on iterator ranges. There are specific range adapters for this exact use case (iterating over map keys or values): boost::adaptors::map_values and boost::adaptors::map_keys.

boost具有在迭代器范围上执行转换的“范围适配器”的概念。这个用例有特定的范围适配器(遍历映射键或值):boost::::adaptors:::map_values和boost:::adaptors:::map_keys。

So you could iterate over map values like this:

你可以像这样迭代映射值:

BOOST_FOREACH(int& size, mmap | boost::adaptors::map_values)
{ 
    ++size;
}

More information here.

更多的信息在这里。

#3

Another option is to use BOOST_FOREACH_PAIR, see my answer here:

另一个选项是使用BOOST_FOREACH_PAIR，请参阅我的答案:

BOOST_FOREACH & templates without typedef

没有类型定义的BOOST_FOREACH和模板

#4

As of C++11 consider using auto keyword:

在c++ 11中考虑使用auto关键字:

std::map<int,int> mmap;    
mmap[1] = 1;
mmap[2] = 2;
mmap[3] = 3;

BOOST_FOREACH(auto& mpair, mmap)
    mpair.second++;

//mmap will contain {2,3,4} here

#1