是否有可能使用boost ::使用的foreach的std ::地图？

问题描述

我觉得非常有用，因为它为我节省了不少著述。例如，假设我想打印列表中的所有元素：

I find boost::foreach very useful as it saves me a lot of writing. For example, let's say I want to print all the elements in a list:

std::list<int> numbers = { 1, 2, 3, 4 };
for (std::list<int>::iterator i = numbers.begin(); i != numbers.end(); ++i)
   cout << *i << " ";

的boost ::的foreach使得code以上多simplier：

boost::foreach makes the code above much simplier:

std::list<int> numbers = { 1, 2, 3, 4 };
BOOST_FOREACH (int i, numbers)
   cout << i << " ";

好多了！不过，我从来没有想出一个办法（如果它是在所有可能的），用它来的std ::地图秒。文档只与类型的例子，如矢量或字符串。

Much better! However I never figured out a way (if it's at all possible) to use it for std::maps. The documentation only has examples with types such as vector or string.

推荐答案

您需要使用：

typedef std::map<int, int> map_type;
map_type map = /* ... */;

BOOST_FOREACH(const map_type::value_type& myPair, map)
{
    // ...
}

原因是，宏需要两个参数。当您尝试内联对定义，您引入第二个逗号，使得宏观三个参数来代替。在preprocessor不尊重任何C ++结构，它只知道文字。

The reason being that the macro expects two parameters. When you try to inline the pair definition, you introduce a second comma, making the macro three parameters instead. The preprocessor doesn't respect any C++ constructs, it only knows text.

所以，当你说 BOOST_FOREACH（对＆LT; INT，INT＆gt;中图）中，preprocessor看到这三个参数的宏：

So when you say BOOST_FOREACH(pair<int, int>, map), the preprocessor sees these three arguments for the macro:

1 对＆LT; INT 结果
 2. INT＆GT; 结果
 3. 地图

这是错误的。这是换每个文档中提到 。

Which is wrong. This is mentioned in the for-each documentation.

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