远程循环与文字列表？ [英] Ranged for loop with literal list?

问题描述

在C ++ 11中，是否有可能写下如下代码：

$ $ $ $ $ $ $ $ $ $ int int [] = {1，5， 6，2，9};
for（int n：ns）{
...
}

  for（int n：{1，5，6，2，9}）{ / VC ++ 11拒绝这种形式
 ... 
} 
  

解决方案

$ b

---

是的，它是有效的。 $ b

ranged-for的定义在 [C ++ 11：6.5.4 / 1] 为我们提供了这个构造的两种语法变体。一个在：的右侧采用表达式，另一个采用 braced-init-list em>。

$ b

您的 braced-init-list 通过 auto 一个 std :: initializer_list ，这是很方便的，因为这些东西可能会迭代。




<

$ p> > for（ for-range-declaration ： braced-init-list ）语句

$ b 让 init 等价于支撑-INIT列表。在每种情况下，基于范围的语句相当于

  {
 auto&& __range = range-init; 
 for（auto __begin = begin-expr，
 __end = end-expr; 
 __begin！= __end; 
 ++ __ begin）{
 for-range-declaration = * __开始; 
 statement 
 
 
 $ / code $ / pre 
 $ b $ 所以，你基本上是这么说的：
 
 
 
 
 
  $ p>  auto ns = {1，5，6，2，9}; 
 for（int n：ns）{
 // ... 
} 
  

（我还没有为这里的通用引用感到困扰。）



这反过来又是等价的到：



  std :: initializer_list< int> ns = {1，5，6，2，9}; 
 for（int n：ns）{
 // ... 
} 
  

现在， GCC 4.8支持这个，但是，因为Visual Studio 11实际上是Visual Studio 2012年，您需要升级以赶上： 不支持初始化列表直到Visual Studio 2013 。

In C++11, is it possible to write the following

int ns[] = { 1, 5, 6, 2, 9 };
for (int n : ns) {
   ...
}

as something like this

for (int n : { 1, 5, 6, 2, 9 }) { // VC++11 rejects this form
   ...
}

解决方案

tl;dr: Upgrade your compiler for great success.

---

Yeah, it's valid.

The definition of ranged-for in [C++11: 6.5.4/1] gives us two variants of syntax for this construct. One takes an expression on the right-hand-side of the :, and the other takes a braced-init-list.

Your braced-init-list deduces (through auto) to a std::initializer_list, which is handy because these things may be iterated over.

[..] for a range-based for statement of the form

for ( for-range-declaration : braced-init-list ) statement

let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent to

{
   auto && __range = range-init;
   for ( auto __begin = begin-expr,
              __end = end-expr;
        __begin != __end;
        ++__begin ) {
      for-range-declaration = *__begin;
      statement
   }
}

[..]

So, you are basically saying:

auto ns = { 1, 5, 6, 2, 9 };
for (int n : ns) {
   // ...
}

^{(I haven't bothered with the universal reference here.)}

which in turn is more-or-less equivalent to:

std::initializer_list<int> ns = { 1, 5, 6, 2, 9 };
for (int n : ns) {
   // ...
}

Now, GCC 4.8 supports this but, since "Visual Studio 11" is in fact Visual Studio 2012, you'll need to upgrade in order to catch up: initialiser lists were not supported at all until Visual Studio 2013.

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