問題描述

for (const auto &s : strs) 是什么意思?冒號 : 的作用是什么?

What does for (const auto &s : strs) mean? What is the function of colon :?

vector<string> &strs;
for (const auto &s : strs){
   //
}

推薦答案

它實際上是一個 C++11 特性，稱為基于范圍的 for 循環".

It's actually a C++11 feature called "range-based for-loops".

在這種情況下，它基本上是一個更容易編寫的替代品:

In this case, it's basically an easier-to-write replacement for:

// Let's assume this vector is not empty.
vector<string> strs;

const vector<string>::iterator end_it = strs.end();

for (vector<string>::iterator it = strs.begin(); it != end_it; ++it) {
  const string& s = *it;
  // Some code here...
}

: 是新語法的一部分.

在左側，您基本上有一個變量聲明，它將綁定到向量的元素，而在右側，您有一個要迭代的變量(也稱為范圍表達式").

On the left you basically have a variable declaration that will be bound to the elements of the vector and one the right you have the variable to iterate on (also called "range expression").

以下是解釋范圍表達式的先決條件的鏈接文檔的摘錄:

Here is an excerpt of the linked documentation that explains the prerequisites for the range-expressions:

range_expression 被評估以確定要迭代的序列或范圍.序列中的每個元素依次被取消引用并分配給具有 range_declaration 中給出的類型和名稱的變量.

range_expression is evaluated to determine the sequence or range to iterate. Each element of the sequence, in turn, is dereferenced and assigned to the variable with the type and name given in range_declaration.

begin_expr 和 end_expr 定義如下:

begin_expr and end_expr are defined as follows:

如果__range是一個數組，則begin_expr是__range，end_expr是(__range + __bound)，其中__bound是數組中元素的個數(如果數組大小未知或類型不完整，程序就出錯了-形成)

If __range is an array, then begin_expr is __range and end_expr is (__range + __bound), where __bound is the number of elements in the array (if the array has unknown size or is of an incomplete type, the program is ill-formed)

如果 __range 的類型是具有 begin 或 end 成員函數之一或兩者的類類型，則 begin_expr 是 __range.begin() 而 end_expr 是 __range.end();

If __range's type is a class type with either or both a begin or an end member function, then begin_expr is __range.begin() and end_expr is __range.end();

否則，begin_expr 是 begin(__range) 和 end_expr 是 end(__range)，它們是通過參數依賴查找找到的，std 作為關聯的命名空間.

Otherwise, begin_expr is begin(__range) and end_expr is end(__range), which are found via argument-dependent lookup with std as an associated namespace.

請注意，由于所有這些，基于范圍的 for 循環也支持迭代 C 數組，因為 std::begin/std::end 也適用于這些數組.

Note that thanks to all this, range-based for loops also support iterating over C arrays as std::begin/std::end works with those too.

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