遍历 C++ STL 映射数据结构：新技术？（迭代范围和 'auto' 关键字）
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Iterating through a C++ STL map data structure: New technique? (Iteration over range and 'auto' keyword)
提问by KT100
So far I have always used an iterator for traversing through all the keys in an STL map as follows:
到目前为止，我一直使用迭代器来遍历 STL 映射中的所有键，如下所示：
for (std::map<char,int>::iterator it=mymap.begin(); it!=mymap.end(); ++it){
std::cout << it->first << " => " << it->second << '\n';
}
Very recently though I came across some code that used a different style to iterate through the keys as shown below. Has this feature been added only recently in revised standard? It seems like a rather interesting way of getting more done with lesser code, as many other languages already provide.
最近，虽然我遇到了一些使用不同样式来迭代键的代码，如下所示。是否最近才在修订标准中添加了此功能？正如许多其他语言已经提供的那样，这似乎是一种用更少的代码完成更多工作的有趣方式。
for (auto& x: mymap) {
std::cout << x.first << " => " << x.second << '\n';
}
Also, I am curious to know the exact implications of using the keyword "auto" here.
另外，我很想知道在此处使用关键字“auto”的确切含义。
回答by Karthik T
This code uses 2 new features from the latest C++ standard (C++11) the auto keyword, for type inference, and the range based for loop.
此代码使用最新 C++ 标准 (C++11) 中的 2 个新功能auto 关键字，用于类型推断和基于范围的 for loop。
Using just autothis can be written as (thanks Ben)
只使用auto这个可以写成（感谢 Ben）
for (auto it=mymap.begin(); it!=mymap.end(); ++it)
Using just range for this can be written as
仅使用范围可以写成
for (std::pair<const char,int>& x: mymap) {
std::cout << x.first << " => " << x.second << '\n';
}
Both of these do the exact same task as your two versions.
这两个版本与您的两个版本执行完全相同的任务。
回答by mcsilvio
The following worked for me:
以下对我有用：
for (auto x: mymap) {
cout << x.first << endl;
}
回答by BoshWash
In addition to the previous answers, C++17 added another approach using structured bindings:
除了前面的答案之外，C++17 添加了另一种使用结构化绑定的方法：
for (auto& [key, value]: mymap) {
std::cout << key << " => " << value << '\n';
}
回答by Alok Save
I am curious to know the exact implications of using the keyword "auto" here.
我很想知道在这里使用关键字“auto”的确切含义。
It enables:
它使：
- Less typing for a typical iterating code
- Less chances of manual errors because compiler deduces the exact type of the iterator.
- 减少典型迭代代码的输入
- 由于编译器推断出迭代器的确切类型，因此手动错误的机会更少。
回答by billz
It's new feature of C++11, it's called Range-Based for Loops, which iterates over all elements of a given range, array, or collection. It's what in other programming languages would be called a foreach loop The general syntax is as follows:
它是 C++11 的新特性，称为Range-Based for Loops，它迭代给定范围、数组或集合的所有元素。这就是其他编程语言中称为 foreach 循环的一般语法如下：
for ( decl : coll ) {
statement
}
Auto: Automatic Type Deduction with auto
Auto: 自动类型推导与 auto
With C++11, you can declare a variable or an object without specifying its specific type by using, for example:
使用 C++11，您可以声明一个变量或对象，而无需指定其特定类型，例如：
auto i = 42; // i has type int
double f();
auto d = f(); // d has type double
