How to iterate over a C++ STL map data structure using the 'auto' keyword?

Question

So far I have always used an iterator for traversing through all the keys in an STL map as follows:

    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.

    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.

Solution 1

This code uses 2 new features from C++11 standard the auto keyword, for type inference, and the range based for loop.

Using just auto this can be written as (thanks Ben)

for (auto it=mymap.begin(); it!=mymap.end(); ++it)

Using just range for this can be written as

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.

Solution 2

In addition to the previous answers, C++17 added another approach using structured bindings:

for (auto& [key, value]: mymap) {
        std::cout << key << " => " << value << '\n';
} 

Solution 3

The following worked for me:

for (auto x: mymap) {
  cout << x.first << endl;
}

Solution 4

I am curious to know the exact implications of using the keyword "auto" here.

It enables:

• Less typing for a typical iterating code
• Less chances of manual errors because compiler deduces the exact type of the iterator.

Solution 5

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:

for ( decl : coll ) {
    statement
}

Auto: Automatic Type Deduction with auto

With C++11, you can declare a variable or an object without specifying its specific type by using, for example:

auto i = 42; // i has type int
double f();
auto d = f(); // d has type double

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Comments

[Ben Voigt]

And the intermediate version would be for (auto it=mymap.begin(); it!=mymap.end(); ++it)

[KT100]

What's the benefit of using 'auto' here?

[Karthik T]

You dont need to type as much. With less boilerplate, the code should become more readable.

[Karthik T]

@KT100 basically at the statement auto it=mymap.begin() the compiler will automatically infer that it is the iterator type for mymap and set the appropriate type, without you needing to figure it out and type it.

[JoergB]

The "just range" loop won't work as posted, as map<char, int>::value_type is pair<const char, int>.

[Karthik T]

@KT100 This was a good example of the advantages of auto :P I wouldnt have messed it up.

[Mooing Duck]

This makes copies of all of the elements, and doesn't actually answer the questions that the poster asked.

[Tolli]

You can find more information and examples of range-based for loops here: en.cppreference.com/w/cpp/language/range-for

[Tolli]

herbsutter.com/2013/08/12/… This explains why you should (almost) always use auto.