Discovering Modern C++: An Intensive Course for Scientists, Engineers, and Programmers (C++ in-Depth Series) by Gottschling Peter

Author:Gottschling, Peter [Gottschling, Peter]
Language: eng
Format: azw3, epub
Publisher: Pearson Education
Published: 2015-12-22T16:00:00+00:00

vv.push_back(v1);

vv.push_back(v2);

vv.push_back(v3);

vv.push_back(v2);

vv.push_back(v1);

They are implicitly converted into reference wrappers (reference_wrapper<T> contains a constructor for T& that is not explicit).

The class contains a method get to get a reference to the actual object so that we can, for instance, print our vector:

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for (const auto& vr : vv) {

copy(begin(vr.get()), end(vr.get()),

ostream_iterator<int>(cout, ", "));

cout endl;

}

Here, the type of vr is const reference_wrapper<vector<int> >&. The wrapper also provides an implicit conversion to the underlying reference type T& that can be used more conveniently:

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for (const vector<int>& vr : vv) {

copy(begin(vr), end(vr), ostream_iterator<int>(cout, ", "));

cout endl;

}

The wrapper is complemented by two helpers: ref and cref, found in the same header. ref yields for an lvalue of type T an object of type reference_wrapper<T> referring to the former. If the argument of ref is already a reference_wrapper<T>, then it is just copied. Likewise, cref yields an object of type reference_wrapper<const T>. These functions are used in several places in the standard library.

We will use them for creating a std::map of references:

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map<int, reference_wrapper<vector<int> > > mv;

Given the length of the wrapper’s type name, it is shorter to declare it with type deduction like this:

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map<int, decltype(ref(v1))> mv;

The usual bracket notation of maps:

mv[4]= ref(v1); // Error

cannot be applied since the wrapper has no default constructor which is called internally in the expression mv[4] before the assignment takes place. Instead of the bracket notation, we should use insert or emplace:

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mv.emplace(make_pair(4, ref(v1)));

mv.emplace(make_pair(7, ref(v2)));

mv.insert(make_pair(8, ref(v3)));

mv.insert(make_pair(9, ref(v2)));

To iterate over the entries, it is again easier to apply type deduction:

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for (const auto& vr : mv) {

cout vr.first ": ";

for (int i : vr.second.get())

cout i ", ";

cout endl;

}

As the bracket operator of our map doesn’t compile, searching for a specific entry is performed with find :

auto& e7= mv.find(7)->second;

This yields a reference to the vector associated with the key 7.

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