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Initializer Lists

CS253 Initializer Lists

Initialization

Consider this code:

    vector<int> v = {11,22,33,44,55};

• What sort of thing is {11,22,33,44,55}?
• What type is it?
• No, not a flowery description—a C++ type.

initializer_list

• {11,22,33,44,55} is an object of the type initializer_list<int>.
• The compiler figures it out, just like it figures out that 2.71828 is a double.
• It’s a real class, with methods and everything.
  • iterator
  • .size()
  • .begin()
  • .end()
  • There is no .empty(), for are no empty initializer lists.
    • If it were empty, what type would it be?
    • initializer_list<what?>

The name

Why is the name so god-awful long?

• You think of a better name.
• We’re introducing a new word to the standard.
• If you call it, say, init, that breaks many user programs that already use init.
• That’s why these names are sometimes unwieldy—to maximize backward compatibility.

Standalone

Since an initializer_list has iterators, .begin(), and .end(), we can use it in a for loop:

for (auto v : {11,22,33,44,55})
    cout << v << ' ';

11 22 33 44 55 

or, more explicitly:

auto il = {11,22,33,44,55};
auto it = il.begin() + 3;
cout << *it << '\n';

44

or, even:

auto il = {11,22,33,44,55};
cout << *(il.begin()+3) << '\n';

44

Interaction with standard containers

So, how does this work?

unordered_set<int> us = {345,678,901,234,567,890};
for (auto v : us)
    cout << v << ' ';

890 567 234 901 678 345 

• The first line must be a ctor, since we are creating us.
• What is the declaration of that ctor?

Constructor using initializer_list

The ctor must be this:

    unordered_set(const initializer_list<value_type> &);

• As you recall, value_type is a typedef for the type being stored by the vector.
• Similarly, the assignment operator must be:

    unordered_set::operator=(const initializer_list<value_type> &)

Not Free

• To use { … } for your container, you must write methods:
  • ctor
  • assignment operator
• It doesn’t just come for free.

Example

class Hundred {
    int data[100];
  public:
    int &operator[](int n) { return data[n]; }
};

Hundred h;
h[1] = 400;
h[4] = 123;
cout << h[1]+h[4] << '\n';

523

Example that fails

class Hundred {
    int data[100];
  public:
    int &operator[](int n) { return data[n]; }
};

Hundred h = {11,22,33,44,55};
cout << h[1]+h[4] << '\n';

c.cc:7: error: could not convert '{11, 22, 33, 44, 55}' from '<brace-enclosed 
   initializer list>' to 'main()::Hundred'

Example

class Hundred {
    int data[100];
  public:
    Hundred(const initializer_list<int> &il) {
        copy(il.begin(), il.end(), data);
    }
    int &operator[](int n) { return data[n]; }
};

Hundred h = {11,22,33,44,55};
cout << h[1]+h[4] << '\n';

77