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Duck Typing

CS253 Duck Typing

Rules for Rulers

• The rules for becoming President of the USA are:
  • natural born citizen
  • thirty-five years old
  • been a resident for fourteen years
  • win the Electoral College

• The rules for becoming king or queen of the UK are:
  • Primogeniture
  • Protestant
• USA: what you can do (well, in theory)
• UK: who you are

Rules for Containers

• The rules for storing something in an STL container are:
  • default ctor
  • copy-ctor
  • assignment operator
  • less-than comparable, for ordered containers

Enforcement

• Who enforces the rules for ruling the USA or UK?
  • The courts
• Who enforces the rules for STL containers?
  • Nobody in particular.

Example

A vector requires that its stored type be copyable, both via copy ctor and assignment operator. For example, you can have a vector<string>, because strings are copyable

string now = "20241122T024848";
vector<string> v = {now, now, now};
cout << v.size() << '\n';

3

Example

How about a non-copyable class?

class Mom {
  public:
    Mom() = default;                        // I like the default ctor.
    Mom(const Mom &) = delete;              // You can’t change Mom!
    Mom &operator=(const Mom &) = delete;   // You can’t change Mom!
};

int main() {
    Mom helen;
    vector<Mom> v = {helen, helen, helen};
}

c.cc:10: error: use of deleted function 'Mom::Mom(const Mom&)'

How enforcement “works”

• STL containers do not contain explict checks for default-constructability, copy construction, and assignability.
• The STL containers merely use these operations.
  • If they work, great!
  • If they fail, well, enjoy your C++ error messages!