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CS253 Duck Typing

🦆 🦆 🦆 The Saying 🦆 🦆 🦆

This saying stresses common sense over formal definition. If I were doing 200mph on I-25 in Luke Skywalker’s speeder, I wouldn’t get far telling a state trooper that it’s “not a car”. The law would probably view it as a car, even without wheels.

Rules for Rulers

• The rules for president of the USA are:
  • natural born citizen
  • thirty-five years old
  • resident for fourteen years
  • winner of the Electoral College
    • not the popular vote

• The rules for king or queen of the UK are:
  • Absolute 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. You can have a vector<string>, because strings are copyable:

string now = "2024-11-21T06:52:42"; // ISO 8601, baby!
vector<string> v;
v.push_back(now);
cout << v.front() << '\n';

2024-11-21T06:52:42

Example

How about a non-copyable class?

vector<istream> v;
cout << "size=" << v.size() << endl;

size=0

vector<istream> v;
v.push_back(cin);

In file included from /usr/include/c++/8/x86_64-redhat-linux/bits/c++allocator.h:33,
                 from /usr/include/c++/8/bits/allocator.h:46,
                 from /usr/include/c++/8/string:41,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/istream:38,
                 from /usr/include/c++/8/sstream:38,
                 from /usr/include/c++/8/complex:45,
                 from /usr/include/c++/8/ccomplex:39,
                 from /usr/include/c++/8/x86_64-redhat-linux/bits/stdc++.h:52,
                 from c.cc:1:
/usr/include/c++/8/ext/new_allocator.h: In instantiation of 'void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::basic_istream<char>; _Args = {const std::basic_istream<char, std::char_traits<char> >&}; _Tp = std::basic_istream<char>]':
/usr/include/c++/8/bits/alloc_traits.h:475:   required from 'static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::basic_istream<char>; _Args = {const std::basic_istream<char, std::char_traits<char> >&}; _Tp = std::basic_istream<char>; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::basic_istream<char> >]'
/usr/include/c++/8/bits/stl_vector.h:1079:   required from 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::basic_istream<char>; _Alloc = std::allocator<std::basic_istream<char> >; std::vector<_Tp, _Alloc>::value_type = std::basic_istream<char>]'
c.cc:2:   required from here
/usr/include/c++/8/ext/new_allocator.h:136: error: use of deleted function 
   'std::basic_istream<_CharT, _Traits>::basic_istream(const 
   std::basic_istream<_CharT, _Traits>&) [with _CharT = char; _Traits = 
   std::char_traits<char>]'

How enforcement “works”

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