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    Argument Dependent Lookup ADL

    What It Does What?

    Situation: Multiple Namespaces

    namespace pixl {
  class point { …
      point(int x, int y); …
  };
  void draw (point const&);
  void print (int);
}
    namespace geom {
  class point { …
      point(double x, double y); …
  };
  void draw (point const&);

}

    With ADL

    Functions are looked up in the namespace(s) of the argument type(s) involved in a call expression:

    pixl::point px {640, 480};
geom::point gm {1.1, 2.3};

draw(px);  // ADL ⇒ call pixl::draw
draw(gm);  // ADL ⇒ call geom::draw

    Argument px has type pixl::point ⇒ ADL looks for function draw in namespace pixl ⇒ matching function found and called.

    Explicit Qualification ⇒ No ADL Without ADL Without ADL

    No ADL is performed if a function call is explicitly qualified with a namespace name.

    Note that explicit qualification is necessary, if none of the function arguments has an associated namespace (like, e.g., fundamental types: int, double, …).

    pixl::draw(px);
geom::draw(gm);

pixl::draw(gm);  //  COMPILER ERROR: expects pixl::point
geom::draw(px);  //  COMPILER ERROR: expects geom::point

// needs qualification
pixl::print(5);
// because 'int' is not in namespace 'pixl'
print(5);        //  COMPILER ERROR: 'print' not found

    Standard Library Example Standard Lib Example Example

    #include <vector>
#include <algorithm>
std::vector<int> v {1,2,3,4};
sort(begin(v), end(v));  //  compiles!
    • argument v's type is std::vector
    • ADL looks for begin/end in namespace std
    • std::begin/std::end found and called
    • return type of begin/end is std::vector::iterator
    • ADL looks for sort in namespace std
    • std::sort found and called

    Why?

    Consistent Member/Non-Member Calls Consistent Function Calls Consistency

    object.fn(); looks up member function in object's class
    fn(object); looks up function in namespace of object's class

    Non-member functions mentioning class X (e.g., taking an argument of type X) that are in the same namespace as X are also part of X's interface.

    Non-Member Operators! Non-Member Operators! Operators!

    Example: Class point in 2 Namespaces
    • want support for expressions like 5 * point{4,1}
    • need functions operator * (NUMBER, point)
    • left hand operand not a point ⇒ operator* cannot be member function of point
    namespace pixl {
  class point { …
      point(int x, int y); …
  };
  // cannot be member function:
  point operator * (int factor, point const&);
}
namespace geom {
  class point { …
      point(double x, double y); …
  };
  point operator * (double factor, point const&);
}
    If there was no ADL
    • would need explicit qualification with ::
      auto p = pixl::operator*(2, px);
    • clumsy call syntax defeats purpose of operators (readability, infix notation, terseness)
    With ADL

    Operator functions are selected based on the namespace of their operand type(s):

    pixl::point px {640, 480};
geom::point gm {1.1, 2.3};

// px has type pixl::point// ⇒ ADL selects pixl::operator*
auto p = 2 * px;  

// gm has type geom::point// ⇒ ADL selects geom::operator*
auto x = 2 * gm;

    When and Where? Where?

    General Rules

    ADL is used to look up name f, if …
    • f is part of a function call expression, like e.g., f(arg)
    • f is the name of a function
    • f is called with at least one argument
    ADL is not used to look up name f, if …
    • the call to f is qualified, e.g., ns::f(arg)
    • the name f itself is contained in an expression like, e.g., (f)(arg)
    • there is already a class member named f in the lookup candidate set
    • there is already a block-scope declaration of function f in the lookup candidate set (types declared with using are ignored)
    • there is already a declaration of any other (variable/object, …) f that is not a function or function template in the lookup candidate set
    ADL is exclusively used for looking up …

    • non-member functions begin and end taking range in

      for(auto const& x : range) { … }

      if member-begin or member-end could not be found in range's class

    • non-member function get needed by structured bindings
    • names that depend on a template parameter (when a template is instantiated)

    When Calling Functions Function Calls

    namespace ns {
  class C { … };
  void f (C);
}
…
ns::C x;
    expression compiles? explanation
    f(x); f is a function name and x has class type ⇒ ADL finds f
    ns::f(x); call explicitly qualified with ns:: ⇒ no ADL
    (f)(x); (f) is not a simple function name, but an expression ⇒ no ADL ⇒ function not found
    using ns::f;
    (f)(x);    		 no ADL, but f was explicitly made visible for regular name lookup

    When Calling Function Templates Calling Function Templates Template Calls

    namespace ns {
  class C { … };
  template<class T> g(T);
  template<int i> h(C);
}
…
ns::C x;
    expression compiles? explanation
    g(x); ADL finds ns::g
    ns::g(x); qualification with ns:: ⇒ no ADL
    g<ns::C>(x); g<ns::C> is not a simple function name ⇒ no ADL ⇒ function template not found
    ns::h<0>(x); qualification with ns:: ⇒ no ADL
    h<0>(x); h<0> is not a simple function name ⇒ no ADL ⇒ function template not found
    using ns::h;
    h<0>(x);    		 function template ns::h was explicitly made visible for regular name lookup

    When Calling Lambdas ⇒ No ADL Calling Lambdas ⇒ No ADL Not For Lambdas

    namespace ns {
  class C { … };
  auto lm = [](C a) { … };  // lambda
}
…
C x;
    expression compiles? explanation
    lm(x); lm is not a function, but a compiler-generated function object ⇒ no ADL ⇒ lm not found
    using ns::lm;
    lm(x);    		 no ADL but object lm was explicitly made visible for regular name lookup

    How Does It Work? How?

    ADL examines the type of each argument in a function call expression f(arg1, arg2, …)
     (in no particular order) and determines the set of associated namespaces and classes which will also be searched for a function (or function template) named f.

    No Associated Namespace For… No Assoc. Namespace For… ADL ignores

    • fundamental types (int, double, …)
    • aliases introduced by using directives (B)
    namespace na {
  class A { … };
  void f(int);
  void g(A);
}
    namespace nb {
  using B = na::A;
  void h(B);

}
    Call ADL Examins Found
    f(0); NO ADL (int doesn't have namespace) NOTHING
    na::f(0); explicit qualification ⇒ NO ADL na::f
    nb::B b;
    h(b);    		 (nb::B ≡ na::A) ⇒ A, na NOTHING
    nb::B b;
    g(b);    		 (nb::B ≡ na::A) ⇒ A, na na::g

    Inline Namespaces

    ADL looks in:
    • enclosing namespace of inline namespace
    • inline namespaces of all previously found namespaces
    namespace lib {
  class A { … };

  inline namespace v2 {
    class B { … };
    void f(A const&);
  }
  namespace v1 {
    class B { … };
    void f(A const&);
  }

  void g(B const&);  // B refers to v2::B
}
    Call ADL Examins Found
    lib::A a;
    f(a);    		 lib → lib::v2 lib::v2::f(lib::A);
    lib::A a;
    lib::v2::f(a);    		 explicit qualification ⇒ NO ADL lib::v2::f(lib::A);
    lib::A a;
    lib::v1::f(a);    		 explicit qualification ⇒ NO ADL lib::v1::f(lib::A);
    lib::B b;
    g(b);    		 lib::v2 → lib lib::g(lib::v2::B);
    lib::v1::B b1;
    g(b1);    		 lib::v1 → lib NOTHING
    no such function
    g(lib::v1::B)

    Classes class C class Args

    ADL looks in:
    • C itself
    • base classes: direct (B) + indirect (A)
    • any class of which C is a member (X)
    • innermost enclosing namespaces of all classes in the set (na, nb, nx)
    Only ADL can find:
    namespace na {
  class A { … };
  void f(A);
}
namespace nb {
  class B : public na::A { … };
}
    namespace nx {
  class X { public: …
      class C : public nb::B { … };
      friend void f(C);
  };
  void f(X);
}
    Call ADL Examins Found
    na::A a;
    f(a);    		 A, na na::f(na::A)
    nb::B b;
    f(b);    		 B, A, nb, na // B → A
    na::f(na::A)
    nx::X x;
    f(x);    		 X, nx nx::f(nx::X)
    nx::X::C c;
    f(c);    		 C, X, B, A,
    nx, nb, na    		 // friend of X
    nx::f(nx::X::C)
    nx::X::C c;
    nx::f(c);    		 explicit qualification ⇒ NO ADL NOTHING
    nx::f(nx::X::C)
    only visible to ADL!

    Enumerations enum (class) E Enumeration Args

    ADL looks in:
    • enclosing namespace of E
    • enclosing class of E
    Only ADL can find:
    namespace ns {
  class C { public: …
    enum class E { x, y, z };
    friend void f(E);
  };
  void f(int, C::E);
}
    Call ADL Examins Found
    f(0,C::E::x); C, ns ns::f(int, ns::C::E);
    f(C::E::x); C, ns // friend of C
    ns::f(ns::C::E);

    Pointers/Addresses Pointer Args

    Raw Pointer (T*) T*

    ADL examins associated namespaces and classes of T

    namespace ns {
  class T { … };
  void g(T*);
}
    Call ADL Examins Found
    ns::T x;
    g(&x);    		 T, ns ns::g(ns::T*)

    Pointer To Class Data Member C::*

    ADL examins all associated namespaces and classes of the member type (A) and the containing class (C)

    // forward declaration of C
namespace nc { class C; } 
namespace na {
  class A { … };
  // pointer &darr; to C member
  void f( A nc::C::* );
}
    namespace nc {
  class C { public: …
    na::A x, y, z;
  };
  // pointer &darr; to C member
  void g( na::A C::* );
}
    Call ADL Examins Found
    na::A nc::C::* p = &nc::C::x;
    f(p);    		 A, na, C, nc na::f
    na::A nc::C::* p = &nc::C::x;
    g(p);    		 A, na, C, nc nc::g

    Function Pointer Y(*)(X)

    ADL examins associated namespaces and classes of all parameter types and the return type
    namespace na {  
  class A { }; 
  void f( void(*)(A) ) {}
}

void g(na::A);
    Call ADL Examins Found
    void(*p)(na::A) = g;
    f(p);    		 A, na na::f

    Pointer To Class Member Function Y(C::*)(X)

    ADL examins associated namespaces and classes of all parameter types, the return type and the containing class type
    namespace na {
  class A { … };
}
namespace nc {
  class C { public: …
    void set(na::A);
  };
  void f( void(nc::C::*)(na::A) );
}
    Call ADL Examins Found
    f( &nc::C::set ); C, nc, A, na nc::f

    Address Of Overloaded Function &(Y(*)(X){)}

    ADL examins every function or function template in the set of overloads (here both f functions)
    namespace na {
  class A { … };
  // set of overloads
  void f(A);
  void f(A, int);

  void g( void(*)(A) );
}
    Call ADL Examins Found
    g( &na::f ); A, na na::g

    Class Template Specializations Template Spec. Args

    ADL looks in:
    • template type itself (X) and its associated namespaces (nx)
    • types of all type template arguments (A1)
    • template template arguments (e.g., P2=A2): classes in which they are members
    • template template arguments (e.g., P2=A2): enclosing namespaces (n2)
    ADL skips arguments for:
    • non-type template parameters (int P3)
    • template template parameters (e.g., P2=A2: ADL skips A2 itself)
    namespace nx {
  template<class P1,            template<class> class P2,            int P3> 
  class X {…};
}
namespace n1 { class A1; }

    // specializations of X used as parameter:
namespace n2 {
  template<class> 
  class A2 { public: …
    friend void h( X<n1::A1,A2,5> const& );
  };
  void g( X<n1::A1,A2,5> const& );
}
namespace nx {
  void f( X<n1::A1,n2::A2,5> const& );
}

    // instantiation:
nx::X< n1::A1, n2::A2, 5> spx {};
    Call ADL Examins Found
    f(spx); X, nx, A1, n1, n2 nx::f
    g(spx); X, nx, A1, n1, n2 n2::g
    h(spx); X, nx, A1, n1, n2 NOTHING

    ADL-Related Idioms Idioms Idioms

    2-Step: ADL + Fallback ADL+Fallback

    template<class T>
void generic_function (…) {
    T x;
    T y;
    …
    using std::swap;
    swap(x, y);
    …
}
    This makes swap a customization point:
    • if custom swap(T&,T&) declared in T's namespace
      ⇒ visible to ADL ⇒ custom swap will be used
    • if no special overload for T exists
      ⇒ std::swap will be used as fallback (made visible with using directive)

    If you want to suppress ADL and make sure that std::swap is always used, namespace-qualify the function call: std::swap(x,y).

    Use the fallback idiom only in generic (template) code and only if a common fallback implementation, like e.g. std::swap exists.

    C++20  If you want to enforce constraints on the arguments through concepts you should use a customization point object to make sure that the constraints can't be bypassed by custom functions.

    Hidden Friends

    namespace math {
  class ratio { …
  public:
    explicit
    ratio(int64_t numerator,           int64_t denominator=1);
    …
    friend ratio operator + (ratio a, ratio b)     { … }
  };
}
    Only ADL can find math::operator+(ratio,ratio)
    • declaring it friend of ratio makes it a member of namespace math
    • it was not directly declared in namespace scope
    • it was not directly defined (=implemented) in namespace scope
    // ADL finds math::operator+(ratio,ratio)
math::ratio x {2,3}, y {1,6};
math::ratio z = x + y;  

    // qualified call ⇒ No ADL ⇒ operator+ not found!
math::ratio x {2,3}, y {1,6};
math::ratio w = math::operator+(x,y);  //  COMPILER ERROR

    Hidden friends are not callable through implicit conversions!

    class B { };

class A { public:
  A() {}
  A(B) {}    // implicit
  A(int) {}  // implicit

  friend void foo(A)   { } 
  friend void foo(B)   { } 
  friend void foo(int) { } 
};

void bar(A) { }
    A a1{ };      // 
A a2{ B{} };  // 
A a3{ 47 };   // 

    bar( A{} );  // 
// implicit conversions:
bar( B{} );  //  bar( A{B{}} )
bar( 123 );  //  bar( A{123} )

    // hidden friend 'foo':
foo( A{} );  // 
// can't find foo(B) & foo(int),
// no implicit conversions to 'A'
foo( B{} );  //  
foo( 123 );  //

    Example: No Constructor Conversions! Example 1

    namespace math {
  class tiny_ratio {
    // no operator+! (on purpose)
    …
  };
  class ratio { 
    …
  public:
    explicit
    ratio(int64_t numerator,           int64_t denominator=1);
    // implicit conversion from tiny_ratio:
    ratio(tiny_ratio r);
    …
    friend ratio operator + (ratio a, ratio b)     { … }
  };
}

    // No implicit conversions possible:
math::tiny_ratio r{1,2}, s{1,4};
auto t = r + s;   //  COMPILER ERROR ⇒ good!

    The hidden friend version of operator+(ratio,ratio) can only be found by ADL, so only if a ratio argument is involved in the call.

    Since we can implicitly construct a ratio from a tiny_ratio any non-hidden function taking ratio as input (by value or non-const reference) could also be called with tiny_ratio objects.

    So, if operator+(ratio,ratio) had been declared in namespace scope instead of as hidden friend, we could use it to add tiny_ratio objects despite the fact that we explicitly did not want that!

    Example: No Operator Conversions! Example 2

    Hidden friends also protect against conversion operators that implicitly convert other types to ratio!

    namespace math {
  class ratio { 
    …
  public: 
    friend ratio operator + (ratio a, ratio b)     { … }
    …
  };
  class my_num {
    …
  public: 
    …
    // conversion operator that can
    // create a 'ratio' from a 'my_num'
    operator ratio() const     { return ratio{…}; }
    // forgot operator + for my_num!
  };
}

    math::my_num u = 23;
math::my_num v = 47;
auto w = u + v ;  //  COMPILER ERROR: operator+ not found

    The hidden friend version of operator+(ratio,ratio) can only be found by ADL, so only if a ratio argument is involved in the call.

    Since my_num::operator ratio() can convert any my_num into a ratio, any non-hidden function taking ratio as input (by value or non-const reference) can also be called with my_num objects.

    So, if operator+(ratio,ratio) had been declared in namespace scope instead of as hidden friend, the last line would have compiled and yielded a math::ratio w!

    Disabling ADL with Niebloids Disabling ADL: Niebloids Niebloids

    Goal: prevent ADL from picking up custom overloads of a generic library function

    Idea: Use Function Object

    namespace lib {
  namespace detail {
    struct foo_fn {
      template<class T>
      void operator() (T const& x) const { … }
    };
  }
  // global function object lib::foo
  inline constexpr const detail::foo_fn foo {};
  // note: inline variable requires C++17
}

    ⇒ Consistent Behavior!

    namespace na {
  class A { … };
}
namespace nb {
  class B { … };
  // custom foo
  void foo(B const& b) {…}
}

    na::A a;
nb::B b;
// with explicit qualification:
lib::foo(a);  // lib::foo → lib::detail::foo
lib::foo(b);  // lib::foo → lib::detail::foo

    // same with using statement:
using lib::foo;
foo(a);      // lib::foo → lib::detail::foo
foo(b);      // lib::foo → lib::detail::foo
    • qualified and unqualified calls have the same effect
    • users cannot override library function's behavior

    C++20's standard library makes use of such function objects, that are also known as Niebloids (named after Eric Niebler).

    Customization Point Objects Customization Objects CPOs
    C++20

    Goal: consistent concept checking on (potential) customization points of a generic library!

    The traditional 2-Step ADL & Fallback Idiom:

    using std::swap; // fallback
    swap(x,y); // ADL may find custom swap

    cannot guarantee that a custom implementation of swap constrains its parameters the same way as the fallback implementation std::swap.

    Idea: Function Object as Single Entry Point Idea: Single-Entry Function Object

    All objects passed to any 'foo' implementation should satisfy concept 'Fooable':

    namespace lib {
  template<class T>   concept Fooable = requires (T x) { 
    {x.size()} -> std::integral; };

  namespace detail {
    inline namespace defaults {
      template<Fooable T>  // constrained!
      void foo(T const& x) { … }
    }
    struct foo_fn {
      template<Fooable T>  // constrained!
      void operator() (T const& x) const {
        // calls custom foo found by ADL
        // or fallback defaults::foo
        foo(x);
      }
    };
  }
  namespace defaults = detail::defaults;
  // global function object lib::foo
  inline constexpr const detail::foo_fn foo {};
}

    Make sure that your default implementation (here lib::defaults::foo) is not unconstrained or underconstrained! Otherwise it might act as catch-all and custom functions (found by ADL) won't be selected! Functions with forwarding reference parameters and/or variadic parameter packs are especially prone to being catch-alls.

    ⇒ Consistent Behavior

    namespace na {
  class A { public: size_t size() const; };
}
namespace nb {
  class B { public: int size() const; };
  void foo(B const& b);
}

    na::A a;
nb::B b;
using lib::foo;
foo(a);      // lib::foo → fallback lib::defaults::foo
foo(b);      // lib::foo → custom   nb::foo

    // explicit qualification has no effect:
lib::foo(a);  // lib::foo → fallback lib::defaults::foo
lib::foo(b);  // lib::foo → custom   nb::foo

    // force default implementation:
lib::defaults::foo(a);
lib::defaults::foo(b);
    • all calls are constrained with Fooable because they all invoke lib::detail::foo_fn::operator()
    • always does the same thing; no need for ADL+Fallback (ADL 2-step)
    • might be surprising that forcing default implementation is not possible with explicit qualification (because lib::foo is an object)

    ⇒ Cannot Bypass Constraints!

    namespace lib {
  template<class T>   concept Fooable = requires (T x) { 
    {x.size()} -> std::integral; };
  …
}

    Calls to lib::foo will never pick up unconstrained custom foo overloads:

    namespace nc {
  class C {   };  // no member 'size()'
  void foo(C const&);
}

    nc::C c;
lib::foo(c);  COMPILER ERROR: C doesn't satisfy 'lib::Fooable'
using lib::foo;
foo(c);       COMPILER ERROR: C doesn't satisfy 'lib::Fooable'
    class D { public: 
  // non-integral return type:
  double size() const { … } 
  friend void foo(D const&) { … }
};

    D d;
lib::foo(d);  COMPILER ERROR: D doesn't satisfy 'lib::Fooable'
using lib::foo;
foo(d);       COMPILER ERROR: D doesn't satisfy 'lib::Fooable'

    Related …

    Last updated: 2020-11-24