Overload resolution

Remarks

Overload resolution happens in several different situations

  • Calls to named overloaded functions. The candidates are all the functions found by name lookup.
  • Calls to class object. The candidates are usually all the overloaded function call operators of the class.
  • Use of an operator. The candidates are the overloaded operator functions at namespace scope, the overloaded operator functions in the left class object (if any) and the built-in operators.
  • Overload resolution to find the correct conversion operator function or constructor to invoke for an initialization
    • For non-list direct initialization (Class c(value)), the candidates are constructors of Class.
    • For non-list copy initialization (Class c = value) and for finding the user defined conversion function to invoke in a user defined conversion sequence. The candidates are the constructors of Class and if the source is a class object, its conversion operator functions.
    • For initialization of a non-class from a class object (Nonclass c = classObject). The candidates are the conversion operator functions of the initializer object.
    • For initializing a reference with a class object (R &r = classObject), when the class has conversion operator functions that yield values that can be bound directly to r. The candidates are such conversion operator functions.
    • For list-initialization of a non-aggregate class object (Class c{1, 2, 3}), the candidates are the initializer list constructors for a first pass through overload resolution. If this doesn't find a viable candidate, a second pass through overload resolution is done, with the constructors of Class as candidates.

Arithmetic promotions and conversions

Converting an integer type to the corresponding promoted type is better than converting it to some other integer type.

void f(int x);
void f(short x);
signed char c = 42;
f(c); // calls f(int); promotion to int is better than conversion to short
short s = 42;
f(s); // calls f(short); exact match is better than promotion to int

Promoting a float to double is better than converting it to some other floating point type.

void f(double x);
void f(long double x);
f(3.14f); // calls f(double); promotion to double is better than conversion to long double

Arithmetic conversions other than promotions are neither better nor worse than each other.

void f(float x);
void f(long double x);
f(3.14); // ambiguous

void g(long x);
void g(long double x);
g(42); // ambiguous
g(3.14); // ambiguous

Therefore, in order to ensure that there will be no ambiguity when calling a function f with either integral or floating-point arguments of any standard type, a total of eight overloads are needed, so that for each possible argument type, either an overload matches exactly or the unique overload with the promoted argument type will be selected.

void f(int x);
void f(unsigned int x);
void f(long x);
void f(unsigned long x);
void f(long long x);
void f(unsigned long long x);
void f(double x);
void f(long double x);

Categorization of argument to parameter cost

Overload resolution partitions the cost of passing an argument to a parameter into one of four different categorizes, called "sequences". Each sequence may include zero, one or several conversions

  • Standard conversion sequence

    void f(int a); f(42);
    
  • User defined conversion sequence

    void f(std::string s); f("hello");
    
  • Ellipsis conversion sequence

    void f(...); f(42);
    
  • List initialization sequence

    void f(std::vector<int> v); f({1, 2, 3});
    

The general principle is that Standard conversion sequences are the cheapest, followed by user defined conversion sequences, followed by ellipsis conversion sequences.

A special case is the list initialization sequence, which does not constitute a conversion (an initializer list is not an expression with a type). Its cost is determined by defining it to be equivalent to one of the other three conversion sequences, depending on the parameter type and form of initializer list.

Exact match

An overload without conversions needed for parameter types or only conversions needed between types that are still considered exact matches is preferred over an overload that requires other conversions in order to call.

void f(int x);
void f(double x);
f(42); // calls f(int)

When an argument binds to a reference to the same type, the match is considered to not require a conversion even if the reference is more cv-qualified.

void f(int& x);
void f(double x);
int x = 42;
f(x); // argument type is int; exact match with int&

void g(const int& x);
void g(int x);
g(x); // ambiguous; both overloads give exact match

For the purposes of overload resolution, the type "array of T" is considered to match exactly with the type "pointer to T", and the function type T is considered to match exactly with the function pointer type T*, even though both require conversions.

void f(int* p);
void f(void* p);

void g(int* p);
void g(int (&p)[100]);

int a[100];
f(a); // calls f(int*); exact match with array-to-pointer conversion
g(a); // ambiguous; both overloads give exact match

Name lookup and access checking

Overload resolution occurs after name lookup. This means that a better-matching function will not be selected by overload resolution if it loses name lookup:

void f(int x);
struct S {
    void f(double x);
    void g() { f(42); } // calls S::f because global f is not visible here,
                        // even though it would be a better match
};

Overload resolution occurs before access checking. An inaccessible function might be selected by overload resolution if it is a better match than an accessible function.

class C {
  public:
    static void f(double x);
  private:
    static void f(int x);
};
C::f(42); // Error! Calls private C::f(int) even though public C::f(double) is viable.

Similarly, overload resolution happens without checking whether the resulting call is well-formed with regards to explicit:

struct X {
    explicit X(int );
    X(char );
};

void foo(X );
foo({4}); // X(int) is better much, but expression is 
          // ill-formed because selected constructor is explicit

Overloading on constness and volatility

Passing a pointer argument to a T* parameter, if possible, is better than passing it to a const T* parameter.

struct Base {};
struct Derived : Base {};
void f(Base* pb);
void f(const Base* pb);
void f(const Derived* pd);
void f(bool b);

Base b;
f(&b); // f(Base*) is better than f(const Base*)
Derived d;
f(&d); // f(const Derived*) is better than f(Base*) though;
       // constness is only a "tie-breaker" rule

Likewise, passing an argument to a T& parameter, if possible, is better than passing it to a const T& parameter, even if both have exact match rank.

void f(int& r);
void f(const int& r);
int x;
f(x); // both overloads match exactly, but f(int&) is still better
const int y = 42;
f(y); // only f(const int&) is viable

This rule applies to const-qualified member functions as well, where it is important for allowing mutable access to non-const objects and immutable access to const objects.

class IntVector {
  public:
    // ...
    int* data() { return m_data; }
    const int* data() const { return m_data; }
  private:
    // ...
    int* m_data;
};
IntVector v1;
int* data1 = v1.data();       // Vector::data() is better than Vector::data() const;
                              // data1 can be used to modify the vector's data
const IntVector v2;
const int* data2 = v2.data(); // only Vector::data() const is viable;
                              // data2 can't be used to modify the vector's data

In the same way, a volatile overload will be less preferred than a non-volatile overload.

class AtomicInt {
  public:
    // ...
    int load();
    int load() volatile;
  private:
    // ...
};
AtomicInt a1;
a1.load(); // non-volatile overload preferred; no side effect
volatile AtomicInt a2;
a2.load(); // only volatile overload is viable; side effect
static_cast<volatile AtomicInt&>(a1).load(); // force volatile semantics for a1

Overloading on Forwarding Reference

You must be very careful when providing a forwarding reference overload as it may match too well:

struct A {
    A() = default;           // #1
    A(A const& ) = default;  // #2

    template <class T>
    A(T&& );                 // #3
};

The intent here was that A is copyable, and that we have this other constructor that might initialize some other member. However:

A a;     // calls #1
A b(a);  // calls #3!

There are two viable matches for the construction call:

A(A const& ); // #2
A(A& );       // #3, with T = A&

Both are Exact Matches, but #3 takes a reference to a less cv-qualified object than #2 does, so it has the better standard conversion sequence and is the best viable function.

The solution here is to always constrain these constructors (e.g. using SFINAE):

template <class T,
    class = std::enable_if_t<!std::is_convertible<std::decay_t<T>*, A*>::value>
    >
A(T&& );

The type trait here is to exclude any A or class publicly and unambiguously derived from A from consideration, which would make this constructor ill-formed in the example described earlier (and hence removed from the overload set). As a result, the copy constructor is invoked - which is what we wanted.

Overloading within a class hierarchy

The following examples will use this class hierarchy:

struct A { int m; };
struct B : A {};
struct C : B {};

The conversion from derived class type to base class type is preferred to user-defined conversions. This applies when passing by value or by reference, as well as when converting pointer-to-derived to pointer-to-base.

struct Unrelated {
    Unrelated(B b);
};
void f(A a);
void f(Unrelated u);
B b;
f(b); // calls f(A)

A pointer conversion from derived class to base class is also better than conversion to void*.

void f(A* p);
void f(void* p);
B b;
f(&b); // calls f(A*)

If there are multiple overloads within the same chain of inheritance, the most derived base class overload is preferred. This is based on a similar principle as virtual dispatch: the "most specialized" implementation is chosen. However, overload resolution always occurs at compile time and will never implicitly down-cast.

void f(const A& a);
void f(const B& b);
C c;
f(c); // calls f(const B&)
B b;
A& r = b;
f(r); // calls f(const A&); the f(const B&) overload is not viable

For pointers to members, which are contravariant with respect to the class, a similar rule applies in the opposite direction: the least derived derived class is preferred.

void f(int B::*p);
void f(int C::*p);
int A::*p = &A::m;
f(p); // calls f(int B::*)

Steps of Overload Resolution

The steps of overload resolution are:

  1. Find candidate functions via name lookup. Unqualified calls will perform both regular unqualified lookup as well as argument-dependent lookup (if applicable).

  2. Filter the set of candidate functions to a set of viable functions. A viable function for which there exists an implicit conversion sequence between the arguments the function is called with and the parameters the function takes.

    void f(char);          // (1)
    void f(int ) = delete; // (2)
    void f();              // (3)
    void f(int& );         // (4)
    
    f(4); // 1,2 are viable (even though 2 is deleted!) 
          // 3 is not viable because the argument lists don't match
          // 4 is not viable because we cannot bind a temporary to 
          //     a non-const lvalue reference
    
  3. Pick the best viable candidate. A viable function F1 is a better function than another viable function F2 if the implicit conversion sequence for each argument in F1 is not worse than the corresponding implicit conversion sequence in F2, and...:

    3.1. For some argument, the implicit conversion sequence for that argument in F1 is a better conversion sequence than for that argument in F2, or

    void f(int );  // (1)
    void f(char ); // (2)
    
    f(4);  // call (1), better conversion sequence
    

    3.2. In a user-defined conversion, the standard conversion sequence from the return of F1 to the destination type is a better conversion sequence than that of the return type of F2, or

    struct A 
    {
        operator int();
        operator double();
    } a;
    
    int i = a; // a.operator int() is better than a.operator double() and a conversion
    float f = a; // ambiguous
    

    3.3. In a direct reference binding, F1 has the same kind of reference by F2 is not, or

    struct A 
    {
        operator X&();  // #1
        operator X&&(); // #2
    };
    A a;
    X& lx = a;  // calls #1
    X&& rx = a; // calls #2
    

    3.4. F1 is not a function template specialization, but F2 is, or

    template <class T> void f(T ); // #1
    void f(int );                  // #2
    
    f(42); // calls #2, the non-template
    

    3.5. F1 and F2 are both function template specializations, but F1 is more specialized than F2.

    template <class T> void f(T );  // #1
    template <class T> void f(T* ); // #2
    
    int* p;
    f(p); // calls #2, more specialized
    

The ordering here is significant. The better conversion sequence check happens before the template vs non-template check. This leads to a common error with overloading on forwarding reference:

struct A {
    A(A const& ); // #1
    
    template <class T>
    A(T&& );      // #2, not constrained
};

A a;
A b(a); // calls #2!
        // #1 is not a template but #2 resolves to
        // A(A& ), which is a less cv-qualified reference than #1
        // which makes it a better implicit conversion sequence

If there's no single best viable candidate at the end, the call is ambiguous:

void f(double ) { }
void f(float ) { }

f(42); // error: ambiguous


2016-07-22
2017-05-05
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