CODEWITHSUNDEEP
c++argument-dependent-lookup
Hylke
Hylke

Reputation: 181

Why doesn't ADL work with functions defined outside of a namespace?

I know that my compiler in the example below will execute the function First::fun(), because of Argument-Dependent name Lookup (ADL)/Koenig lookup and in order to execute Second::fun() this needs to be explicitly called in the main function.

#include <iostream>
using namespace std;

namespace First
{
    enum Enum
    {
        FIRST
    };

    void fun(First::Enum symbol)
    {
        cout << "First fun\n";
    }
}

namespace Second
{
    void fun(First::Enum symbol)
    {
        cout << "Second fun\n";
    }
}

int main()
{
    fun(First::FIRST);  // Calls First::fun()
}

However, when adding another function fun() outside of the namespaces (see code below) and calling fun() without a prefixed namespace the compiler gives an ambiguity error. The functions inside the namespaces can still be called by explicitly prefixing the namespace, but fun() is unreachable. Why doesn't the compiler prefer the function outside of the namespaces when none are explicitly called? Is there a specific reason this behaviour is avoided?

// ^ Namespaces are still here

fun(First::Enum symbol)
{
    cout << "No namespace fun\n";
}    

int main()
{
    fun(First::FIRST);  // Doesn't compile: ambiguity!
}

EDIT

As Yksisarvinen rightfully pointed out, the global fun() can still be called by prefixing with the global namespace: ::fun(First::FIRST);.

However, this still leaves me with the question: Why doesn't the compiler prefer the global fun() in the ambiguous call?

Upvotes: 2

Views: 300

Answers (1)

songyuanyao
songyuanyao

Reputation: 172924

Why doesn't the compiler prefer the global fun() in the ambiguous call?

The global fun is found by unqualified name lookup, and First::fun is found by ADL, both are put in overload set and overload resolution can't select one.

These function names are looked up in the namespaces of their arguments in addition to the scopes and namespaces considered by the usual unqualified name lookup.

Upvotes: 5

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