call function without namespace, is it a feature or a bug?

January 24, 2024

namespace ns {
    void f() {}

    struct Foo {};
    void foof(Foo) {};
}
int main() {
    // f(); // cannot find f
    foof(ns::Foo{}); // call to ns::foof(Foo)
}

I found we can leave out namespace when calling a function with type defined in the same namespace.
I didn’t find this rule in cppreference namespace. Is it a feature or a compiler bug?

I tried several gcc and clang compiler and they all work.

>Solution :

Is it a feature or a compiler bug?

It’s a c++ feature called Argument Dependant Lookup, or ADL.

From cppreference:

Argument-dependent lookup, also known as ADL, or Koenig lookup 1, is
the set of rules for looking up the unqualified function names in
function-call expressions, including implicit function calls to
overloaded operators. 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.

In your case since you specified the namespace for the Foo argument, the compiler was able to lookup the function foof in the same namespace.