ADL in constructor initialization list

Question

How can I enable ADL in a constructor initialization list? For example, let's say that I have a bignum that has a namespace-level abs function. Now I want to write a class Foo that initializes its members with the absolute values of the instances passed to the constructor; it should the use namespace-level abs if it exists and std::abs otherwise:

template<typename T>
struct Foo
{
    T _data;

    Foo(T data):
        _data(abs(data)) // I want find abs with ADL here
    {}
};

Using declarations are forbidden at class scope anyway and I don't want to "pollute" the namespace. How can I enable the ADL so that it works in the constructor initialization list?

Answer 1

You can solve this directly from within the initialiser list, by using a lambda expression: it allows you to write the required helper function inline, and that helper function can contain using std::abs;.

template<typename T>
struct Foo
{
    T _data;

    Foo(T data):
        _data([&](){ using std::abs; return abs(data); }())
    {}
};