ADL (Argument-Dependent Lookup) is the rule that when calling a function with unqualified name, the compiler searches for candidates not just in local and global scopes but also in the namespaces of the function's arguments. This allows writing std::swap(a, b) and having the compiler find custom swap overloads in the same namespace as the arguments.

Use ADL to find overloaded functions for generic algorithms; qualify with std:: when you want only the standard library version.

This example shows ADL finding custom overloads based on argument namespaces.

// compile: g++ -o adl adl.cpp
// run: ./adl
// description: ADL finds overloads in argument namespaces
 
#include <iostream>
#include <utility>
 
namespace MyLib {
    class Widget {
    public:
        int value = 42;
    };
 
    void process(const Widget& w) {
        std::cout << "MyLib::process(Widget): " << w.value << "\n";
    }
}
 
void process(int x) {
    std::cout << "global process(int): " << x << "\n";
}
 
int main() {
    MyLib::Widget w;
    process(w);     // ADL finds MyLib::process because w is in MyLib
    process(42);    // global process(int)
 
    return 0;
}