Kenneth Laskoski wrote:

Why is this nested namespace noncopyable_ necessary? The comment "// protection from unintended ADL" seems greek to me. Can someone please enlighten me and point out a practical consequence of simply defining "namespace boost { class noncopyable {...}; }"

I think this was the case few releases ago until someone probably got bitten by the following scenario. Consider this example. #include <iostream> namespace boost { // Imagine that the noncopyable is in the boost namespace as you suggest class noncopyable { /* ... */ }; // Imagine that you also included the following function from boost. // possibly - you have no idea as it is hidden somewhere deep in the // include chain template<typename T> void some_func( T ) { std::cout << "Boost func."; } } class MyClass : public boost::noncopyable { }; // If // 1. you forget to include header which declares your own // implementation of void some_func( MyClass ) or // 2. your some_func is in some other namespace which you // forget to explicitly qualify // ADL will resolve function call to the boost::some_func // and the program will successfully compile. int main() { MyClass m; some_func_x( m ); } HTH