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Non-member non-friend interfaces

12345678910111213141516171819202122232425namespace ns { class foo { public: void member() { // Uses private data } private: // Private data }; void non_member(foo obj) { obj.member(); } } int main() { ns::foo obj; non_member(obj); }

This sample is licensed under the CC0 Public Domain Dedication.


Reduce dependencies on internal class details and improve encapsulation.


The foo class, defined on lines 3–13, has a single member function, member (lines 6–9), that requires access to foo’s private data.

The function non_member on lines 15–18 is also logically part of foo’s interface, yet has been defined as a non-member because it can be implemented in terms of member. This approach improves encapsulation by reducing the number of functions that are dependent on the private members of a class.

On line 24, argument-dependent lookup (ADL) allows us to call non_member without qualifying it with the ns namespace. The ADL rule specifies that the name of a function will be looked up in the namespaces of its arguments. As we are passing an ns::foo to non_member, the function is found in the ns namespace.


  • Joseph Mansfield

Last Updated

03 April 2015


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