C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 14:
[14.5] Should my class declare a member function or a friend function?

Use a member when you can, and a friend when you have to.

Sometimes friends are syntactically better (e.g., in class Fred, friend functions allow the Fred parameter to be second, while members require it to be first). Another good use of friend functions are the binary infix arithmetic operators. E.g., aComplex + aComplex should be defined as a friend rather than a member if you want to allow aFloat + aComplex as well (member functions don't allow promotion of the left hand argument, since that would change the class of the object that is the recipient of the member function invocation).

In other cases, choose a member function over a friend function.