C templates and friends

Asked 10 years, 8 months ago. Active 10 years, 8 months ago. Viewed times. Sorry for the confusion. Adrian Adrian Cubbi: ohh, so when i define it inside of the class, there is no need for forward declaration, perfect explination, thank you : — Adrian. Add a comment. Active Oldest Votes. Member templates that are functions are discussed in Member Function Templates.

Nested class templates are declared as class templates inside the scope of the outer class. They can be defined inside or outside of the enclosing class. Class templates can have friends. A class or class template, function, or function template can be a friend to a template class. Friends can also be specializations of a class template or function template, but not partial specializations. In the following example, a friend function is defined as a function template within the class template.

This code produces a version of the friend function for every instantiation of the template. This construct is useful if your friend function depends on the same template parameters as the class does. There are many threads in the forum about the Friends and Templates.

The snag happens when the compiler sees the friend lines way up in the class definition proper. At that moment it does not yet know the friend functions are themselves templates; it assumes they are non-templates like this:. I must confess that i don't know much about compiler technology. It has a template parameter T and triangle brackets I am trying to think what other type it can be other than template function.

I am also confused on the assumption compiler make. Why does compile make this assumption but not a template function - which seems more reasonable to me. Whenever you declare friend function in class, which is template declared before , you have to signal compiler that, declared function is template by placing empty angle brackets or by qualifying it for e. If you don't do that either then declared friend function is supposed as declaration of new function if there is no previous declared non-template function.

Stack Overflow for Teams — Collaborate and share knowledge with a private group. The rules for how the compiler performs type deduction in function templates are based on the rules for ordinary functions. In the minimum template above, note that the type parameter T is not qualified in any way until it is used in the function call parameters, where the const and reference qualifiers are added. There is no practical limit to the number of type parameters. Separate multiple parameters by commas:.

The keyword class is equivalent to typename in this context. You can express the previous example as:. You can use the ellipsis operator Any built-in or user-defined type can be used as a type argument. The primary restriction when using templates is that a type argument must support any operations that are applied to the type parameters. For example, if we call minimum using MyClass as in this example:.

There is no inherent requirement that the type arguments for any particular template all belong to the same object hierarchy, although you can define a template that enforces such a restriction.

Note that the arguments must be pointers. The basic requirements that std::vector and other standard library containers impose on elements of T is that T be copy-assignable and copy-constructible. For example, you can provide a constant integral value to specify the length of an array, as with this example that is similar to the std::array class in the Standard Library:.

Note the syntax in the template declaration.