In modern C++ the standards have a wonderfully named extended friend declaration feature to address the class declarations which is a template parameter as a friend. This extended friend declaration is available with the C++ compiler that has C++11 and above standards. In this post, we explain a friend and extended friend declaration in modern C++.
Table of Contents
What is a friend declaration in modern C++?
The friend declaration is used in a class body with the friend keyword and grants a function or another class access to private and protected members of the class where the friend declaration appears.
C++11 extends the current language to support a wider range of friend declarations. Now, in modern C++has the extended friend declaration feature to address class declarations which is a template parameter as a friend. This extended friend declaration is available with a C++ compiler that has C++11 and above standards.
Syntaxes for the friend declaration (since C++11):
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friend typename_specifier ; |
and
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friend simple_type_specifier ; |
Is there a simple example of a friend declaration in modern C++??
Here is a simple friend declaration C++ example.
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class Ta; class Tx { friend Ta; }; |
How do I make a function declaration with friend declaration in modern C++?
In C++, we can declare functions with friend. Here is the syntax:
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friend function_declaration |
and we can declare the F() function with friend as shown below.
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float F(float m) { return m*9.81; // F=m.g } class Tx { friend float F(float m); }; |
How to create a function definition with friend declaration in modern C++?
In C++, we can define functions with friend. Here is the syntax how we can do,
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friend function_definition |
and we can declare a reset() function with friend as below.
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class Tx { int i; friend void reset( Tx& T) { T.i = 0; } }; |
What is an elaborated class specifier with friend declaration in modern C++?
According to the C++ standard, an elaborated type specifier must be used in a friend declaration for a class. That means the class-key of the elaborated_ type_specifier is required. In C++, we can use elaborated class specifier with friend. Here is the syntax.
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friend elaborated_class_specifier ; |
How to use a template with friend declaration in modern C++?
Here is an template example that uses friend.
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template <class Tx> class Tw { friend Tw; }; |
How to use a namespace with friend declaration in a class in modern C++?
Here is a friend example that is used in a class of a namespace.
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class Ts; namespace Ns { class Tx { friend Ts; }; } |
Is there a full example of an extended friend declaration in modern C++?
Here is a full example to extended friend declaration in modern C++.
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#include <iostream> class Ta; class Tb { friend Ta; }; //------------------------------------ class Tx { friend class Ta; }; //------------------------------------ template <class Tz> class Ty { friend Ty; }; //------------------------------------ class Ts; namespace Ns { class Tx { friend Ts; }; } //------------------------------------ float F(float m) { return m*9.81; // F=m.g } class Z { friend float F(float m); }; //------------------------------------ class Txx { int i; friend void reset( Txx& T) { T.i = 0; } }; //------------------------------------ template <class Tx> class Tw { friend Tw; }; //------------------------------------ int main() { class Tb b1; class Tx x1; class Z z1; class Txx xx1; Ns::Tx o1; return 0; } |
For more information about the extended friend declarations feature, see Extended friend declarations Proposal document.
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