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The following is a declaration of the concept "equality_comparable" from the <concepts> header of a C++20 standard library. This concept is satisfied by any type T such that for lvalues a and b of type T, the expressions a==b and a!=b as well as the reverse b==a and b!=a compile, and their results are convertible to a type that satisfies the concept "boolean-testable":
/* will print the provided char string as expected using ADL derived from the argument type std::cout */ operator << (std:: cout, "Hi there") /* calls a ostream member function of the operator<< taking a void const*, which will print the address of the provided char string instead of the content of the char string */ std:: cout. operator ...
To elaborate on the above example, consider a base class with no virtual functions. Whenever the base class calls another member function, it will always call its own base class functions. When we derive a class from this base class, we inherit all the member variables and member functions that were not overridden (no constructors or destructors).
The C++ Standard Library provides base classes unary_function and binary_function to simplify the definition of adaptable unary functions and adaptable binary functions. Adaptable function objects are important, because they can be used by function object adaptors: function objects that transform or manipulate other function objects.
Function identity now also includes trailing requires-clauses (P1971) Constrained non-template functions have been removed <compare> is now available in freestanding implementations [134] std::spans typedef was changed from index_type to size_type to be consistent with the rest of the standard library [135]
A closure-constructing operator creates a function object from a part of the program: the part of code given as an argument to the operator is part of the function, and so is the lexical environment: the bindings of the lexically visible variables are captured and stored in the function object, which is more commonly called a closure.
Notice that the type of the result can be regarded as everything past the first supplied argument. This is a consequence of currying, which is made possible by Haskell's support for first-class functions; this function requires two inputs where one argument is supplied and the function is "curried" to produce a function for the argument not supplied.
The allocator is required to have two member functions, A::construct and A::destroy (both functions have been deprecated in C++17, and removed in C++20), which handles object construction and destruction, respectively. The semantics of the functions should be equivalent to the following: [11]