C++ Concepts

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• Data Type: Values and operations on those values.
• Expression
  • Sequence of operators and operands.
  • Involves same or different data types.
  • Semantics include value categories.
• Value Categories
  • It is either an lvalue or an rvalue
  • lvalue
    • Can be taken by address.
  • rvalue
    • Cannot be taken by address.
    • Usually temporary, lifetime in block scope, and/or a literal (12, nullptr, ‘a’)

    int x = 12 x is an lvalue and 12 is an rvalue 12 = x is illegal because rvalue cannot be used as an assignment

    caller see changes, callee can change data

    f(T&) takes an lvalue reference f(t) legal because t is an lvalue f(T()) illegal because T() is an rvalue and is temporary

    callee cannot change data

    f(const T&) takes an lvalue or rvalue reference f(t) legal because t is an lvalue f(T()) legal because the param is immutable and guarantees that the arg won’t change

    caller cannot see changes, callee can change data

    f(T&&) takes an rvalue reference f(t) illegal because t is an lvalue f(T()) legal because T() is an rvalue

Traits

Whether in a template or elsewhere, ::* is a C++ token, only usable in a type expression, in the context class_name::*. It declares a pointer to member.