16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.5 Constraints on programs [constraints] Function arguments [res.on.arguments]

Each of the following applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
  • If an argument to a function has an invalid value (such as a value outside the domain of the function or a pointer invalid for its intended use), the behavior is undefined.
  • If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
  • If a function argument is bound to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument, except that the argument passed to a move assignment operator may be a reference to *this ([lib.types.movedfrom]).
    [Note 1: 
    If the type of a parameter is a forwarding reference ([temp.deduct.call]) that is deduced to an lvalue reference type, then the argument is not bound to an rvalue reference.
    — end note]
    [Note 2: 
    If a program casts an lvalue to an xvalue while passing that lvalue to a library function (e.g., by calling the function with the argument std​::​move(x)), the program is effectively asking that function to treat that lvalue as a temporary object.
    The implementation is free to optimize away aliasing checks which would possibly be needed if the argument was an lvalue.
    — end note]