c++ - 什么是未命名的左值？

Question

我在草稿里看到了N4268下面划线部分的一个叫做“未命名左值”的概念

[ Note: Temporaries, unnamed lvalues, and named lvalues with no linkage are A temporary object is not an acceptable template-arguments when the corresponding template-parameter has reference type. [ Example: ... ] ]

我搜索了很多，但 stackoverflow 和 google 都没有给我答案。我只找到这篇关于值(value)类别的帖子

What are rvalues, lvalues, xvalues, glvalues, and prvalues

但这并没有帮助。

Answer 1

并不是所有的左值表达式都有名字。事实上，N4296 中的措辞给了我们一些例子:

For a non-type template-parameter of reference or pointer type, the value of the constant expression shall not refer to (or for a pointer type, shall not be the address of):

• a subobject (1.8),
• a temporary object (12.2),
• a string literal (2.14.5),
• the result of a typeid expression (5.2.8), or
• a predefined func variable (8.4.1).

此外，cppreference的关于值类别的信息详细介绍了左值:

An lvalue is an expression that identifies a non-temporary object or a non-member function.

The following expressions are lvalues:

• The name of a variable or function in scope, regardless of type, such as std::cin or std::endl. Even if the variable's type is rvalue reference, the expression consisting of its name is an lvalue expression.

• Function call or overloaded operator expression if the function's or overloaded operator's return type is an lvalue reference, such as std::getline(std::cin, str) or std::cout << 1 or str1 = str2 or ++iter

• Built-in pre-increment and pre-decrement, dereference, assignment and compound assignment, subscript (except on an array xvalue), member access (except for non-static non-reference members of xvalues, member enumerators, and non-static member functions), member access through pointer to data member if the left-hand operand is lvalue, comma operator if the right-hand operand is lvalue, ternary conditional if the second and third operands are lvalues.

• Cast expression to lvalue reference type.

• String literal

然而，这一切都无关紧要，因为更改的范围是提案 Allow constant evaluation for all non-type template arguments .