Identifiers

Identifier

An identifier is an arbitrarily long sequence of digits, underscores, lowercase and uppercase Latin letters, and most Unicode characters (disallowed are control characters and characters in the basic source character set). A valid identifier must begin with a non-digit character (Latin letter, underscore, or Unicode non-digit character). Identifiers are case-sensitive (lowercase and uppercase letters are distinct), and every character is significant.

In expressions

An identifier that names a variable, a function, or an enumerator can be used as an expression. The expression consisting of just the identifier returns the entity named by the identifier. The value category of the expression is lvalue if the identifier names a function, a variable, or a data member, and prvalue otherwise (e.g. an enumerator is a prvalue expression).

Within the body of a non-static member function, each identifier that names a non-static member is implicitly transformed to a class member access expression this->member.

这里有两个注意点，一个是value category,有以下几种lvalue,prvalue(pure rvalue)和xvalue。函数，变量和成员变量的value category是lvalue，其他的value category是prvalue。关于详细的value category官网有详细的解释，感兴趣的可以看下。后面可能会单独列出。另外一个是在非静态成员函数内，非静态成员变量是被隐式地转换成

this->member访问的。

Unqualified identifiers

Besides suitably declared identifiers, the following can be used in expressions in the same role:

• an overloaded operator name in function notation, such as operator+ or operator new;
• a user-defined conversion function name, such as operator bool;
• a user-defined literal operator name, such as operator "" _km;
• a template name followed by its argument list, such as MyTemplate<int>;
• the character ~ followed by a class name, such as ~MyClass;
• the character ~ followed by a decltype specifier, such as ~decltype(str).

Together with identifiers they are known as unqualified id-expressions.

对于qualified identifiers编译器执行的是qualified name lookup,对于unqualified identifiers编译器执行的是unqualified name lookup。

关于overloaded operator是一些重载操作;user-defined conversion function则是一些用户自定义的转换函数--Enables implicit conversion or explicit conversion from a class type to another type.如

struct X {
    //implicit conversion
    operator int() const { return 7; }
};

执行以下语句是可以的
X x;
int n = static_cast<int>(x);   // OK: sets n to 7
int m = x;                     // OK: sets m to 7