Primary Expressions

 

The latest version of this topic can be found at Primary Expressions.

Primary expressions are the building blocks of more complex expressions. They are literals, names, and names qualified by the scope-resolution operator (::). A primary expression may have any of the following forms:

  
      literal  
      this  
:: namename( expression )  

A literal is a constant primary expression. Its type depends on the form of its specification. See Literals for complete information about specifying literals.

The this keyword is a pointer to a class object. It is available within nonstatic member functions and points to the instance of the class for which the function was invoked. The this keyword cannot be used outside the body of a class-member function.

The type of the this pointer is type *const (where type is the class name) within functions not specifically modifying the this pointer. The following example shows member function declarations and the types of this:

// expre_Primary_Expressions.cpp  
// compile with: /LD  
class Example  
{  
public:  
    void Func();          //  * const this  
    void Func() const;    //  const * const this  
    void Func() volatile; //  volatile * const this  
};  

See Type of this Pointer for more information about modifying the type of the this pointer.

The scope-resolution operator (::) followed by a name constitutes a primary expression. Such names must be names at global scope, not member names. The type of this expression is determined by the declaration of the name. It is an l-value (that is, it can appear on the left hand side of an assignment operator expression) if the declaring name is an l-value. The scope-resolution operator allows a global name to be referred to, even if that name is hidden in the current scope. See Scope for an example of how to use the scope-resolution operator.

An expression enclosed in parentheses is a primary expression whose type and value are identical to those of the unparenthesized expression. It is an l-value if the unparenthesized expression is an l-value.

In the context of the primary expression syntax given above, name means anything in the syntax described for Names, although when using the scope-resolution operator before the name, types of names that can only occur in a class are not allowed. This includes user-defined conversion function names, and destructor names.

Examples of primary expressions include:

100 // literal  
'c' // literal  
this // in a member function, a pointer to the class instance  
::func // a global function  
::operator + // a global operator function  
::A::B // a global qualified name  
( i + 1 ) // a parenthesized expression  

The examples below are all considered names, and hence primary expressions, in various forms:

MyClass // a identifier  
MyClass::f // a qualified name  
operator = // an operator function name  
operator char* // a conversion operator function name  
~MyClass // a destructor name  
A::B   // a qualified name  
A<int> // a template id  

Types of Expressions

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