Compound literal expressions

A compound literal is a postfix expression that provides an unnamed object whose value is given by an initializer list. The C99 language feature allows you to pass parameters to functions without the need for temporary variables. It is useful for specifying constants of an aggregate type (arrays, structures, and unions) when only one instance of such types is needed. C++ only To be compatible with C99, the XL C/C++ compiler supports this feature as an IBM extension.

The syntax for a compound literal resembles that of a cast expression. However, a compound literal is an lvalue, while the result of a cast expression is not. Furthermore, a cast can only convert to scalar types or void, whereas a compound literal results in an object of the specified type.


Compound literal syntax

                       .-,----------------.      
                       V                  |      
>>-(--type_name--)--{----initializer_list-+--}-----------------><

The type_name can be any data type, including IBM extension begins vector IBM extension ends and user-defined types. It can be an array of unknown size, but not a variable length array. If the type is an array of unknown size, the size is determined by the initializer list.

The following example passes a constant structure variable of type point containing two integer members to the function drawline:

drawline((struct point){6,7});

If the compound literal occurs outside the body of a function, the initializer list must consist of constant expressions, and the unnamed object has static storage duration. If the compound literal occurs within the body of a function, the initializer list need not consist of constant expressions, and the unnamed object has automatic storage duration.

IBM extension For compatibility with GNU C, a static variable can be initialized with a compound literal of the same type, provided that all the initializers in the initializer list are constant expressions.

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