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6.5 Return Statements

  1. A return_statement is used to complete the execution of the innermost enclosing subprogram_body, entry_body, or accept_statement.

    Syntax 

  2. return_statement ::= return [expression];

    Name Resolution Rules

  3. The expression, if any, of a return_statement is called the return expression. The result subtype of a function is the subtype denoted by the subtype_mark after the reserved word return in the profile of the function. The expected type for a return expression is the result type of the corresponding function.

    Legality Rules

  4. A return_statement shall be within a callable construct, and it applies to the innermost one. A return_statement shall not be within a body that is within the construct to which the return_statement applies.
  5. A function body shall contain at least one return_statement that applies to the function body, unless the function contains code_statements. A return_statement shall include a return expression if and only if it applies to a function body.

    Dynamic Semantics

  6. For the execution of a return_statement, the expression (if any) is first evaluated and converted to the result subtype.
  7. If the result type is class-wide, then the tag of the result is the tag of the value of the expression.
  8. If the result type is a specific tagged type:
    1. If it is limited, then a check is made that the tag of the value of the return expression identifies the result type. Constraint_Error is raised if this check fails.
    2. If it is nonlimited, then the tag of the result is that of the result type.

  1. A type is a return-by-reference type if it is a descendant of one of the following:
    1. a tagged limited type;
    2. a task or protected type;
    3. a nonprivate type with the reserved word limited in its declaration;
    4. a composite type with a subcomponent of a return-by-reference type;
    5. a private type whose full type is a return-by-reference type.

  1. If the result type is a return-by-reference type, then a check is made that the return expression is one of the following:
    1. a name that denotes an object view whose accessibility level is not deeper than that of the master that elaborated the function body; or
    2. a parenthesized expression or qualified_expression whose operand is one of these kinds of expressions.

  1. The exception Program_Error is raised if this check fails.
  2. For a function with a return-by-reference result type the result is returned by reference; that is, the function call denotes a constant view of the object associated with the value of the return expression. For any other function, the result is returned by copy; that is, the converted value is assigned into an anonymous constant created at the point of the return_statement, and the function call denotes that object.
  3. Finally, a transfer of control is performed which completes the execution of the callable construct to which the return_statement applies, and returns to the caller.

    Examples

  4. Examples of return statements: 
  5. return;
-- in a procedure body, entry_body, or accept_statement

return Key_Value(Last_Index);
-- in a function body

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