Ada Resource Association
News and resources for the Ada programming language
Ada Reference ManualLegal Information
Contents   Index   References   Search   Previous   Next 

6.5 Return Statements

1/2
A simple_return_statement or extended_return_statement (collectively called a return statement) is used to complete the execution of the innermost enclosing subprogram_body, entry_body, or accept_statement.

Syntax

2/2
simple_return_statement ::= return [expression];
2.1/3
extended_return_object_declaration ::= 
    defining_identifier : [aliased][constantreturn_subtype_indication [:= expression]
2.2/3
extended_return_statement ::= 
    return extended_return_object_declaration [do
        handled_sequence_of_statements
    end return];
2.3/2
return_subtype_indication ::= subtype_indication | access_definition

Name Resolution Rules

3/2
The result subtype of a function is the subtype denoted by the subtype_mark, or defined by the access_definition, after the reserved word return in the profile of the function. The expected type for the expression, if any, of a simple_return_statement is the result type of the corresponding function. The expected type for the expression of an extended_return_statement is that of the return_subtype_indication.

Legality Rules

4/2
A return statement shall be within a callable construct, and it applies to the innermost callable construct or extended_return_statement that contains it. A return statement shall not be within a body that is within the construct to which the return statement applies.
5/3
A function body shall contain at least one return statement that applies to the function body, unless the function contains code_statements. A simple_return_statement shall include an expression if and only if it applies to a function body. An extended_return_statement shall apply to a function body. An extended_return_statement with the reserved word constant shall include an expression.
5.1/2
  For an extended_return_statement that applies to a function body:
5.2/3
If the result subtype of the function is defined by a subtype_mark, the return_subtype_indication shall be a subtype_indication. The type of the subtype_indication shall be covered by the result type of the function. The subtype defined by the subtype_indication shall be statically compatible with the result subtype of the function; if the result type of the function is elementary, the two subtypes shall statically match. If the result subtype of the function is indefinite, then the subtype defined by the subtype_indication shall be a definite subtype, or there shall be an expression.
5.3/2
If the result subtype of the function is defined by an access_definition, the return_subtype_indication shall be an access_definition. The subtype defined by the access_definition shall statically match the result subtype of the function. The accessibility level of this anonymous access subtype is that of the result subtype.
5.4/3
If the result subtype of the function is class-wide, the accessibility level of the type of the subtype defined by the return_subtype_indication shall not be statically deeper than that of the master that elaborated the function body. 
5.5/3
  For any return statement that applies to a function body:
5.6/3
If the result subtype of the function is limited, then the expression of the return statement (if any) shall meet the restrictions described in 7.5
5.7/3
If the result subtype of the function is class-wide, the accessibility level of the type of the expression (if any) of the return statement shall not be statically deeper than that of the master that elaborated the function body.
5.8/3
If the subtype determined by the expression of the simple_return_statement or by the return_subtype_indication has one or more access discriminants, the accessibility level of the anonymous access type of each access discriminant shall not be statically deeper than that of the master that elaborated the function body.
5.9/3
  If the keyword aliased is present in an extended_return_object_declaration, the type of the extended return object shall be immutably limited. 

Static Semantics

5.10/3
   Within an extended_return_statement, the return object is declared with the given defining_identifier, with the nominal subtype defined by the return_subtype_indication. An extended_return_statement with the reserved word constant is a full constant declaration that declares the return object to be a constant object. 

Dynamic Semantics

5.11/3
   For the execution of an extended_return_statement, the subtype_indication or access_definition is elaborated. This creates the nominal subtype of the return object. If there is an expression, it is evaluated and converted to the nominal subtype (which might raise Constraint_Error — see 4.6); the return object is created and the converted value is assigned to the return object. Otherwise, the return object is created and initialized by default as for a stand-alone object of its nominal subtype (see 3.3.1). If the nominal subtype is indefinite, the return object is constrained by its initial value. A check is made that the value of the return object belongs to the function result subtype. Constraint_Error is raised if this check fails.
6/2
For the execution of a simple_return_statement, the expression (if any) is first evaluated, converted to the result subtype, and then is assigned to the anonymous return object.
7/2
If the return object has any parts that are tasks, the activation of those tasks does not occur until after the function returns (see 9.2). 
8/3
If the result type of a function is a specific tagged type, the tag of the return object is that of the result type. If the result type is class-wide, the tag of the return object is that of the type of the subtype_indication if it is specific, or otherwise that of the value of the expression. A check is made that the master of the type identified by the tag of the result includes the elaboration of the master that elaborated the function body. If this check fails, Program_Error is raised.
8.1/3
  If the result subtype of the function is defined by an access_definition designating a specific tagged type T, a check is made that the result value is null or the tag of the object designated by the result value identifies T. Constraint_Error is raised if this check fails. 
Paragraphs 9 through 20 were deleted. 
21/3
 If any part of the specific type of the return object of a function (or coextension thereof) has one or more access discriminants whose value is not constrained by the result subtype of the function, a check is made that the accessibility level of the anonymous access type of each access discriminant, as determined by the expression or the return_subtype_indication of the return statement, is not deeper than the level of the master of the call (see 3.10.2). If this check fails, Program_Error is raised.
22/3
 For the execution of an extended_return_statement, the handled_sequence_of_statements is executed. Within this handled_sequence_of_statements, the execution of a simple_return_statement that applies to the extended_return_statement causes a transfer of control that completes the extended_return_statement. Upon completion of a return statement that applies to a callable construct by the normal completion of a simple_return_statement or by reaching the end return of an extended_return_statement, a transfer of control is performed which completes the execution of the callable construct, and returns to the caller.
23/2
 In the case of a function, the function_call denotes a constant view of the return object. 

Implementation Permissions

24/3
 For a function call used to initialize a composite object with a constrained nominal subtype or used to initialize a return object that is built in place into such an object:
24.1/3
If the result subtype of the function is constrained, and conversion of an object of this subtype to the subtype of the object being initialized would raise Constraint_Error, then Constraint_Error may be raised before calling the function.
24.2/3
If the result subtype of the function is unconstrained, and a return statement is executed such that the return object is known to be constrained, and conversion of the return object to the subtype of the object being initialized would raise Constraint_Error, then Constraint_Error may be raised at the point of the call (after abandoning the execution of the function body). 

Examples

25
Examples of return statements: 
26/2
return;                         -- in a procedure body, entry_body,
                                -- accept_statement, or extended_return_statement
27
return Key_Value(Last_Index);   -- in a function body
28/2
return Node : Cell do           -- in a function body, see 3.10.1 for Cell
   Node.Value := Result;
   Node.Succ := Next_Node;
end return;

Contents   Index   References   Search   Previous   Next 
Ada-Europe Ada 2005 and 2012 Editions sponsored in part by Ada-Europe