6.5 Return Statements

To be honest: The same applies to generic functions. 

Reason: {AI95-00318-02} The requirement that a function body has to have at least one return statement is a “helpful” restriction. There has been some interest in lifting this restriction, or allowing a raise statement to substitute for the return statement. However, there was enough interest in leaving it as is that we decided not to change it. 

Ramification: {AI95-00318-02} A return statement can apply to an extended_return_statement, so a simple_return_statement without an expression can be given in one. However, neither simple_return_statement with an expression nor an extended_return_statement can be given inside an extended_return_statement, as they must apply (directly) to a function body. 

Discussion: In other words, if limited, the expression must produce a “new” object, rather than being the name of a preexisting object (which would imply copying). 

Discussion: We know that if the result type is class wide, then there must be an expression of the return statement. Similarly, if the result subtype is unconstrained, then either the return_subtype_indication (if any) is constrained, or there must be an expression.

Ramification: If the result type is controlled or has a controlled part, appropriate calls on Initialize or Adjust are performed prior to executing the handled_sequence_of_statements, except when the initial expression is an aggregate (which requires build-in-place with no call on Adjust).

If the return statement is left without resulting in a return (for example, due to an exception propagated from the expression or the handled_sequence_of_statements, or a goto out of the handled_sequence_of_statements), the return object is finalized prior to leaving the return statement.

Ramification: The conversion might raise Constraint_Error — (see 4.6). 

Proof: This is specified by the rules in 9.2. 

Reason: Only the caller can know when task activations should take place, as it depends on the context of the call. If the function is being used to initialize the component of some larger object, then that entire object must be initialized before any task activations. Even after the outer object is fully initialized, task activations are still postponed until the begin at the end of the declarative part if the function is being used to initialize part of a declared object. 

Ramification: {AI95-00318-02} The first sentence is true even if the tag of the expression is different, which could happen if the expression were a view conversion or a dereference of an access value. Note that for a limited type, because of the restriction to aggregates and function calls (and no conversions), the tag will already match.

Reason: {AI95-00318-02} The first rule ensures that a function whose result type is a specific tagged type always returns an object whose tag is that of the result type. This is important for dispatching on controlling result, and allows the caller to allocate the appropriate amount of space to hold the value being returned (assuming there are no discriminants).

The check prevents the returned object from outliving its type. Note that this check cannot fail for a specific tagged type, as the tag represents the function's type, which necessarily must be declared outside of the function. 

Reason: The check prevents the returned object (for a nonlimited type) from outliving the object designated by one of its discriminants. The check is made on the values of the discriminants, which may come from the return_subtype_indication (if constrained), or the expression, but it is never necessary to check both. 

Reason: Without such a permission, it would be very difficult to implement “build-in-place” semantics. Such an exception is not handleable within the function, because in the return-by-copy case, the constraint check to verify that the result satisfies the constraints of the object being initialized happens after the function returns, and we want the semantics to change as little as possible when switching between return-by-copy and build-in-place. This implies further that upon detecting such a situation, the implementation may need to simulate a goto to a point outside any local exception handlers prior to raising the exception. 

Ramification: This permission is allowed during the evaluation of the expression of an extended_return_statement, because the return_subtype_indication may be unconstrained and the expression then would provide the constraints. 

{AI95-00318-02} {incompatibilities with Ada 83} In Ada 95, if the result type of a function has a part that is a task, then an attempt to return a local variable will raise Program_Error. This is illegal in Ada 2005, see below. In Ada 83, if a function returns a local variable containing a task, execution is erroneous according to AI83-00867. However, there are other situations where functions that return tasks (or that return a variant record only one of whose variants includes a task) are correct in Ada 83 but will raise Program_Error according to the new rules.

The rule change was made because there will be more types (protected types, limited controlled types) in Ada 95 for which it will be meaningless to return a local variable, and making all of these erroneous is unacceptable. The current rule was felt to be the simplest that kept upward incompatibilities to situations involving returning tasks, which are quite rare. 

This clause has been moved here from chapter 5, since it has mainly to do with subprograms.

A function now creates an anonymous object. This is necessary so that controlled types will work.

{AI95-00318-02} We have clarified that a return statement applies to a callable construct, not to a callable entity.

{AI95-00318-02} There is no need to mention generics in the rules about where a return statement can appear and what it applies to; the phrase “body of a subprogram or generic subprogram” is syntactic, and refers exactly to “subprogram_body”.

{AI95-00318-02} {incompatibilities with Ada 95} The entire business about return-by-reference types has been dropped. Instead, the expression of a return statement of a limited type can only be an aggregate or function_call (see 7.5). This means that returning a global object or type_conversion, legal in Ada 95, is now illegal. Such functions can be converted to use anonymous access return types by adding access in the function definition and return statement, adding .all in uses, and adding aliased in the object declarations. This has the advantage of making the reference return semantics much clearer to the casual reader.

We changed these rules so that functions, combined with the new rules for limited types (7.5), can be used as build-in-place constructors for limited types. This reduces the differences between limited and nonlimited types, which will make limited types useful in more circumstances. 

{AI95-00318-02} {extensions to Ada 95} The extended_return_statement is new. This provides a name for the object being returned, which reduces the copying needed to return complex objects (including no copying at all for limited objects). It also allows component-by-component construction of the return object. 

{AI95-00318-02} The wording was updated to support anonymous access return subtypes.

{AI95-00318-02} The term “return expression” was dropped because reviewers found it confusing when applied to the default expression of an extended_return_statement.

{AI95-00344-01} {AI95-00416-01} Added accessibility checks to class-wide return statements. These checks could not fail in Ada 95 (as all of the types had to be declared at the same level, so the tagged type would necessarily have been at the same level as the type of the object).

{AI95-00402-01} {AI95-00416-01} Added accessibility checks to return statements for types with access discriminants. Since such types have to be limited in Ada 95, the expression of a return statement would have been illegal in order for this check to fail.

{AI95-00416-01} Added an Implementation Permission allowing early raising of Constraint_Error if the result cannot fit in the ultimate object. This gives implementations more flexibility to do built-in-place returns, and is essential for limited types (which cannot be built in a temporary).