CHAPTER 7

Blocks and Statements

The sequence of execution of a T program is controlled by a sequence of statements, which are executed for their effect and do not have values.

Some statements contain other statements as part of their structure; such other statements are substatements of the statement. In the same manner, some statements contain expressions (§8) as part of their structure.

Sequences of statements are organized into blocks. There are two primary blocks defined for a T program: the main block, which is explained in section (§7.1), and statement blocks which are explained in section (§7.2).

Statements that will be familiar to C and C++ programmers are the block (§7.3), empty (§7.4), if (§7.5), while (§7.6), return (§7.7), and delete (§7.8) statements.

7.1 Main Block

A program shall contain a global construct called main (the main block), which is the designated start of the execution of a program. Exactly one main block must exist for every T program. The block is entered when program execution starts. Program execution continues until a return statement in the main block is executed or the end of the main block is reached (or a run-time error is encountered). If the end of the main block is reached, then the result is as if the program executed a return of 0. The declaration of a main block includes the definition of a return type, which must be int. The return value exists to provide a way to pass a single status value to the surrounding environment.

All implementations of the main block will have the following definition:

MainFunctionDeclaration:

    int main() { MainBlockStatementsopt }

T does not allow for arguments to be passed into the main block.

The main block can exist at the beginning of the source file, at the end of the source file or in between any two class definitions within the source file.

Variable declarations can be provided at the outermost level of the main block (not at an inner block). The scope of a variable declared in the main block is from its declaration point to the end of the main block.

7.2 Statements

There are many kinds of statements in the T programming language. Most correspond to statements in the C and C++ languages. Statements are given by the following grammar:

Statement:

    Block

    EmptyStatement (7.4)

    ExpressionStatement (7.5)

    IfThenElseStatement (7.6)

    WhileStatement (7.7)

    ReturnStatement (7.8)

    DeleteExpression (7.9)

7.3 Blocks

A block is a sequence of statements within braces.

Block:

    { BlockStatementsopt }



BlockStatements:

    BlockStatement

    BlockStatements BlockStatement



BlockStatement:

    Statement

A block is executed by executing each of the statements in order from first to last (left to right). It is possible for a block to terminate early through a return statement.

7.4 The Empty Statement

An empty statement does nothing.

EmptyStatement:

    ;

7.5 Expression Statements

Certain kinds of expressions may be used as statements by following them with semicolons:

ExpressionStatement:

    StatementExpression ;



StatementExpression:

    Assignment

    MethodInvocation

An expression statement is executed by evaluating the expression; if the expression has a value, the value is discarded.

7.6 The if-then-else Statement

The if-then-else statement allows a conditional choice of two statements, executing one or the other but not both.

IfThenElseStatement:

    if ( Expression ) Statement else Statement

The Expression must have type int, or a compile-time error occurs.

An if-then-else statement is executed by first evaluating the Expression. Execution continues by making a choice based on the resulting value:

• If the value is not 0, then the first contained Statement (the one before the else keyword) is executed.
• If the value is 0, then the second contained Statement (the one after the else keyword) is executed.

7.7 The while Statement

The while statement executes an Expression and a Statement repeatedly until the value of the Expression is 0.

WhileStatement:

    while ( Expression ) Statement

The Expression must have type int, or a compile-time error occurs. A while statement is executed by first evaluating the Expression. Execution continues by making a choice based on the resulting value:

• If the value is not 0, then the contained Statement is executed.
  • If execution of the Statement completes, then the entire while statement is executed again, beginning by re-evaluating the Expression. (Note that the Statement can be exited early, via a return statement.)
• If the value of the Expression is 0, no further action is taken.

If the value of the Expression is 0 the first time it is evaluated, then the Statement is not executed.

7.8 The return Statement

A return statement returns control to the invoker of a method (§5.4, §8.10), returns control to the invoker of a constructor (§5.5, §8.7), returns control to the invoker of a destructor (§5.6, §7.9), or terminates the main block (§7.1), and is given by the following grammar:

ReturnStatement:

    return expression ;

    return;

Use of a return without an expression shall only be used within a constructor or destructor.

If a return statement is contained within a method, the value of the Expression becomes the value of the method invocation. More precisely, execution of such a return statement first evaluates the Expression. The value produced by the Expression is communicated to the invoker. A return statement with no Expression is not allowed in this context and will result in a compile-time error.

It is possible to return from the middle of a while or if-then-else block.

A compile-time error occurs if the type of the return expression is not convertible by assignment conversion to the return type of the enclosing method, or to int if the return statement is in the main block.

A run-time error occurs if the return type is an array or reference type and the run-time type of the return expression is not assignable to that type. That is, for reference types, the run-time type of the return expression must be the same type as the return type, or it must be a subclass of that type. For array types,

• the component types are both int; or
• the component types are both reference types and the component type of the return expression must be the same as the component type of the return type, or it must be a subclass of that type; or
• the component types are both array types in which case these rules are applied recursively.

7.9 The delete Statement

The DeleteExpression operator destroys a class or array object.

DeleteExpression:

    delete Expression

The Expression must denote a reference to a reference type. If not, a compile-time error is produced. A delete that attempts to access a null reference will result in a run-time error. The use of a reference (by a delete or in any other context) that has been deleted will result in undefined behavior.

The delete statement first invokes the destructor for the class of the object being deleted. Destructors automatically call the destructor of its class' immediate superclass, except for the destructor for the primordial Object class. The memory for the object is reclaimed after all superclass destructors have completed.

7.10 The out Statement

The out statement is a rudimentary mechanism for printing integers and it provides the only way to generate output in the T programming language. Its syntax is described by the following grammar:

OutputStatement:

    out Expression ;

If the Expression has type int then the out statement will print to stdout the ASCII equivalent of the integer Expression, followed by a newline (ASCII LF) character. All ASCII equivalents of integer expressions will be represented in decimal with no leading 0's and a single leading "-" if the integer is negative.

It is a compile-time error for the Expression to have a type other than int.

Author(s): Brian Mehlman (§7.1-7.5, §7.8-7.9), and Yu Li (§7.6-7.7)