[N3643] Statement expressions (draft)

The following paper contains a rough draft of wording for standardizing popular statement expressions extension.
With a side goal to enable them for evaluation as constant expressions.

Potential wording

6.8.7.3 The continue statement

A continue statement with an identifier operand shall appear within the body of a statement named by the label with the corresponding identifier.

6.8.7.4 The break statement

A break statement with an identifier operand shall appear within the body of a switch or iteration statement named by the label with the corresponding identifier.

6.8.? Statement expression

Syntax

statement-expression:
  ( compound-statement )

Description

A statement expression allows a block of statements to be treated as an expression, enabling the evaluation of the last statement within the block to yield a value.

Constraints

Jumps via jumps statements shall not enter into any statement-expression.

Jumps via jumps statements shall not exit statement-expression.

Semantics

Evaluation of a statement-expression is not interleaved with the evaluation of other parts of the containing expression.

The value and type of statement-expression are the same as of the last statement within its compound-statement, as if they were returned via a return statement. 1

If the compound-statement is empty, or its last statement is not an expression, the statement-expressions is a void expressions.

Objects declared within the compound-statement have block scope, and their lifetime is confined to the statement-expression.

For a statement-expression to be a constant expression2, it can consists only of statements evaluated during translation (like constant expressions, contstexpr declarations, or static_assert).

EXAMPLE 1   Statement expression as function-like macro 


      #define BIT(type, n) ({ \
          static_assert(n >= 0 && n < sizeof(type) * CHAR_BIT); \
          (type)1 << (n); \
      })

      int arr[BIT(unsigned int, 2)];

EXAMPLE 2   Statement expression as constant expression 


      unsigned char foo[({
          static_assert(sizeof (int) >= 4);
          constexpr auto s = sizeof (int);
          10 * s;
      })];
  1. Requirements of F.6 may apply
  2. Thus be eligible for use in file-scope initializations, static assertions, or other contexts requiring translation-time evaluation.

J.5.? Jumping out of statement expression

Implementations may allow jumping out of statement expressions via means of jumps statements.

 

References