对于 C++14 中不确定值和未定义行为的使用，C++ 标准是否发生了变化?

Has C++ standard changed with respect to the use of indeterminate values and undefined behavior in C++14?(对于 C++14 中不确定值和未定义行为的使用，C++ 标准是否发生了变化?)

问题描述

As covered in Does initialization entail lvalue-to-rvalue conversion? Is int x = x; UB? the C++ standard has a surprising example in section 3.3.2 Point of declaration in which an int is initialized with it's own indeterminate value:

int x = 12;
{ int x = x; }

Here the second x is initialized with its own (indeterminate) value. — end example ]

Which Johannes answer to this question indicates is undefined behavior since it requires an lvalue-to-rvalue conversion.

In the latest C++14 draft standard N3936 which can be found here this example has changed to:

unsigned char x = 12;
{ unsigned char x = x; }

Here the second x is initialized with its own (indeterminate) value. — end example ]

Has something changed in C++14 with respect to indeterminate values and undefined behavior that has driven this change in the example?

Yes, this change was driven by changes in the language which makes it undefined behavior if an indeterminate value is produced by an evaluation but with some exceptions for unsigned narrow characters.

Defect report 1787 whose proposed text can be found in N3914¹ was recently accepted in 2014 and is incorporated in the latest working draft N3936:

The most interesting change with respect to indeterminate values would be to section 8.5 paragraph 12 which goes from:

If no initializer is specified for an object, the object is default-initialized; if no initialization is performed, an object with automatic or dynamic storage duration has indeterminate value. [ Note: Objects with static or thread storage duration are zero-initialized, see 3.6.2. — end note ]

to (emphasis mine):

If no initializer is specified for an object, the object is default-initialized. When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced (5.17 [expr.ass]). [Note: Objects with static or thread storage duration are zero-initialized, see 3.6.2 [basic.start.init]. —end note] If an indeterminate value is produced by an evaluation, the behavior is undefined except in the following cases:

• If an indeterminate value of unsigned narrow character type (3.9.1 [basic.fundamental]) is produced by the evaluation of:

  • the second or third operand of a conditional expression (5.16 [expr.cond]),

  • the right operand of a comma (5.18 [expr.comma]),

  • the operand of a cast or conversion to an unsigned narrow character type (4.7 [conv.integral], 5.2.3 [expr.type.conv], 5.2.9 [expr.static.cast], 5.4 [expr.cast]), or

  • a discarded-value expression (Clause 5 [expr]),

  then the result of the operation is an indeterminate value.

• If an indeterminate value of unsigned narrow character type (3.9.1 [basic.fundamental]) is produced by the evaluation of the right operand of a simple assignment operator (5.17 [expr.ass]) whose first operand is an lvalue of unsigned narrow character type, an indeterminate value replaces the value of the object referred to by the left operand.

• If an indeterminate value of unsigned narrow character type (3.9.1 [basic.fundamental]) is produced by the evaluation of the initialization expression when initializing an object of unsigned narrow character type, that object is initialized to an indeterminate value.

and included the following example:

[ Example:

int f(bool b) {
  unsigned char c;
  unsigned char d = c; // OK, d has an indeterminate value
  int e = d;           // undefined behavior
  return b ? d : 0;    // undefined behavior if b is true
}

— end example ]

We can find this text in N3936 which is the current working draft and N3937 is the C++14 DIS.

Prior to C++1y

It is interesting to note that prior to this draft unlike C which has always had a well specified notion of what uses of indeterminate values were undefined C++ used the term indeterminate value without even defining it (assuming we can not borrow definition from C99) and also see defect report 616. We had to rely on the underspecified lvalue-to-rvalue conversion which in draft C++11 standard is covered in section 4.1 Lvalue-to-rvalue conversion paragraph 1 which says:

[...]if the object is uninitialized, a program that necessitates this conversion has undefined behavior.[...]

Footnotes:

1. 1787 is a revision of defect report 616, we can find that information in N3903

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