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Show Lecture.NotFullySpecified as a slide show.

CS253 Not Fully Specified

What the language definition does not say.

The C++ standard defines several kinds of not-fully specified things:

Implementation-defined (§1.9.2):

A choice made by the compiler, must be documented

Unspecified behavior (§1.9.3):

A choice made by the compiler, need not be documented

Undefined behavior (§1.9.4):

All bets are off!

Implementation-defined behavior

A choice made by the compiler, which must be documented.

// Size of variables:
cout << sizeof(int) << '\n';

4

// Maximum value of a double:
double d = 6e307;
cout << d << '\n' << d*2 << '\n' << d*3;

6e+307
1.2e+308
inf

Such choices are often heavily influenced by the hardware.

Implementation-defined behavior examples

// Signed overflow:
short s = 32767;
cout << ++s;

-32768

// Character set:
switch ('$') {
    case 0x24: cout << "ASCII or UTF-8\n"; break;
    case 0x5b: cout << "EBCDIC\n";         break;
    default:   cout << "WTF!?\n";          break;
}

ASCII or UTF-8

Implementation-defined behavior examples

// The result of shifting a negative signed value right:
cout << (-1 >> 4) << '\n';

-1

// The result of system():
system("date");

Sun Apr 28 16:16:12 MDT 2024

Unspecified behavior

A choice made by the compiler, need not be documented or consistent, generally a “this-or-that” sort of choice.

// Order of evaluation of an expression (mostly):

int foo() { cout << "foo"; return 0; }
int bar() { cout << "bar"; return 0; }

int main() {
    return foo()*bar();
}

foobar

Unspecified behavior examples

// Comparing addresses of different objects:
int a,b;
cout << boolalpha << (&a < &b);

false

// Order of evaluation of function arguments:
int foo() { cout << "foo"; return 0; }
int bar() { cout << "bar"; return 0; }

void ignore_arguments(int, int) { }

int main() {
    ignore_arguments(foo(), bar());
}

barfoo

Unspecified behavior examples

I suspect that byte order (little-endian, big-endian) is unspecified, since a program can’t detect byte order without an unspecified operation:

int word = 0x12345678;
short *usp = reinterpret_cast<short *>(&word);
cout << hex << *usp << '\n';

5678

Undefined behavior

With undefined behavior, all bets are off! Anything can happen. Consistency is not required. Warnings are not required.

// Uninitialized & out-of-range values:
int a[135];
a[30] = 0;
for (int i=1; ; i*=2)
    cout << i << ": " << a[i] << endl;

1: 0
2: 0
4: 0
8: 0
16: 0
32: 0
64: 0
128: 4196656
256: 1382438695
512: 0
1024: 0
2048: 0
SIGSEGV: Segmentation fault

Undefined behavior examples

// Dereferencing a null pointer:
cout << "This will certainly be displayed!\n";
int *p = nullptr;
cout << *p << '\n';

SIGSEGV: Segmentation fault

// Shifting too far:
int amount=35;
cout << (1<<amount);

8

Undefined behavior examples

// Multiple writes to the same location
// in a single expression:
int a = 0;
cout << ++a + ++a << '\n';

c.cc:4: warning: operation on 'a' may be undefined
4

int b;
cout << b << '\n';

c.cc:2: warning: 'b' is used uninitialized in this function
0

g++ notices some undefined behavior, not all. This is a QOI (Quality Of Implementation) aspect, but not a standards-conformance issue.

But, why??

C++ is quite concerned about efficiency.

• The size of an int is implementation-defined so that the compiler can use the natural size provided by the architecture.
• The size of an double is implementation-defined so that the compiler can use the available hardware floating-point format.
• Some things are unspecified to give the compiler maximum freedom to generate fast code. Perhaps the compiler evaluates function arguments right-to-left because that’s the easiest order to push them onto the stack.

C++’s attitude is “You break the rules, you pay the price.” It doesn’t hold your hand.

Not just theoretical

Information from the Unisys C Compiler Programming Reference Manual:

Type	Bits	sizeof	Signed Range	Unsigned Max
char	9	1	−255 to 255	511
short	18	2	−217+1 to 217−1	218−1
int	36	4	−235+1 to 235−1	236−2
long	36	4	−235+1 to 235−1	236−2
long long	72	8	−271+1 to 271−1

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