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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");

Fri Nov 22 05:47:27 MST 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:
long long a[25][6];
a[0][0] = 0;
for (int j=0; j<8; j++) {
    for (int i=0; i<30; i++)
        cout << a[i][j] << ' ';
    cout << '\n';
}

0 140735868298136 7813586406938797358 2 0 73728 139703012354264 0 256 139703012328208 139703012328304 0 0 0 0 2 0 0 0 0 0 0 4196016 1 2 140735868298112 4902899136 0 0 0 
139703014666800 139703012452408 140735868297776 384 139703012375694 139703001832736 281470681751424 32 1000 139703012328248 139703012328344 0 0 0 0 0 0 0 0 0 0 0 139702998159452 4196464 4196573 4294967321 4196262 0 139703012405786 4196078 
1 1 140735868297824 4196640 139703009622192 0 1 140735868298136 139703012328304 139703012328208 0 0 0 0 0 0 0 0 0 0 0 0 140735868297824 7813586406938797358 139703012404432 4196496 0 1616078941733922814 0 140735868298104 
6295584 0 6294984 6295921 32 0 140735868298120 140735868298120 139703012328344 139703012328248 0 0 0 0 0 0 0 0 0 0 0 0 1 4295032831 0 139702998067173 -1616148622835308546 1696747404146518014 0 139703014666544 
1 139702997864728 1 4196016 139703009622166 139703007951824 140735868298136 0 139703012328304 139703012328344 0 0 0 0 0 0 0 0 0 0 0 5153960759809 140735868297824 140735868297840 4196496 139703009443168 4196032 140733193388032 0 1 
140735868298120 139702998159600 139703012375694 139703001832736 281470681751424 139703008716384 139703012375694 1000 139703012328344 0 0 0 0 0 0 0 0 0 0 0 0 139703001847112 6294984 4196486 4196032 140735868298120 140735868298112 0 0 140735868306098 
140735868298136 7813586406938797358 2 0 73728 139703012354264 0 256 139703012328208 139703012328304 0 0 0 0 2 0 0 0 0 0 0 4196016 1 2 140735868298112 4902899136 0 0 0 0 
139703012452408 140735868297776 384 139703012375694 139703001832736 281470681751424 32 1000 139703012328248 139703012328344 0 0 0 0 0 0 0 0 0 0 0 139702998159452 4196464 4196573 30064771096 4196262 0 139703012405786 4196078 140735868306106 

Undefined behavior examples

cout << "Hello, world!\n";
int *p = nullptr;
cout << *p << '\n'; // Kaboom!

SIGSEGV: Segmentation fault

Why don’t we see the desired output?

Undefined behavior examples

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

8

The standard states that you can’t shift more than the word size, and can’t shift a negative amount. Why?

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.

Things Be Changin’

This is undefined behavior in C++14:

int i=5;
i = i++;
cout << i;

c.cc:2: warning: operation on 'i' may be undefined
5

C++17, regarding assignment, says “The right operand is sequenced before the left operand”, so ++ finishes before =, and this output is guaranteed to be 6:

// c++17
int i=5;
i = i++;
cout << i;

c.cc:3: warning: operation on 'i' may be undefined
5

Looks like the compiler hasn’t caught up to the standard.

Not just theoretical

Information from the Unisys C Compiler Programming Reference Manual: