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Not Fully Specified

What the language definition does not say.

Different types of odd stuff

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

A choice made by the compiler, must be documented.

• sizeof(int)
• maximum value of a double
• maximum identifier length (though there is a mininum maximum length)
• what happens when a signed int overflows
• what values the character set uses
  • In ASCII, 'i'+1 == 'j'
  • In EBCDIC, 'i'+8 == 'j'

Unspecified

A choice made by the compiler, need not be documented or consistent.

// Order of evaluation of an expression (mostly):
int foo() { cout << "foo\n"; return 1; }
int bar() { cout << "bar\n"; return 1; }
int main() {
    cout << foo()+bar() << '\n';
}

foo
bar
2

// Order of evaluation of function arguments:
int foo() { cout << "foo\n"; return 1; }
int bar() { cout << "bar\n"; return 1; }
void ignore_arguments(int, int) { }
int main() {
    ignore_arguments(foo(), bar());
}

bar
foo

Undefined behavior

All bets are off! Anything can happen. Warnings are not required.

int *p = nullptr;
cout << "alpha\n";
cout << *p << '\n';

SIGSEGV: Segmentation fault

int n = 1067316150;
cout << * (float *) &n << '\n';

1.234

int a = 0;
cout << ++a + ++a << '\n';

c.cc:2: warning: operation on 'a' may be undefined
4

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 in order to give the compiler the maximum freedom to generate code that is the fastest. 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.