Is the operation "false < true" well defined?
Does the C++ specification define:
In other words, are the results from the following operations defined by the specification?
false < false
false < true
true < false
true < true
On my setup (Centos 7, gcc 4.8.2) , the code below spits out what I'd expect (given C's history of representing false as 0 and true as 1):
false < false = false
false < true = true
true < false = false
true < true = false
Whilst I'm pretty sure most (all?) compilers will give the same output, is this legislated by the C++ specification? Or is an obfuscating, but specification-compliant compiler allowed to decide that true is less than false?
#include <iostream>
const char * s(bool a)
{
return (a ? "true" : "false");
}
void test(bool a, bool b)
{
std::cout << s(a) << " < " << s(b) << " = " << s(a < b) << std::endl;
}
int main(int argc, char* argv[])
{
test(false, false);
test(false, true);
test(true, false);
test(true, true);
return 0;
}
TL;DR:
The operations are well defined according to the draft C++ standard.
Details
We can see that by going to the draft C++ standard section 5.9 Relational operators which says (emphasis mine going forward):
The operands shall have arithmetic , enumeration, or pointer type , or type std::nullptr_t. The operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) all yield false or true. The type of the result is bool
and bools are arithematic types from 3.9.1 Fundamental types
Types bool , char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types.
and
Integral and floating types are collectively called arithmetic types.
and true and false are boolean literals from 2.14.6 Boolean literals:
boolean-literal:
false
true
Going back to section 5.9 to see the mechanics of the relational operators further, it says:
The usual arithmetic conversions are performed on operands of arithmetic or enumeration type.
the usual arithmetic conversions are covered in section 5 which says:
Otherwise, the integral promotions (4.5) shall be performed on both operands
and section 4.5 says:
A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one.
and so the expressions:
false < false
false < true
true < false
true < true
using these rules become:
0 < 0
0 < 1
1 < 0
1 < 1
Boolean values are subject to the usual integer promotions, with false defined as 0 and true defined as 1 . That makes all the comparisons well defined.
According to the C++ Standard (5.9 Relational operators)
2 The usual arithmetic conversions are performed on operands of arithmetic or enumeration type.
and
1...The type of the result is bool.
and (3.9.1 Fundamental types)
6 Values of type bool are either true or false.49 [ Note: There are no signed, unsigned, short, or long bool types or values. —end note ] Values of type bool participate in integral promotions (4.5).
and (4.5 Integral promotions)
6 A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one .
So in all your examples true is converted to int 1 and false is converted to int 0
These expressions
false < false
false < true
true < false
true < true
are entirely equivalent to
0 < 0
0 < 1
1 < 0
1 < 1
上一篇: sizeof（）在c中的返回值？