操作是否为“假<"?真的"定义明确?
C++ 规范是否定义:
Does the C++ specification define:
- 布尔参数是否存在小于"运算符，如果存在，
- 4 个参数排列的结果?
换句话说，规范定义了以下操作的结果吗?
In other words, are the results from the following operations defined by the specification?
false < false
false < true
true < false
true < true
在我的设置 (Centos 7, gcc 4.8.2) 中，下面的代码给出了我的预期(鉴于 C 将 false 表示为 0 并将 true 表示为 1 的历史):
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
虽然我很确定大多数(所有?)编译器都会给出相同的输出，但这是由 C++ 规范规定的吗?还是允许混淆但符合规范的编译器决定真小于假?
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;
}
推荐答案
TL;DR:
这些操作根据 C++ 标准草案进行了很好的定义.
The operations are well defined according to the draft C++ standard.
详情
我们可以通过转到 C++ 标准草案 部分 5.9 关系运算符 说(强调我的未来):
We can see that by going to the draft C++ standard section 5.9 Relational operators which says (emphasis mine going forward):
操作数应具有算术、枚举或指针类型，或类型std::nullptr_t.运营商<(小于), > (大于), <= (小于或等于), 和 >= (大于或等于) 所有产量虚假或真实.结果的类型是bool
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
和 bool 是 3.9.1 基本类型中的算术类型
and bools are arithematic types from 3.9.1 Fundamental types
类型bool、char、char16_t、char32_t、wchar_t，有符号和无符号整数类型统称为称为整数类型.
Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types.
和
整数和浮点类型统称为算术类型.
Integral and floating types are collectively called arithmetic types.
和 true 和 false 是来自 2.14.6 的布尔文字:
and true and false are boolean literals from 2.14.6 Boolean literals:
boolean-literal:
false
true
回到 5.9 部分，进一步了解关系运算符的机制，它说:
Going back to section 5.9 to see the mechanics of the relational operators further, it says:
通常的算术转换是在算术或枚举类型的操作数上执行的.
The usual arithmetic conversions are performed on operands of arithmetic or enumeration type.
常用的算术转换在 5 部分中有介绍，其中说:
the usual arithmetic conversions are covered in section 5 which says:
否则，应在两个操作数上执行积分提升 (4.5)
Otherwise, the integral promotions (4.5) shall be performed on both operands
4.5 节说:
bool 类型的纯右值可以转换为 int 类型的纯右值，false 变为 0 和 true合而为一.
A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one.
等等表达式:
false < false
false < true
true < false
true < true
使用这些规则变成:
0 < 0
0 < 1
1 < 0
1 < 1
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