在 C++ 中从 double/int/string 自动转换为 bool

Automatic conversion from double/int/string to bool in C++(在 C++ 中从 double/int/string 自动转换为 bool)

问题描述

我是一名 Java 程序员，一直在尝试学习一点 C++ 以扩展我的知识.这是我认为由于隐式转换而起作用的一个小代码片段，但我想知道它引用了规范的哪一部分，以及什么时候我必须注意的其他规则它涉及隐式转换.是否有制定隐式转换规则的文档/链接/站点?

I'm a Java programmer who has been trying to learn a bit of C++ on the side to expand on my knowledge. Here is a small code snippet which I think works due to implicit conversion but I'd like to know which part of the specification does it refer to and what are the other rules which I must be aware of when it comes to implicit conversion. Is there a document/link/site out there which lays down the implicit conversion rules?

#include <vector>
#include <iostream>
#include <iterator>

int main(void) {
  using namespace std;
  vector<bool> a;
  a.push_back("asdf");
  a.push_back("");  
  a.push_back(12);  
  a.push_back(0.0);  
  copy(a.begin(), a.end(), ostream_iterator<bool>(cout, "
"));
  return 0;
}

/*
output:

1
1
1
0
*/

TIA，
佐助

推荐答案

指针和整数以及布尔值都是整数类型.前三个都是指针或整数，因为它们都是非零的，所以它们转换为布尔值 true.double 类型的第四个值转换为零整数值，因此 false.

Pointers and integers, and also booleans, are integral types. The first three are all either pointers or integers, and since they are all non-zero, they convert to the boolean value true. The fourth value of type double converts to a zero integral value and hence false.

无法表示为整数值(如无穷大和 NaN)的双精度数的转换未定义.

Conversion of doubles that are not representable as integral values (like infinity and NaN) is undefined.

有关详细信息，请参阅 4.9，有关布尔转换"，请参阅 4.12:

See 4.9 for details, and also 4.12 for "Boolean conversions":

算术纯右值、无范围枚举、指针或指向成员类型的指针可以转换为bool 类型的纯右值.将零值、空指针值或空成员指针值转换为 false；任何其他值都将转换为 true.

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

你的 0.0 是零值的算术类型.

Your 0.0 is an arithmetic type of zero value.

也许你可能不熟悉 C++ 中的字符串字面量:"" 表示数组 char[1] { 0 }，而这个数组(一个元素)衰减为指向其第一个元素的指针，该元素必然是非空指针.类似地，asdf"表示一个数组char[5] { 'a', 's', 'd', 'f', 0 }，这又一次衰减为a(非空)指向其第一个元素的指针.角色的实际价值完全无关紧要.

Perhaps you may not be familiar with string literals in C++: "" denotes the array char[1] { 0 }, and this array (of one element) decays to a pointer to its first element, which is necessarily a non-null pointer. Similarly, "asdf" denotes an array char[5] { 'a', 's', 'd', 'f', 0 }, and again this decays to a (non-null) pointer to its first element. The actual value of the characters is entirely immaterial.

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