从双/ INT /字符串自动转换在C ++ bool的 [英] Automatic conversion from double/int/string to bool in C++

问题描述

我是谁一直努力学习一点C ++就在旁边对我的知识扩展的Java程序员。这里是一个小code段，我的认为的作品由于隐式转换，但我想知道哪些规范的一部分它是指什么是其他规则，我必须知道，当涉及到隐式转换。是否有一个文件/链接/站点那里它规定了隐式转换规则？

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, "\n"));
  return 0;
}

/*
output:

1
1
1
0
*/

TIA，结果
佐助

TIA,
sasuke

推荐答案

指针和整数，并且也布尔值，是的整型的。前三个都是要么指针或整数，因为它们是所有非零，他们转换为布尔值真正。类型的第四个值双击转换成一个零积分值，因此假。

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.

这不会再presentable作为积分值（如无穷大和NaN）双打的转换是不确定的。

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

请参阅4.9的详细信息，并且还4.12布尔转换：

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

算术，无范围枚举，指针，或指针到成员A型prvalue可以被转换成一个
  bool类型的prvalue。零值，空指针值或空成员指针值转换为false;
  任何其他值转换为真实的。

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.

您 0.0 是一个算术类型的零值。

Your 0.0 is an arithmetic type of zero value.

也许你可能不熟悉C字符串++：，表示数组的char [1] {0} ，和这个阵列（一个元件）衰变到一个指针到它的第一个元素，这是必然的非空指针。同样，ASDF表示数组的char [5] {A，S，D，F，0} ，再此衰减到一个（非空）指针到它的第一个元素。字符的实际值是完全无关紧要的。

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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