英文:
when define variable with bool data type and we will use cin to get user input.expected 0 or 1
问题
当我写x等于3时，我超出了预期，因为我没有写零或一，但尽管如此，这个非零值隐式地转换为布尔值，所以输出是1。
#include <iostream>
using namespace std;
int main() {
bool x;
cin >> x;
cout << x << "\n";
return 0;
}
当我写x等于文本"hallo"时，我也超出了预期，因为我没有写零或一，但在这种情况下，这个非零值没有隐式地转换为布尔值，所以输出是0。难道不应该预期文本"hallo"会转换为布尔值，因此输出应该是1吗？
假设输出将是1，因为文本值"hallo"被转换为布尔值，所以输出应该是1。因此，如果你告诉我cin >>从一开始就期望输入零或一的值，那么你违反了预期，所以输出是零。我会回答你说，当我输入x的值为3时，我也违反了预期，因为我应该输入零或一的值，然而输出是1，那么当我输入文本值时为什么不会发生这种情况，为什么它不会隐式转换为布尔值呢？
英文:
When I write that x is 3
I thus exceeded expectations because I did not write a zero or a 1
but nevertheless this non-zero value is implicitly converted into a Boolean value so ouput is 1.
#include <iostream>
using namespace std;
int main() {
bool x;
cin>>x;
cout << x<< "\n";
return 0;
}
When I write that x is text hallo
I thus exceeded expectations because I did not write a zero or a 1
but in this case this non-zero value is not implicitly converted into a Boolean value
so output is 0
Isn't it supposed this text hallo is converted to boolean value so output is 1.
It is assumed that the output will be 1 because the text value hallo is converted to boolean value so output is 1. So if you told me that the cin >> expects from the beginning to enter a value of zero or one .. then you violated the expectations, so the output is zero. I will reply to you and say that when I entered that the value of x is 3, I also violated expectations because I was supposed to enter a value of one or zero, and yet the output was one, so why did this not happen when I entered the text value and why was it not implicitly converted to a boolean value
答案1
得分: 1
一般而言，您需要区分流上是否设置了std::boolalpha。
如果没有设置，那么bool会像其他无符号整数一样对待，在这种情况下，范围从0到1（包括1）。如果我们根本无法读取任何数值（这对于任何非数值初始字符都是如此），则结果为0（即false）；如果我们可以读取但值太大以至于无法表示（对于bool来说，任何大于1的数都会被转换为一个巨大的正数），则结果是整数的最大值，对于bool来说是1（即true）。在这两种情况下，流的失败位将被设置，您可以通过检查流的状态来检查它：
bool x;
if (std::cin >> x)
std::cout << "good\n";
else
std::cout << "fail\n";
std::cout << x << "\n";
如果应用了std::boolalpha，情况就会改变 - 此时任何匹配true或false的文本都会转换为相应的值，而任何其他文本都会导致false，同时设置了失败位。
参考 std::num_get::get，其中 operator>> 也有涉及。
英文:
In general: You need to differentiate, is std::boolalpha set on the stream or not?
If not, then bool is treated like any other unsigned integral, in given case with range from 0 up to including 1. If we cannot read any numeric value at all (which is the case for any non-numeric initial character) then the result set is 0 (i.e. false); if we can read but the value is too large to be represented (which for bool any number > 1 – and negative numbers will be converted to a huge positive one) the result is the maximum value of the integral, which for bool is 1 (i.e. true). In both cases the fail-bit of the stream will be set, and you can check it by checking the stream state:
bool x;
if(std::cin >> x)
std::cout << "good\n";
else
std::cout << "fail\n";
std::cout << x << "\n";
Matter changes if you apply std::boolalpha – then any text matching true or false is converted to the corresponding value while any other text results in false, again with the fail-bit set.
See std::num_get::get, which operator>> refers to.
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