C ++打印布尔值，显示什么? [英] C++ printing boolean, what is displayed?

问题描述

我将bool打印到这样的输出流中:

I print a bool to an output stream like this:

#include <iostream>

int main()
{
    std::cout << false << std::endl;
}

标准是否要求流中有特定结果(例如0代表false)?

Does the standard require a specific result on the stream (e.g. 0 for false)?

推荐答案

标准流具有boolalpha标志，该标志确定要显示的内容-如果为假，它们将显示为0和1.正确时，它们将显示为false和true.

The standard streams have a boolalpha flag that determines what gets displayed -- when it's false, they'll display as 0 and 1. When it's true, they'll display as false and true.

还有一个std::boolalpha操纵器来设置标志，因此:

There's also an std::boolalpha manipulator to set the flag, so this:

#include <iostream>
#include <iomanip>

int main() {
    std::cout<<false<<"\n";
    std::cout << std::boolalpha;   
    std::cout<<false<<"\n";
    return 0;
}

...产生如下输出:

...produces output like:

0
false

就其价值而言，将boolalpha设置为true时产生的实际单词是本地化的，也就是说，<locale>具有用于处理数字转换的num_put类别，因此，如果向右注入流语言环境，它可以/将打印出true和false，因为它们在该语言环境中表示.例如

For what it's worth, the actual word produced when boolalpha is set to true is localized--that is, <locale> has a num_put category that handles numeric conversions, so if you imbue a stream with the right locale, it can/will print out true and false as they're represented in that locale. For example,

#include <iostream>
#include <iomanip>
#include <locale>

int main() {
    std::cout.imbue(std::locale("fr"));

    std::cout << false << "\n";
    std::cout << std::boolalpha;
    std::cout << false << "\n";
    return 0;
}

...并且至少在理论上(假设您的编译器/标准库接受"fr"作为法语"的标识符)，它可能会打印出faux而不是false.但是，我应该补充一点，对此的真正支持充其量是不均衡的，即使是Dinkumware/Microsoft库(通常在这方面也很出色)都会为我检查过的每种语言打印false.

...and at least in theory (assuming your compiler/standard library accept "fr" as an identifier for "French") it might print out faux instead of false. I should add, however, that real support for this is uneven at best--even the Dinkumware/Microsoft library (usually quite good in this respect) prints false for every language I've checked.

虽然使用的名称是在numpunct构面中定义的，所以如果您确实希望它们针对特定语言正确打印，则可以创建numpunct构面来做到这一点.例如，(我相信)至少对于法语来说是相当准确的样子:

The names that get used are defined in a numpunct facet though, so if you really want them to print out correctly for particular language, you can create a numpunct facet to do that. For example, one that (I believe) is at least reasonably accurate for French would look like this:

#include <array>
#include <string>
#include <locale>
#include <ios>
#include <iostream>

class my_fr : public std::numpunct< char > {
protected:
    char do_decimal_point() const { return ','; }
    char do_thousands_sep() const { return '.'; }
    std::string do_grouping() const { return "\3"; }
    std::string do_truename() const { return "vrai";  }
    std::string do_falsename() const { return "faux"; }
};

int main() {
    std::cout.imbue(std::locale(std::locale(), new my_fr));

    std::cout << false << "\n";
    std::cout << std::boolalpha;
    std::cout << false << "\n";
    return 0;
}

结果是(如您可能预期的那样):

And the result is (as you'd probably expect):

0
faux

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