将 `nullptr` 分配给 `bool` 类型.哪个编译器是正确的? [英] Assigned `nullptr` to `bool` type. Which compiler is correct?

问题描述

我有以下代码片段将 nullptr 分配给 bool 类型.

I have a following snippet of code that assigned nullptr to bool type.

#include <iostream>

int main()
{
    bool b = nullptr;
    std::cout << b;
}

在 clang 3.8.0 中工作正常.它给出了一个输出0.Clang 演示

In clang 3.8.0 working fine. it's give an output 0. Clang Demo

但是g++ 5.4.0报错:

source_file.cpp: In function ‘int main()’:
source_file.cpp:5:18: error: converting to ‘bool’ from ‘std::nullptr_t’ requires direct-initialization [-fpermissive]
         bool b = nullptr;

哪个编译器是正确的?

推荐答案

来自 C++ 标准(4.12 布尔转换)

From the C++ Standard (4.12 Boolean conversions)

1 算术、无范围枚举、指针或指针的纯右值to 成员类型可以转换为 bool 类型的纯右值.一个零值、空指针值或空成员指针值被转换为假；任何其他值都将转换为 true.为直接初始化 (8.5)，std::nullptr_t 类型的纯右值可以是转换为 bool 类型的纯右值；结果值为假.

1 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. For direct-initialization (8.5), a prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

所以这个声明

bool b( nullptr );

有效并且这个

bool b = nullptr;

错了.

我自己已经在 指出了这个问题isocpp

这篇关于将 `nullptr` 分配给 `bool` 类型.哪个编译器是正确的?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！