static_cast< T>(-1)是生成无数字限制的全一位数据的正确方法吗? [英] Is static_cast<T>(-1) the right way to generate all-one-bits data without numeric_limits?

问题描述

我正在无法访问C ++标准库(特别是不能访问std::numeric_limits)的环境中编写C ++代码.假设我要实现

I'm writing C++ code in an environment in which I don't have access to the C++ standard library, specifically not to std::numeric_limits. Suppose I want to implement

template <typename T> constexpr T all_ones( /* ... */ )

重点放在无符号整数类型上，我该放在哪里?具体来说，static_cast<T>(-1)是否足够好? (我猜其他类型可以根据其大小将其视为无符号字符数组.)

Focusing on unsigned integral types, what do I put there? Specifically, is static_cast<T>(-1) good enough? (Other types I could treat as an array of unsigned chars based on their size I guess.)

推荐答案

在0上使用bitwise NOT运算符~.

T allOnes = ~(T)0;

A static_cast<T>(-1)采用二进制补码，这是不可移植的.如果您只关心未签名的类型，那么 hvd的答案是可行的方法.

A static_cast<T>(-1) assumes two's complement, which is not portable. If you are only concerned about unsigned types, hvd's answer is the way to go.

工作示例: https://ideone.com/iV28u0

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