C++ 中的固定宽度整数文字？

Question

C++11 首先通过用户定义的文字引入了对在 C++ 中定义新文字的支持。 C++11 或更高版本是否也为 <cstdint> 中的类型预定义固定宽度整数文字的后缀？

Answer 1

No. 从 C++14 开始，标准定义的唯一文字后缀由 <chrono>、<complex> 和 [=13= 提供] headers 在标准库中。 <chrono> header 定义了 h、min、s、ms、us、ns 后缀持续时间，<complex> 定义虚数的 i、il 和 if 后缀，<string> 定义 [=27] 的 s 后缀=] 文字。

但是，可以很容易地定义自己的 fixed-width 字面值，如下所示：

#include <cstdint>

constexpr std::int8_t operator "" _int8(unsigned long long v)
{ return static_cast<std::int8_t>(v); }

constexpr std::uint8_t operator "" _uint8(unsigned long long v)
{ return static_cast<std::uint8_t>(v); }

constexpr std::int16_t operator "" _int16(unsigned long long v)
{ return static_cast<std::int16_t>(v); }

constexpr std::uint16_t operator "" _uint16(unsigned long long v)
{ return static_cast<std::uint16_t>(v); }

constexpr std::int32_t operator "" _int32(unsigned long long v)
{ return static_cast<std::int32_t>(v); }

constexpr std::uint32_t operator "" _uint32(unsigned long long v)
{ return static_cast<std::uint32_t>(v); }

constexpr std::int64_t operator "" _int64(unsigned long long v)
{ return static_cast<std::int64_t>(v); }

constexpr std::uint64_t operator "" _uint64(unsigned long long v)
{ return static_cast<std::uint64_t>(v); }

constexpr std::int_fast8_t operator "" _int_fast8(unsigned long long v)
{ return static_cast<std::int_fast8_t>(v); }

constexpr std::uint_fast8_t operator "" _uint_fast8(unsigned long long v)
{ return static_cast<std::uint_fast8_t>(v); }

constexpr std::int_fast16_t operator "" _int_fast16(unsigned long long v)
{ return static_cast<std::int_fast16_t>(v); }

constexpr std::uint_fast16_t operator "" _uint_fast16(unsigned long long v)
{ return static_cast<std::uint_fast16_t>(v); }

constexpr std::int_fast32_t operator "" _int_fast32(unsigned long long v)
{ return static_cast<std::int_fast32_t>(v); }

constexpr std::uint_fast32_t operator "" _uint_fast32(unsigned long long v)
{ return static_cast<std::uint_fast32_t>(v); }

constexpr std::int_fast64_t operator "" _int_fast64(unsigned long long v)
{ return static_cast<std::int_fast64_t>(v); }

constexpr std::uint_fast64_t operator "" _uint_fast64(unsigned long long v)
{ return static_cast<std::uint_fast64_t>(v); }

constexpr std::int_least8_t operator "" _int_least8(unsigned long long v)
{ return static_cast<std::int_least8_t>(v); }

constexpr std::uint_least8_t operator "" _uint_least8(unsigned long long v)
{ return static_cast<std::uint_least8_t>(v); }

constexpr std::int_least16_t operator "" _int_least16(unsigned long long v)
{ return static_cast<std::int_least16_t>(v); }

constexpr std::uint_least16_t operator "" _uint_least16(unsigned long long v)
{ return static_cast<std::uint_least16_t>(v); }

constexpr std::int_least32_t operator "" _int_least32(unsigned long long v)
{ return static_cast<std::int_least32_t>(v); }

constexpr std::uint_least32_t operator "" _uint_least32(unsigned long long v)
{ return static_cast<std::uint_least32_t>(v); }

constexpr std::int_least64_t operator "" _int_least64(unsigned long long v)
{ return static_cast<std::int_least64_t>(v); }

constexpr std::uint_least64_t operator "" _uint_least64(unsigned long long v)
{ return static_cast<std::uint_least64_t>(v); }

constexpr std::intmax_t operator "" _intmax(unsigned long long v)
{ return static_cast<std::intmax_t>(v); }

constexpr std::uintmax_t operator "" _uintmax(unsigned long long v)
{ return static_cast<std::uintmax_t>(v); }

constexpr std::intptr_t operator "" _intptr(unsigned long long v)
{ return static_cast<std::intptr_t>(v); }

constexpr std::uintptr_t operator "" _uintptr(unsigned long long v)
{ return static_cast<std::uintptr_t>(v); }

警告：如果用在不适合 unsigned long long 的文字上，上面的代码会默默地给出错误的结果，如果文字值会溢出, 不符合要求的类型，例如999_int8。 A better implementation (GPL-3 licensed) would probably have to parse the literal character-by-character and static_assert on overflow, like this.

使用用户定义文字的缺点是需要在后缀前加上下划线 _，因为根据 §17.6.4.3.4，没有下划线的后缀是为将来的标准化保留的。