“错误：在转换的常量表达式中不允许从'double'转换为'std::size_t'（又名'unsigned long'）”是什么意思？

Question

#define Tag std::bitset<32-8-log2(4*_N)>
#define Offset std::bitset<log2(4*_N)>

I tried to write a code in cpp with the following #define functions and i ended up with the error.

error: conversion from 'double' to 'std::size_t' (aka 'unsigned long') is not allowed in a converted constant expression
Tag* _tagBits;

Answer 1

You could define your own constexpr (naive) log2:

#include <iostream>
#include <bitset>

constexpr std::size_t log2(std::size_t N)
{
    return N <= 1 ? 0 : 1+log2(N/2);
}

int main()
{
    std::bitset<1+log2(13)> b;
    std::cout << b << '\n'; // 0000
}