A handy method to verify if a number n is a power of two (like 2, 4, 8, etc.) is to use the following test:
bool test = n & (n - 1) == 0;
This operation can be very efficient because it only involves subtraction, a bitwise AND, and a comparison. If this expression is evaluated to true, then the number n is indeed a power of two.
Another method uses the std::popcount (population count) function, which is part of the C++20 standard library, for the test:
bool test = std::popcount(n) == 1; // (Since C++20)
This function counts the number of set bits (1s) in. If the hardware supports a popcount instruction (POPCNT), this function can be very fast.
In C++, you generally “pay for what you use”. For this test there is no use for counting.
What is the better method, in terms of CPU efficiency?
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bool test = std::has_single_bit(n);
std::has_single_bit should be converted to the most efficient sequence for the current platform
template< class T > constexpr bool has_single_bit( T x ) noexcept;(since C++20)
Checks if x is an integral power of two.
This overload participates in overload resolution only if T is an unsigned integer type (that is,
unsigned char,unsigned short,unsigned int,unsigned long,unsigned long long, or an extended unsigned integer type).
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