std::generate_canonical

Defined in header `<random>`
template< class RealType, size_t bits, class Generator > RealType generate_canonical( Generator& g );		(since C++11)

Generates a random floating point number in range [0, 1).

To generate enough entropy, generate_canonical() will call g() exactly k times, where \(k = max(1, \lceil \frac{b}{log_2 R} \rceil)\)k = max(1, ⌈ b / log
2 R ⌉) and.

b = std::min<std::size_t>(bits, std::numeric_limits<RealType>::digits)
R = g.max() - g.min() + 1.

Parameters

generator to use to acquire entropy

Return value

Floating point value in range [0, 1).

Exceptions

None except from those thrown by g.

Notes

Some existing implementations have a bug where they may occasionally return 1.0 if RealType is float GCC #63176 LLVM #18767. This is LWG issue 2524

Example

produce random numbers with 10 bits of randomness: this may produce only k*R distinct values.

#include <random>
#include <iostream>
 
int main()
{
    std::random_device rd;
    std::mt19937 gen(rd());
    for(int n=0; n<10; ++n) {
        std::cout << std::generate_canonical<double, 10>(gen) << ' ';
    }
}

Possible output:

0.208143 0.824147 0.0278604 0.343183 0.0173263 0.864057 0.647037 0.539467 0.0583497 0.609219