Question

Is there a way to convert a std::bitset<64> to a double without using any external library (Boost, etc.)? I am using a bitset to represent a genome in a genetic algorithm and I need a way to convert a set of bits to a double.

Answer 1

The C++11 road:

union Converter { uint64_t i; double d; };

double convert(std::bitset<64> const& bs) {
    Converter c;
    c.i = bs.to_ullong();
    return c.d;
}

EDIT: As noted in the comments, we can use char* aliasing as it is unspecified instead of being undefined.

double convert(std::bitset<64> const& bs) {
    static_assert(sizeof(uint64_t) == sizeof(double), "Cannot use this!");

    uint64_t const u = bs.to_ullong();
    double d;

    // Aliases to `char*` are explicitly allowed in the Standard (and only them)
    char const* cu = reinterpret_cast<char const*>(&u);
    char* cd = reinterpret_cast<char*>(&d);

    // Copy the bitwise representation from u to d
    memcpy(cd, cu, sizeof(u));

    return d;
}

C++11 is still required for to_ullong.