[c++] Can't use modulus on doubles?

I have a program in C++ (compiled using g++). I'm trying to apply two doubles as operands to the modulus function, but I get the following error:

error: invalid operands of types 'double' and 'double' to binary 'operator%'

Here's the code:

int main() {
    double x = 6.3;
    double y = 2;
    double z = x % y;
}

Use fmod() from <cmath>. If you do not want to include the C header file:

template<typename T, typename U>
constexpr double dmod (T x, U mod)
{
    return !mod ? x : x - mod * static_cast<long long>(x / mod);
}

//Usage:
double z = dmod<double, unsigned int>(14.3, 4);
double z = dmod<long, float>(14, 4.6);
//This also works:
double z = dmod(14.7, 0.3);
double z = dmod(14.7, 0);
double z = dmod(0, 0.3f);
double z = dmod(myFirstVariable, someOtherVariable);

You can implement your own modulus function to do that for you:

double dmod(double x, double y) {
    return x - (int)(x/y) * y;
}

Then you can simply use dmod(6.3, 2) to get the remainder, 0.3.

fmod(x, y) is the function you use.