|| operator in c++ for vector norm

I'm trying to write a own "library" for vectors where I can define operators such as "*" for scalar multiplication and "+" for vector addition etc. I think I got most of it right, but now I'm trying to define a operator for the l2 norm of a vector using "| |", meaning this:

Vector v;
// let v be (3, 4)
double a = |v|;
// then a should be 5 (sqrt(16 + 9))

but then I get this error:

main.cpp:11:17: error: no match for 'operator|' (operand types are 'Vector' and 'int')
     double a = v|2|;
               ~^~
main.cpp:11:20: error: expected primary-expression before ';' token
     double a = v|2|;

Now here I'm struggling to define the operator ||...

I tried doing something like this:

double Vector::operator||(int){
//  here I used the scalar product to calculate the norm
    double d = (*this) * (*this);
    return sqrt(d);
}

or I tried defining it as friend function with two parameters. I think the main problem is what parameters I have to give the operator because it always requiers two (or one if its a member function). I just cant figure out a way to do it...

Has anyone of you an idea how solve this or is there no solution for this and I have to use a normal function?

Thanks in advance :)

You can't do that, because C++ doesn't support such a syntax. The best you can do is to create a function called something like magnitude and do your calculations in it. Like this:

template <typename T>
T magnitude(std::vector<T> const& vec) { ... }

double a = magnitude(v);