ArgMin for vector<double> in C++?

I'd like to find the index of the minimum value in a C++ std::vector<double>. Here's a somewhat verbose implementation of this:

//find index of smallest value in the vector
int argMin(std::vector<double> vec)
{
    std::vector<double>::iterator mins = std::min_element(vec.begin(), vec.end()); //returns all mins
    double min = mins[0]; //select the zeroth min if multiple mins exist
    for(int i=0; i < vec.size(); i++)
    {
        //Note: could use fabs( (min - vec[i]) < 0.01) if worried about floating-point precision
        if(vec[i] == min)    
            return i;
    }
    return -1;
}

(Let me know if you notice any mistakes in the above implementation. I tested it, but my testing is not at all exhaustive.)

I think the above implementation is probably a wheel-reinvention; I'd like to use built-in code if possible. Is there a one-line call to an STL function for this? Or, can someone suggest a more concise implementation?