Data input string and double

I'm in a project implementing a C ++ calculator that uses variables. So, if I define a variable x = 5, and I type x ^ 2 then it returns the value of 25. For this, I have to store the variable name and the corresponding value. The user would type a name or a double and depending on the entry, store the value or the name. There is my difficulty.

I am trying to input data in C ++ with the use of class and operator overload. However, I can not do that. I have a class called "variable":

class variavel
{
    public:
        std::string nome;
        double valor;
        friend std::istream& operator>> (std::istream& os, variavel &v);
};

What I want to do is something like:

std::istream& operator>> (std::istream& os, variavel &v)
{
    os >> v.nome;
    try
    {
        v.valor = ConverteParaDouble(v.nome); /* lança uma exceção se não for um numero */
        v.nome = ""; /* nao executa se lançar a exceção */
    }
    catch(excecao &e) /* captura a exceção */
    {
        /* Outros comandos */
    }
}

However, I do not know how to do this, be it convert (tried atoi , stod ) or throw the exception. For the stod documentation ( click here ), it would be necessary, but always when I put the code:

std::istream& operator>> (std::istream& os, variavel &v)
{
    os >> v.nome;
    try
    {
        v.valor = std::stod(v.nome);
        v.nome = "";
    }
    catch(const std::invalid_argument& ia)
    {
        v.nome = "deu ruim";    
    }
}

But whenever I try to compile, it says stod is not in std . How do you arrange this?