Trouble with identifying why the IDE is saying undeclared identifier in C++

问题

I have a class at the moment with these lines of code. What I am struggling with is the function write in that my IDE says "Use of undeclared identifier for root"

Why is this?

template<typename T>
    class X
    {
    public:
        const void write(std::ostream & output);

    private:
        std::unique_ptr< TreeNode<Ty> > root;


    };


    const void write(std::ostream & output)
    {
        root->write(output);
    }

Edit to show more full extent of the code with the Template.

回答1:

Try using the scope resolution operator to tell the compiler that your write function belongs to class X:

const void X::write(std::ostream & output)
{
    root->write(output);
}

Edit 1: templates
With templates, the syntax becomes:

template<typename T>
const void
X<T>::write(std::ostream & output)
{
    root->write(output);
}

来源：https://stackoverflow.com/questions/46943651/trouble-with-identifying-why-the-ide-is-saying-undeclared-identifier-in-c