警告C4172返回本地变量或临时地址。 但是我返回一个输入参数值

Question

I designed a template class which can operate several kinds of strings, such as CString and std::string. I have been using it long time, and it works fine. But the "return str[i]" in "at" function led to a warning C4172, that means I return an address of local varible. But the str isn't a local varible. It should be passed from the outside. How can I make the compile not prompt the warning?

template<class TSTR, typename TCHR>
    class stringOpr
    {
    public:
        typedef typename TSTR TSTRING;
        typedef typename TCHR TCHARTYPE;
        virtual const TCHARTYPE& at(const TSTRING& str, int i) const
        {
            return str[i];//Warning C4172
        }
...
};

Answer 1

That warning makes sense. If there is a MyString that the operator of it didn't return a reference.

class MyString
{
char operator[](int i) const;
};

This function will crash. In this case, you should rewrite your "at" function to fit a returning reference. If you really want to return a reference, do this,

#pragma warning(disable : 4172)    
virtual const TCHARTYPE& at(const TSTRING& str, int i) const
{
    return str[i];
#pragma warning(default : 4172)
}

Ignore the warning, and re-enable it after this.

/ ---------------------------------------------------- /

CString::operator[] really doesn't return a reference. So I make a concession

virtual /*const*/ TCHARTYPE/*&*/ at(const TSTRING& str, int i) const
        {
            return str[i];
        }