reason for _funname() and funname()

Hi I have seen this before but I am confused as to why people do so. If there is a technical term to better define it please do let me know

If you look at the code below

Code:

Code:

class _Correl
{
public:
    double Nom;
    double DeNom1, DeNom2;

    _Correl( double val ) { Nom = DeNom1 = DeNom2 = val; }
    void operator=( double val ) { Nom = DeNom1 = DeNom2 = val; }
    operator double() { return Nom / sqrt( DeNom1 * DeNom2 ); }
};

class Correl
{
public:
    typedef _Correl result_type;

    Correl() { mean1 = mean2 = 0; }
    Correl( double _mean1, double _mean2 = 0 ) { mean1 = _mean1; mean2 = _mean2; }

    template<class Val> result_type operator () ( result_type s, Val val1, Val val2 )
        {
            double v1 = val1 - mean1;
            double v2 = val2 - mean2;
            s.Nom += v1 * v2;
            s.DeNom1 += v1 * v1;
            s.DeNom2 += v2 * v2;
            return s;
        }
protected:
    double mean1;
    double mean2;
};

there are terms like "class Correl" and "typdef _Correl"

"mean1 = _mean1" "double _mean1"

Now I do understand that mean1 and _mean1 are different
but why do codders (at higher level) use "_name of (variable/function etc.).

I have seen this in header files of VC++ as well?

If there is some documentation that I could refer on this matter I would like to know.

Thanks
Nipun

From C99 (not the C++ standard, but you can be pretty sure something similar appears)

7.1.3 Reserved identifiers

[#1] Each header declares or defines all identifiers listed
in its associated subclause, and optionally declares or
defines identifiers listed in its associated future library
directions subclause and identifiers which are always
reserved either for any use or for use as file scope
identifiers.

-- All identifiers that begin with an underscore and
either an uppercase letter or another underscore are
always reserved for any use.

-- All identifiers that begin with an underscore are
always reserved for use as identifiers with file scope
in both the ordinary and tag name spaces.

> I have seen this in header files of VC++ as well?
Which is the only place you should see them, and where they are allowed to be.
Defining any symbol with a leading underscore, or using such-like from a system header file is non-standard usage.