Naming convention - underscore in C++ and C# variables

It's common to see a _var variable name in a class field. What does the underscore mean? Is there a reference for all these special naming conventions?

Solution 1:

The underscore is simply a convention; nothing more. As such, its use is always somewhat different to each person. Here's how I understand them for the two languages in question:

In C++, an underscore usually indicates a private member variable.

In C#, I usually see it used only when defining the underlying private member variable for a public property. Other private member variables would not have an underscore. This usage has largely gone to the wayside with the advent of automatic properties though.

Before:

private string _name;
public string Name
{
    get { return this._name; }
    set { this._name = value; }
}

After:

public string Name { get; set; }

Solution 2:

It is best practice to NOT use UNDERSCORES before any variable name or parameter name in C++

Names beginning with an underscore or a double underscore are RESERVED for the C++ implementers. Names with an underscore are reserved for the library to work.

If you have a read at the C++ Coding Standard, you will see that in the very first page it says:

"Don't overlegislate naming, but do use a consistent naming convention: There are only two must-dos: a) never use "underhanded names," ones that begin with an underscore or that contain a double underscore;" (p2 , C++ Coding Standards, Herb Sutter and Andrei Alexandrescu)

More specifically, the ISO working draft states the actual rules:

In addition, some identifiers are reserved for use by C ++ implementations and shall not be used otherwise; no diagnostic is required. (a) Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use. (b) Each identifier that begins with an underscore is reserved to the implementation for use as a name in the global namespace.

It is best practice to avoid starting a symbol with an underscore in case you accidentally wander into one of the above limitations.

You can see it for yourself why such use of underscores can be disastrous when developing a software:

Try compiling a simple helloWorld.cpp program like this:

g++ -E helloWorld.cpp

You will see all that happens in the background. Here is a snippet:

   ios_base::iostate __err = ios_base::iostate(ios_base::goodbit);
   try
     {
       __streambuf_type* __sb = this->rdbuf();
       if (__sb)
  {
    if (__sb->pubsync() == -1)
      __err |= ios_base::badbit;
    else
      __ret = 0;
  }

You can see how many names begin with double underscore!

Also if you look at virtual member functions, you will see that *_vptr is the pointer generated for the virtual table which automatically gets created when you use one or more virtual member functions in your class! But that's another story...

If you use underscores you might get into conflict issues and you WILL HAVE NO IDEA what's causing it, until it's too late.