C++ – Trailing underscores for member variables in C++

cmembernaming-conventions

I've seen people use a trailing underscore for member variables in classes, for instance in the renowned C++ FAQ Lite.

I think that it's purpose is not to mark variables as members, that's what "m_" is for. It's actual purpose is to make it possible to have an accessor method named like the field, like this:

class Foo {
public:
    bar the_bar() { return the_bar_; }
private:
    bar the_bar_;
}

Having accessors omit the "get_" part is common in the STL and boost, and I'm trying to develop a coding style as close to these as possible, but I can't really see them using the underscore trick. I wasn't able to find an accessor in STL or boost that would just return a private variable.

I have a few questions I'm hoping you will be able to answer:

  1. Where does this convention come from? Smalltalk? Objective-C? Microsoft? I'm wondering.
  2. Would I use the trailing underscore for all private members or just as a workaround in case I want to name a function like a variable?
  3. Can you point me to STL or boost code that demonstrates trailing underscores for member variables?
  4. Does anybody know what Stroustrup's views on the issue are?
  5. Can you point me to further discussion of the issue?

Best Answer

In C++,

  1. identifiers starting with an underscore, followed by a capital character
  2. identifiers having two consecutive underscores anywhere
  3. identifiers in the global namespace starting with an underscore

are reserved to the implementation. (More about this can be found here.) Rather than trying to remember these rules, many simply do not use identifiers starting with an underscore. That's why the trailing underscore was invented.

However, C++ itself is old, and builds on 40 years of C (both of which never had a single company behind them), and has a standard library that has "grown" over several decades, rather than brought into being in a single act of creation. This makes for the existence of a lot of differing naming conventions. Trailing underscore for privates (or only for private data) is but one, many use other ones (not few among them arguing that, if you need underscores to tell private members from local variables, your code isn't clear enough).

As for getters/setters - they are an abomination, and a sure sign of "quasi classes", which I hate.

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