[c++] Why use prefixes on member variables in C++ classes

A lot of C++ code uses syntactical conventions for marking up member variables. Common examples include

• m_memberName for public members (where public members are used at all)
• _memberName for private members or all members

Others try to enforce using this->member whenever a member variable is used.

In my experience, most larger code bases fail at applying such rules consistently.

In other languages, these conventions are far less widespread. I see it only occasionally in Java or C# code. I think I have never seen it in Ruby or Python code. Thus, there seems to be a trend with more modern languages to not use special markup for member variables.

Is this convention still useful today in C++ or is it just an anachronism. Especially as it is used so inconsistently across libraries. Haven't the other languages shown that one can do without member prefixes?

I think that, if you need prefixes to distinguish class members from member function parameters and local variables, either the function is too big or the variables are badly named. If it doesn't fit on the screen so you can easily see what is what, refactor.

Given that they often are declared far from where they are used, I find that naming conventions for global constants (and global variables, although IMO there's rarely ever a need to use those) make sense. But otherwise, I don't see much need.

That said, I used to put an underscore at the end of all private class members. Since all my data is private, this implies members have a trailing underscore. I usually don't do this anymore in new code bases, but since, as a programmer, you mostly work with old code, I still do this a lot. I'm not sure whether my tolerance for this habit comes from the fact that I used to do this always and am still doing it regularly or whether it really makes more sense than the marking of member variables.

I'm all in favour of prefixes done well.

I think (System) Hungarian notation is responsible for most of the "bad rap" that prefixes get.

This notation is largely pointless in strongly typed languages e.g. in C++ "lpsz" to tell you that your string is a long pointer to a nul terminated string, when: segmented architecture is ancient history, C++ strings are by common convention pointers to nul-terminated char arrays, and it's not really all that difficult to know that "customerName" is a string!

However, I do use prefixes to specify the usage of a variable (essentially "Apps Hungarian", although I prefer to avoid the term Hungarian due to it having a bad and unfair association with System Hungarian), and this is a very handy timesaving and bug-reducing approach.

I use:

• m for members
• c for constants/readonlys
• p for pointer (and pp for pointer to pointer)
• v for volatile
• s for static
• i for indexes and iterators
• e for events

Where I wish to make the type clear, I use standard suffixes (e.g. List, ComboBox, etc).

This makes the programmer aware of the usage of the variable whenever they see/use it. Arguably the most important case is "p" for pointer (because the usage changes from var. to var-> and you have to be much more careful with pointers - NULLs, pointer arithmetic, etc), but all the others are very handy.

For example, you can use the same variable name in multiple ways in a single function: (here a C++ example, but it applies equally to many languages)

MyClass::MyClass(int numItems)
{
    mNumItems = numItems;
    for (int iItem = 0; iItem < mNumItems; iItem++)
    {
        Item *pItem = new Item();
        itemList[iItem] = pItem;
    }
}

You can see here:

• No confusion between member and parameter
• No confusion between index/iterator and items
• Use of a set of clearly related variables (item list, pointer, and index) that avoid the many pitfalls of generic (vague) names like "count", "index".
• Prefixes reduce typing (shorter, and work better with auto-completion) than alternatives like "itemIndex" and "itemPtr"

Another great point of "iName" iterators is that I never index an array with the wrong index, and if I copy a loop inside another loop I don't have to refactor one of the loop index variables.

Compare this unrealistically simple example:

for (int i = 0; i < 100; i++)
    for (int j = 0; j < 5; j++)
        list[i].score += other[j].score;

(which is hard to read and often leads to use of "i" where "j" was intended)

with:

for (int iCompany = 0; iCompany < numCompanies; iCompany++)
    for (int iUser = 0; iUser < numUsers; iUser++)
       companyList[iCompany].score += userList[iUser].score;

(which is much more readable, and removes all confusion over indexing. With auto-complete in modern IDEs, this is also quick and easy to type)

The next benefit is that code snippets don't require any context to be understood. I can copy two lines of code into an email or a document, and anyone reading that snippet can tell the difference between all the members, constants, pointers, indexes, etc. I don't have to add "oh, and be careful because 'data' is a pointer to a pointer", because it's called 'ppData'.

And for the same reason, I don't have to move my eyes out of a line of code in order to understand it. I don't have to search through the code to find if 'data' is a local, parameter, member, or constant. I don't have to move my hand to the mouse so I can hover the pointer over 'data' and then wait for a tooltip (that sometimes never appears) to pop up. So programmers can read and understand the code significantly faster, because they don't waste time searching up and down or waiting.

(If you don't think you waste time searching up and down to work stuff out, find some code you wrote a year ago and haven't looked at since. Open the file and jump about half way down without reading it. See how far you can read from this point before you don't know if something is a member, parameter or local. Now jump to another random location... This is what we all do all day long when we are single stepping through someone else's code or trying to understand how to call their function)

The 'm' prefix also avoids the (IMHO) ugly and wordy "this->" notation, and the inconsistency that it guarantees (even if you are careful you'll usually end up with a mixture of 'this->data' and 'data' in the same class, because nothing enforces a consistent spelling of the name).

'this' notation is intended to resolve ambiguity - but why would anyone deliberately write code that can be ambiguous? Ambiguity will lead to a bug sooner or later. And in some languages 'this' can't be used for static members, so you have to introduce 'special cases' in your coding style. I prefer to have a single simple coding rule that applies everywhere - explicit, unambiguous and consistent.

The last major benefit is with Intellisense and auto-completion. Try using Intellisense on a Windows Form to find an event - you have to scroll through hundreds of mysterious base class methods that you will never need to call to find the events. But if every event had an "e" prefix, they would automatically be listed in a group under "e". Thus, prefixing works to group the members, consts, events, etc in the intellisense list, making it much quicker and easier to find the names you want. (Usually, a method might have around 20-50 values (locals, params, members, consts, events) that are accessible in its scope. But after typing the prefix (I want to use an index now, so I type 'i...'), I am presented with only 2-5 auto-complete options. The 'extra typing' people attribute to prefixes and meaningful names drastically reduces the search space and measurably accelerates development speed)

I'm a lazy programmer, and the above convention saves me a lot of work. I can code faster and I make far fewer mistakes because I know how every variable should be used.

Arguments against

So, what are the cons? Typical arguments against prefixes are:

• "Prefix schemes are bad/evil". I agree that "m_lpsz" and its ilk are poorly thought out and wholly useless. That's why I'd advise using a well designed notation designed to support your requirements, rather than copying something that is inappropriate for your context. (Use the right tool for the job).

• "If I change the usage of something I have to rename it". Yes, of course you do, that's what refactoring is all about, and why IDEs have refactoring tools to do this job quickly and painlessly. Even without prefixes, changing the usage of a variable almost certainly means its name ought to be changed.

• "Prefixes just confuse me". As does every tool until you learn how to use it. Once your brain has become used to the naming patterns, it will filter the information out automatically and you won't really mind that the prefixes are there any more. But you have to use a scheme like this solidly for a week or two before you'll really become "fluent". And that's when a lot of people look at old code and start to wonder how they ever managed without a good prefix scheme.

• "I can just look at the code to work this stuff out". Yes, but you don't need to waste time looking elsewhere in the code or remembering every little detail of it when the answer is right on the spot your eye is already focussed on.

• (Some of) that information can be found by just waiting for a tooltip to pop up on my variable. Yes. Where supported, for some types of prefix, when your code compiles cleanly, after a wait, you can read through a description and find the information the prefix would have conveyed instantly. I feel that the prefix is a simpler, more reliable and more efficient approach.

• "It's more typing". Really? One whole character more? Or is it - with IDE auto-completion tools, it will often reduce typing, because each prefix character narrows the search space significantly. Press "e" and the three events in your class pop up in intellisense. Press "c" and the five constants are listed.

• "I can use this-> instead of m". Well, yes, you can. But that's just a much uglier and more verbose prefix! Only it carries a far greater risk (especially in teams) because to the compiler it is optional, and therefore its usage is frequently inconsistent. m on the other hand is brief, clear, explicit and not optional, so it's much harder to make mistakes using it.

When you have a big method or code blocks, it's convenient to know immediately if you use a local variable or a member. it's to avoid errors and for better clearness !

You should never need such a prefix. If such a prefix offers you any advantage, your coding style in general needs fixing, and it's not the prefix that's keeping your code from being clear. Typical bad variable names include "other" or "2". You do not fix that with requiring it to be mOther, you fix it by getting the developer to think about what that variable is doing there in the context of that function. Perhaps he meant remoteSide, or newValue, or secondTestListener or something in that scope.

It's an effective anachronism that's still propagated too far. Stop prefixing your variables and give them proper names whose clarity reflects how long they're used. Up to 5 lines you could call it "i" without confusion; beyond 50 lines you need a pretty long name.

I like variable names to give only a meaning to the values they contain, and leave how they are declared/implemented out of the name. I want to know what the value means, period. Maybe I've done more than an average amount of refactoring, but I find that embedding how something is implemented in the name makes refactoring more tedious than it needs to be. Prefixes indicating where or how object members are declared are implementation specific.

color = Red;

Most of the time, I don't care if Red is an enum, a struct, or whatever, and if the function is so large that I can't remember if color was declared locally or is a member, it's probably time to break the function into smaller logical units.

If your cyclomatic complexity is so great that you can't keep track of what is going on in the code without implementation-specific clues embedded in the names of things, most likely you need to reduce the complexity of your function/method.

Mostly, I only use 'this' in constructors and initializers.

I almost never use prefixes in front of my variable names. If you're using a decent enough IDE you should be able to refactor and find references easily. I use very clear names and am not afraid of having long variable names. I've never had trouble with scope either with this philosophy.

The only time I use a prefix would be on the signature line. I'll prefix parameters to a method with _ so I can program defensively around them.

Our project has always used "its" as a prefix for member data, and "the" as a prefix for parameters, with no prefix for locals. It's a little cutesy, but it was adopted by the early developers of our system because they saw it used as a convention by some commercial source libraries we were using at the time (either XVT or RogueWave - maybe both). So you'd get something like this:

void
MyClass::SetName(const RWCString &theName)
{
   itsName = theName;
}

The big reason I see for scoping prefixes (and no others - I hate Hungarian notation) is that it prevents you from getting into trouble by writing code where you think you're referring to one variable, but you're really referring to another variable with the same name defined in the local scope. It also avoids the problem of coming up with a variable names to represent that same concept, but with different scopes, like the example above. In that case, you would have to come up with some prefix or different name for the parameter "theName" anyway - why not make a consistent rule that applies everywhere.

Just using this-> isn't really good enough - we're not as interested in reducing ambiguity as we are in reducing coding errors, and masking names with locally scoped identifiers can be a pain. Granted, some compilers may have the option to raise warnings for cases where you've masked the name in a larger scope, but those warnings may become a nuisance if you're working with a large set of third party libraries that happen to have chosen names for unused variables that occasionally collide with your own.

As for the its/the itself - I honestly find it easier to type than underscores (as a touch typist, I avoid underscores whenever possible - too much stretching off the home rows), and I find it more readable than a mysterious underscore.

I generally don't use a prefix for member variables.

I used to use a m prefix, until someone pointed out that "C++ already has a standard prefix for member access: this->.

So that's what I use now. That is, when there is ambiguity, I add the this-> prefix, but usually, no ambiguity exists, and I can just refer directly to the variable name.

To me, that's the best of both worlds. I have a prefix I can use when I need it, and I'm free to leave it out whenever possible.

Of course, the obvious counter to this is "yes, but then you can't see at a glance whether a variable is a class member or not".

To which I say "so what? If you need to know that, your class probably has too much state. Or the function is too big and complicated".

In practice, I've found that this works extremely well. As an added bonus it allows me to promote a local variable to a class member (or the other way around) easily, without having to rename it.

And best of all, it is consistent! I don't have to do anything special or remember any conventions to maintain consistency.

By the way, you shouldn't use leading underscores for your class members. You get uncomfortably close to names that are reserved by the implementation.

The standard reserves all names starting with double underscore or underscore followed by capital letter. It also reserves all names starting with a single underscore in the global namespace.

So a class member with a leading underscore followed by a lower-case letter is legal, but sooner or late you're going to do the same to an identifier starting with upper-case, or otherwise break one of the above rules.

So it's easier to just avoid leading underscores. Use a postfix underscore, or a m_ or just m prefix if you want to encode scope in the variable name.

The main reason for a member prefix is to distinguish between a member function local and a member variable with the same name. This is useful if you use getters with the name of the thing.

Consider:

class person
{
public:
    person(const std::string& full_name)
        : full_name_(full_name)
    {}

    const std::string& full_name() const { return full_name_; }
private:
    std::string full_name_;
};

The member variable could not be called full_name in this case. You need to rename the member function to get_full_name() or decorate the member variable somehow.

I use m_ for member variables just to take advantage of Intellisense and related IDE-functionality. When I'm coding the implementation of a class I can type m_ and see the combobox with all m_ members grouped together.

But I could live without m_ 's without problem, of course. It's just my style of work.

In python leading double underscores are used to emulate private members. For more details see this answer

According to JOINT STRIKE FIGHTER AIR VEHICLE C++ CODING STANDARDS (december 2005):

AV Rule 67

Public and protected data should only be used in structs—not classes. Rationale: A class is able to maintain its invariant by controlling access to its data. However, a class cannot control access to its members if those members non-private. Hence all data in a class should be private.

Thus, the "m" prefix becomes unuseful as all data should be private.

But it is a good habit to use the p prefix before a pointer as it is a dangerous variable.

Some responses focus on refactoring, rather than naming conventions, as the way to improve readability. I don't feel that one can replace the other.

I've known programmers who are uncomfortable with using local declarations; they prefer to place all the declarations at the top of a block (as in C), so they know where to find them. I've found that, where scoping allows for it, declaring variables where they're first used decreases the time that I spend glancing backwards to find the declarations. (This is true for me even for small functions.) That makes it easier for me to understand the code I'm looking at.

I hope it's clear enough how this relates to member naming conventions: When members are uniformly prefixed, I never have to look back at all; I know the declaration won't even be found in the source file.

I'm sure that I didn't start out preferring these styles. Yet over time, working in environments where they were used consistently, I optimized my thinking to take advantage of them. I think it's possible that many folks who currently feel uncomfortable with them would also come to prefer them, given consistent usage.

I don't think one syntax has real value over another. It all boils down, like you mentionned, to uniformity across the source files.

The only point where I find such rules interesting is when I need 2 things named identicaly, for example :

void myFunc(int index){
  this->index = index;
}

void myFunc(int index){
  m_index = index;
}

I use it to differentiate the two. Also when I wrap calls, like from windows Dll, RecvPacket(...) from the Dll might be wrapped in RecvPacket(...) in my code. In these particular occasions using a prefix like "_" might make the two look alike, easy to identify which is which, but different for the compiler

I use it because VC++'s Intellisense can't tell when to show private members when accessing out of the class. The only indication is a little "lock" symbol on the field icon in the Intellisense list. It just makes it easier to identify private members(fields) easier. Also a habit from C# to be honest.

class Person {
   std::string m_Name;
public:
   std::string Name() { return m_Name; }
   void SetName(std::string name) { m_Name = name; }
};

int main() {
  Person *p = new Person();
  p->Name(); // valid
  p->m_Name; // invalid, compiler throws error. but intellisense doesn't know this..
  return 1;
}

When reading through a member function, knowing who "owns" each variable is absolutely essential to understanding the meaning of the variable. In a function like this:

void Foo::bar( int apples )
{
    int bananas = apples + grapes;
    melons = grapes * bananas;
    spuds += melons;
}

...it's easy enough to see where apples and bananas are coming from, but what about grapes, melons, and spuds? Should we look in the global namespace? In the class declaration? Is the variable a member of this object or a member of this object's class? Without knowing the answer to these questions, you can't understand the code. And in a longer function, even the declarations of local variables like apples and bananas can get lost in the shuffle.

Prepending a consistent label for globals, member variables, and static member variables (perhaps g_, m_, and s_ respectively) instantly clarifies the situation.

void Foo::bar( int apples )
{
    int bananas = apples + g_grapes;
    m_melons = g_grapes * bananas;
    s_spuds += m_melons;
}

These may take some getting used to at first—but then, what in programming doesn't? There was a day when even { and } looked weird to you. And once you get used to them, they help you understand the code much more quickly.

(Using "this->" in place of m_ makes sense, but is even more long-winded and visually disruptive. I don't see it as a good alternative for marking up all uses of member variables.)

A possible objection to the above argument would be to extend the argument to types. It might also be true that knowing the type of a variable "is absolutely essential to understanding the meaning of the variable." If that is so, why not add a prefix to each variable name that identifies its type? With that logic, you end up with Hungarian notation. But many people find Hungarian notation laborious, ugly, and unhelpful.

void Foo::bar( int iApples )
{
    int iBananas = iApples + g_fGrapes;
    m_fMelons = g_fGrapes * iBananas;
    s_dSpuds += m_fMelons;
}

Hungarian does tell us something new about the code. We now understand that there are several implicit casts in the Foo::bar() function. The problem with the code now is that the value of the information added by Hungarian prefixes is small relative to the visual cost. The C++ type system includes many features to help types either work well together or to raise a compiler warning or error. The compiler helps us deal with types—we don't need notation to do so. We can infer easily enough that the variables in Foo::bar() are probably numeric, and if that's all we know, that's good enough for gaining a general understanding of the function. Therefore the value of knowing the precise type of each variable is relatively low. Yet the ugliness of a variable like "s_dSpuds" (or even just "dSpuds") is great. So, a cost-benefit analysis rejects Hungarian notation, whereas the benefit of g_, s_, and m_ overwhelms the cost in the eyes of many programmers.

Those conventions are just that. Most shops use code conventions to ease code readability so anyone can easily look at a piece of code and quickly decipher between things such as public and private members.

Code Complete recommends m_varname for member variables.

While I've never thought the m_ notation useful, I would give McConnell's opinion weight in building a standard.

I prefer postfix underscores, like such:

class Foo
{
   private:
      int bar_;

   public:
      int bar() { return bar_; }
};

I can't say how widespred it is, but speaking personally, I always (and have always) prefixed my member variables with 'm'. E.g.:

class Person {
   .... 
   private:
       std::string mName;
};

It's the only form of prefixing I do use (I'm very anti Hungarian notation) but it has stood me in good stead over the years. As an aside, I generally detest the use of underscores in names (or anywhere else for that matter), but do make an exception for preprocessor macro names, as they are usually all uppercase.

As others have already said, the importance is to be colloquial (adapt naming styles and conventions to the code base in which you're writing) and to be consistent.

For years I have worked on a large code base that uses both the "this->" convention as well as using a postfix underscore notation for member variables. Throughout the years I've also worked on smaller projects, some of which did not have any sort of convention for naming member variables, and other which had differing conventions for naming member variables. Of those smaller projects, I've consistently found those which lacked any convention to be the most difficult to jump into quickly and understand.

I'm very anal-retentive about naming. I will agonize over the name to be ascribed to a class or variable to the point that, if I cannot come up with something that I feel is "good", I will choose to name it something nonsensical and provide a comment describing what it really is. That way, at least the name means exactly what I intend it to mean--nothing more and nothing less. And often, after using it for a little while, I discover what the name should really be and can go back and modify or refactor appropriately.

One last point on the topic of an IDE doing the work--that's all nice and good, but IDEs are often not available in environments where I have perform the most urgent work. Sometimes the only thing available at that point is a copy of 'vi'. Also, I've seen many cases where IDE code completion has propagated stupidity such as incorrect spelling in names. Thus, I prefer to not have to rely on an IDE crutch.

The original idea for prefixes on C++ member variables was to store additional type information that the compiler didn't know about. So for example, you could have a string that's a fixed length of chars, and another that's variable and terminated by a '\0'. To the compiler they're both char *, but if you try to copy from one to the other you get in huge trouble. So, off the top of my head,

char *aszFred = "Hi I'm a null-terminated string";
char *arrWilma = {'O', 'o', 'p', 's'};

where "asz" means this variable is "ascii string (zero-terminated) and "arr" means this variable is a character array.

Then the magic happens. The compiler will be perfectly happy with this statement:

strcpy(arrWilma, aszFred);

But you, as a human, can look at it and say "hey, those variables aren't really the same type, I can't do that".

Unfortunately a lot places use standards such as "m_" for member variables, "i" for integers no matter how used, "cp" for char pointers. In other words they're duplicating what the compiler knows, and making the code hard to read at the same time. I believe this pernicious practice should be outlawed by statute and subject to harsh penalties.

Finally, there's two points I should mention:

• Judicious use of C++ features allows the compiler to know the information you had to encode in raw C-style variables. You can make classes that will only allow valid operations. This should be done as much as practical.
• If your code blocks are so long that you forget what type a variable is before you use it, they are way too long. Don't use names, re-organize.

Lately I have been tending to prefer m_ prefix instead of having no prefix at all, the reasons isn't so much that its important to flag member variables, but that it avoids ambiguity, say you have code like:

void set_foo(int foo) { foo = foo; }

That of cause doesn't work, only one foo allowed. So your options are:

• this->foo = foo;

  I don't like it, as it causes parameter shadowing, you no longer can use g++ -Wshadow warnings, its also longer to type then m_. You also still run into naming conflicts between variables and functions when you have a int foo; and a int foo();.

• foo = foo_; or foo = arg_foo;

  Been using that for a while, but it makes the argument lists ugly, documentation shouldn't have do deal with name disambiguity in the implementation. Naming conflicts between variables and functions also exist here.

• m_foo = foo;

  API Documentation stays clean, you don't get ambiguity between member functions and variables and its shorter to type then this->. Only disadvantage is that it makes POD structures ugly, but as POD structures don't suffer from the name ambiguity in the first place, one doesn't need to use it with them. Having a unique prefix also makes a few search&replace operations easier.

• foo_ = foo;

  Most of the advantages of m_ apply, but I reject it for aesthetic reasons, a trailing or leading underscore just makes the variable look incomplete and unbalanced. m_ just looks better. Using m_ is also more extendable, as you can use g_ for globals and s_ for statics.

PS: The reason why you don't see m_ in Python or Ruby is because both languages enforce the their own prefix, Ruby uses @ for member variables and Python requires self..

Many of those conventions are from a time without sophisticated editors. I would recommend using a proper IDE that allows you to color every kind of variable. Color is by far easier to spot than any prefix.

If you need to get even more detail on a variable any modern IDE should be able to show it to you by moving the caret or cursor over it. And if you use a variable in a wrong way (for instance a pointer with the . operator) you will get an error, anyway.

Other languages will use coding conventions, they just tend to be different. C# for example has probably two different styles that people tend to use, either one of the C++ methods (_variable, mVariable or other prefix such as Hungarian notation), or what I refer to as the StyleCop method.

private int privateMember;
public int PublicMember;

public int Function(int parameter)
{
  // StyleCop enforces using this. for class members.
  this.privateMember = parameter;
}

In the end, it becomes what people know, and what looks best. I personally think code is more readable without Hungarian notation, but it can become easier to find a variable with intellisense for example if the Hungarian notation is attached.

In my example above, you don't need an m prefix for member variables because prefixing your usage with this. indicates the same thing in a compiler-enforced method.

This doesn't necessarily mean the other methods are bad, people stick to what works.

IMO, this is personal. I'm not putting any prefixes at all. Anyway, if code is meaned to be public, I think it should better has some prefixes, so it can be more readable.

Often large companies are using it's own so called 'developer rules'.
Btw, the funniest yet smartest i saw was DRY KISS (Dont Repeat Yourself. Keep It Simple, Stupid). :-)

Others try to enforce using this->member whenever a member variable is used

That is usually because there is no prefix. The compiler needs enough information to resolve the variable in question, be it a unique name because of the prefix, or via the this keyword.

So, yes, I think prefixes are still useful. I, for one, would prefer to type '_' to access a member rather than 'this->'.

It is useful to differentiate between member variables and local variables due to memory management. Broadly speaking, heap-allocated member variables should be destroyed in the destructor, while heap-allocated local variables should be destroyed within that scope. Applying a naming convention to member variables facilitates correct memory management.