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The question of understanding a large code has previously been well answered. But I feel I should ask this question again to ask the problems I have been facing.
I have just started a student job. I am a beginner programmer and just learned about classes two months back. At the job though, I have been handed a code that is part of a big software. I understand what that code is supposed to do (to read a file). But after spending a few weeks trying to understand the code and modify it to achieve our desired results, I have come to the conclusion that I need to understand each line of that code. The code is about 1300 lines.
Now when i start reading the code, I find that, for example, a variable is defined as:
VarType VarName
Now VarType is not a type like int or float. It is a user defined type so i have to go the class to see what this type is.
In the next line, I see a function being called, like points.interpolate(x);
Now i have to go into another class and see what the interpolate function does.
This happens a lot which means even if I try to understand a small part of the code, I have to go to 3 or 4 different classes and keep them in mind all at one time without losing the main objective and that is tough.
I may not be a skilled programmer but I want to be able to do this. Can I have some suggestions how i should approach this?
Also (I will sound really stupid when I ask this) what is a debugger? I hope this gives you an idea of where I stand (and the need to ask this question again). :(
With any luck, those functions and classes should have at least some documentation to describe what they do. You do not need to do know how they work to understand what they do. When you see the use of interpolate, don't start looking at how it works, otherwise you end up in a deep depth-first-search through the code base. Instead, read its documentation, and that should tell you everything you need to know to understand the code that uses it.
If there is no documentation, I feel for you. I can suggest two tips:
Make general assumptions about what a function or class will do from its name, return type and arguments and the surrounding code that uses it until something happens that contradicts those assumptions. I can make a pretty good guess about what interpolate does without reading how it works. This only works when the names of the functions or classes are sufficiently self-documenting.
If you need a deep understanding of how some code works, start from the bottom and work upwards. Doing this means that you won't end up having to remember where you were in some high level code as you search through the code base. Get a good understanding of the low level fundamental classes before you attempt to understand the high level application of those types.
This also means that you will understand the functions and classes in a generic sense, rather than in the context of the code that led you to them. When you find points.interpolate(x), instead of wondering what interpolate does to these specific points with this specific x argument, find out what it does in general. Later, you will be able to apply your new-found knowledge to any code that uses the same function.
Nonetheless, I wouldn't worry about 1300 lines of code. That's basically a small project. It's only larger than examples and college assignments. If you take these tips into account, that amount of code should be easily manageable.
A debugger is a program that helps you debug your code. Common features of debuggers allow you to step through your code line-by-line and watch as the values of variables change. You can also set up breakpoints in your code that are of interest and the debugger will let you know when it's hit them. Some debuggers even let you change code while executing. There are many different debuggers that all have different sets of features.
Try making assumptions about what the code does based on its title. For example, assume that the interpolate function correctly interpolates your point; only go digging in that bit of code if the output looks suspicious.
First, consider getting an editor/IDE that has the following features:
parens/brackets/braces matching
collapsing/uncollapsing of blocks of code between curly braces
type highlighting (in tooltips)
macro expansion (in tooltips or in a separate window/panel)
function prototype expansion (in tooltips or in a separate window/panel)
quick navigation to types, functions and classes and back
opening the same file in multiple windows/panels at different positions
search for all mentions/uses of a specific type, variable, function or class and presentation of that as a list
call tree/graph construction/navigation
regex search in addition to simple search
bookmarks?
Source Insight is one of such tools. There must be others.
Second, consider annotating the code as you go through it. While doing this, note (write down) the following:
invariants (what's always true or must always be true)
assumptions (what may not be true, e.g. missing checks/validations or unwarranted expectations), think "what if"
objectives (the what) of a piece of code
peculiarities/details of implementation (the how; e.g. whether exceptions are thrown and which, which error codes are returned and when)
a simplified call tree/graph to see the code flow
do the same for data flow
Draw diagrams (in ASCII or on paper/board); I sometimes photograph my papers or the board. Specifically, draw block diagrams and state machines.
Work with code at different levels of abstraction/detail. Zoom in to see the details, zoom out to see the structure. Collapse/uncollapse blocks of code and branches of the call tree/graph.
Also, have a checklist of what you are going to do. Check the items you've done. Add more as necessary. Assign priorities to work items, if it's appropriate.
A debugger is a program that lets you execute your program step by step and examine its state (variables). It also lets you modify the state and that may be useful at times too.
You may use a debugger to understand your code if you're not very well familiar with it or with the programming language.
Another thing that may come in handy is writing tests or input data test sets for your program. They may reveal problems and limitations in terms of logic and performance.
Also, don't neglect documentation and people! If there's something or someone that can give you more information about the project/code, use that something or someone. Ask for advice.
I know this sounds like a lot, but you'll end up doing some of this at some point anyway. Just wait for a big enough project. :)
You may basically needs to understand what is the functionality of a function being called at first, then understand what is input and output to that function, for example, if you really needs to understand how interpolate is done, you can then go to the details. Usually, the name of the functions are self-explainable, you can get a feeling about what the function does from its name if the code is well written.
Another thing you may want to try is to run some toy examples to go through the code, you can use some of the debuggers or IDE that can help you navigate through the code. Understanding large-scale code takes time and experience, just be patience.
"Try the Debugger Approach"
[Update : A debugger is a special program that lets you pause a running program to examine the state of program (Variable Values/Which function is running/Who is the parent function etc.,)]
The way I do it is by Step Debugging the code, for the usecase I want to understand.
If you are using an Advanced/Mordern IDE then setting breakpoints at the entry point (like main() or a point of interest) is fairly easy. And from there on just enter into the function you want to examine or overstep the function.
To give you a step by step approach
Setup a break point in the main() methods (entry points) starting expression.
Run the program with debugging active
The program will break at the break point.
Now, if step over until you come across a function/expression that seems interesting. (say, your points.interpolate(x); ) function
Step into the function, and examine the program state like the variables and function stack, in live.
Avoid complex system Libraries. Just Step over/Step out. (Example: Avoid something like MathLib.boringComputaion() )
Repeat until the program exits.
I found out that this way of learning is very rapid and gives you a quick understanding of any complex/large piece of software.
Use Eclipse, or if you cant then try GDB if its C/C++. Every popular programming language has a decent Debugger.
Understand the basic debugging operations like will be a benifit:
Setting-up a breakpoint.
Stopping at a breakpoint.
Examine/Watch Variables.
Examine Function Stack (the hierarchy of function calls)
Single-Step - Stepping to next Line in Code.
Step-Into a function.
Step-Out of a function.
Step-over a function.
Jumping to the next breakpoint (point of interest).
Hope, it helps!
Many great answer have already been given. I thought to add my understanding as a former student (not too long ago) and what I learned to help me understand code. This particularly helped me because I began a project to convert a database I wrote in Java many years ago to c++.
1. **Code Reading** - Do not underestimate this important task. The ability to write code
does not always translate into the ability to read it -- and reading it can be more
frustrating than writing it.
Take your time and carefully discover what each line of the codes does. This will certainly help you avoid making assumptions unless you come across code that you are familiar with and can gloss over it.
2. Don't hesitate to follow references, locate declarations, and uncover definitions of
code elements you are reading. What you learn about how a particular variable,
method call, or class are defined all contribute to learning and ultimately to you
being able to perform your task.
This is particularly important because detective, and effective detective work, are essential parts of being bale to understand the small parts of the code so that you can, in the future, grasp the larger parts with less difficulty.
Others have already posted information about what a debugger is and you will find it is an invaluable asset at tracking down code errors and, I think, helps with code reading, knowledge gain, and understanding so you can be a successful programmer.
Here is a link to a debugger tutorial utilizing Visual Studio and may give you a strong understanding of at least the process at hand.
I have to interview some C++ candidates over the next few weeks and as the most senior programmer in the company I'm expected to try and figure out whether these people know what they are doing.
So has anybody got any suggestions?
Personally I hate being left in a room to fill out some C++ questions so I'd rather do a more complex test that I can chat with the interviewee about their approaches and so forth as we go along. ie it doesn't matter whether they get the right answers or not its how they approach the problem that interests me. I don't care whether they understand obscure features of the language but I do care that they have a good solid understanding of pointers as well as understanding the under lying differences between pointers and references. I would also love to see how they approach optimisation of a given problem because solid fast code is a must, in my opinion.
So any suggestions along these lines would be greatly appreciated!
I wouldn't make them write code. Instead, I'd give them a couple of code snippets to review.
For example, the first would be about design by contract: See if they know what preconditions, postconditions and invariants are. Do a couple of small mistakes, such as never initializing an integer field but asserting that it is >= 0 in the invariant, and see if they spot them.
The second would be to give them bool contains(char * inString, char c). Implement it with a trivial loop. Then ask whether there are any mistakes. Of course, your code here does not check for null in the input parameter inString (even if the very previous question talked about preconditions!). Also, the loop finishes at character 0. Of course, the candidate should spot the possible problems and insist on using std::string instead of this char * crap. It's important because if they do complain, you'll know that they won't add their own char *'s to new code.
An alternative which addresses containers: give them a std::vector<int> and code which searches for prime numbers or counts the odd numbers or something. Make some small mistake. See if they find any issues and they understand the code. Ask in which situation a std::set would be better (when you are going to search elements quite systematically and original order of entrance doesn't matter.).
Discuss everything live, letting them think a couple minutes. Capture the essence of what they say. Don't focus on "coverage" (how many things they spot) because some people may be stressed. Listen to what they actually say, and see if it makes any sense.
I disagree with writing code in interviews. I'd never ask anyone to write code. I know my handwritten code would probably suck in a situation like that. Actually, I have seldom been asked to do so, but when I have, I haven't been hired.
This one is a great complex task, even though it is looking quite harmless.
I believe that a C++ programmer needs more than just generic programming skills, because...
In C++ it's harder to shoot yourself in the foot, but when you do, you blow off your whole leg.
Writing bug-free, maintainable C++ code places a much higher demand on a few areas than most languages.
One thing I'll call "pedanticness". You know how some people can spot spelling errors in something at a glance? A C++ programmer needs to be able to spot simple bugs while they read or write code (whether the code is their own or not). A programmer who relies on the "compile and test" technique just to get rid of simple bugs is incompatible with the C++ language, because those bugs don't always lead to immediate failure in C++.
C++ programmers also need a good knowledge of low-level stuff. Pointers, memory allocators, blocking, deadlocks. And "nitty gritty" C++ issues, like multiple inheritance and method hiding and such, particularly if they need to maintain other people's code.
Finally, C++ programmers need to be able to write code that's easy for other people to use. Can they design stuff well?
A good test for the first two areas is "Here's some C++ code I got off the internet. Find the bugs, and identify the unneccessary bits." (There's lots of really bad C++ code available on the internet, and often the programmer does unnecessary things due to a faulty understanding of how to be "safe" in C++.)
The last area you can test with more generic interview questions.
A few questions can allow you to know a lot about a candidate:
Differences between a pointer and a reference, when would you use each?
Why would you make a destructor virtual?
Where is the construction order of a class attributes defined?
Copy constructor and operator=. When would you implement them? When would you make them private?
When would you use smart pointers? what things would you take into account to decide which?
Where else have you seen RAII idiom?
When would you make a parameter const? when a method?
When would you make an attribute mutable?
What is virtual inheritance?
What is template specialization?
What are traits?
What are policies?
What is SFINAE?
What do you know about C++Ox standard?
What boost libraries have you used?
What C++ books have you read? (Sutter? Alexandrescu?)
Some short exercises (no more than 10 minutes) about STL containers, memory management, slicing, etc. would also be useful. I would allow him to do it in a computer with a ready environment. It's important to observe the agility.
Checkout Joel's Guerrilla guide to interviewing. Seems a lot like what you are looking for.
"Write a program that receives 3 integers in the range of 0..2^32-1, and validates if they represent valid edges of a triangle".
It seems to be a simple question. The input is considered valid if the sum of any two edges is greater than the third edge. However, there are some pitfalls, that a good programmer will handle:
The correct type to use should be unsigned long. Many "programmers" will fail here.
Zero values should be considered as non-valid.
Overflow should be avoided: "if (a+b <= c) return false" is problematic since a+b may cause an overflow.
if (a <= c-b) is also bad solution since c-b may be negative. Not a good thing for unsigned types.
if (c > b) { if (a <= c-b) return false; } else { if (a <= b-c) return false; } This looks much better, but it will not work correctly if (a >= b+c).
A good programmer must be detail oriented. This simple exercise will help checking if he is.
Depending on what your organisation's pre-screening is like, assume that the person knows nothing at all about C++ and has just put in on their CV because it makes them look supertechnical. Seriously. Start with something simple, like reversing a string. I have had candidates who couldn't even write a function prototype for this !!
Do not forget to also test for code bigotry. I know I don't want anyone working for or with me that isn't a flexible and consequently practical programmer both in their attitude to the programming language, but also in their approach to problem solving.
Denying any type of preconceptions
Understanding the value of the
exceptions in any Best Practices
Being capable of refusing long term
habits in favor of something else if
the need arises
These are characteristics dear to me. The manner of testing for these is not ideal if the interviews aren't lengthy or don't involve presenting code. But showing code snippets with purposely debatable techniques while offering a use case scenario and asking the candidate how they feel about the solution is one way.
This article offers some general ideas that are relevant regardless of what language you're working with.
Don't test only the C++ and overall technical skills! Those are of course important, but they are nothing if people don't listen, don't answer properly or don't follow the commitments they made.
Check at most for the ability to clearly communicate. If people cant tell you what roughly they did in their former jobs within a few minutes, they will also be unable to report about their work at your place etc.
In a recent company we invited people for interviews in groups of about 3 people together. They were surprised, but nobody was angry about that. It was very interesting, because people had to communicate not only with us, but also with others in the same position. In case we were interested further, we arranged a second interview.
You can choose potentially problematic task and see how they approach it. Ask them to write a smart pointer for example, you'll see if they understand pointers, references and templates in one step :) Usually they are stressed so they will do mistakes, those mistakes might help you find out how good they problem solving skills are, what paths would they use to fix a mistake and so on. The only problem with this approach is that sometimes interviewee just don't know anything about the task and you would have to quickly figure out something easier. If they do perfect code you can discuss their choices but when there's nothing to look at it is depressing for both of you.
Here is my answer to a similar question geared towards C#, but notice that my answer is language agnostic. My interview question is, in fact, in C. I rarely interview a person with the goal of finding out if they can program. I want to find out if they can think, problem solve, collaborate, communicate, understand something new, and so on. In the meantime, I circle around trying to see if they "get it" in terms of the big picture of software engineering. I use programming questions because that's a common basis and an easy ruse.
Get Codility.com to screen out non-programming programmers, this will get you a limited number of mostly reasoable candidates. Sit for an hour with each of them and try to build something together (a micro web server, a script for processing some of your data, a simple GUI). Pay attention to communication skills, i.e. how much effort does it take to understand the candidate. Ask the candidate for recommendation of books related to the subject (C++ software development in your case). Follow Guerilla Guide to Interviewing, i.e. answer yourself honestly, if the person is smart and gets things done. Good luck.
Check 10 C++ Interview Questions by Tests4Geeks.
It's an addition to their pre-interview C++ test and it has really usefull questions. Many people have been working on these interview questions so it's quite balanced and has no tricky or syntax questions.
Idea is quite simple - first you weed out incompetent candidates using the test, then you use article questions in real-life interview.
Whatever you do, pairing would be a good idea. Come up with a good program and pair with the guy and work towards solving the problem. IMHO, that a very good idea
So has anybody got any suggestions?
I'd recommend getting a copy of this:
http://www.amazon.co.uk/Programming-Interviews-Exposed-Secrets-Programmer/dp/047012167X/ref=sr_1_1?ie=UTF8&s=books&qid=1252499175&sr=8-1
ie it doesn't matter whether they get the right answers or not its how they approach the problem that interests me
You could ask the candidate to come up with a UML design to a common problem. If they show you a design pattern, then you can talk through the pros/cons of the pattern. You could then ask them to produce some code for one of the classes.
This would help you determine their technical knowledge level and their communication abilities.
I do care that they have a good solid understanding of pointers as well as understanding the under lying differences between pointers and references
Linked list problems are good for determining whether a candidate has a solid grasp of pointers.
As for references, you could show them some code that does not use references correctly, and ask them to describe the problem.
e.g show them a class definition that contains a reference member variable, and the implementation of the constructor with the reference initialization missing.
I would also love to see how they approach optimisation of a given problem because solid fast code is a must, in my opinion.
I'd start off simple...
Show them a code example that passes strings to a function by value. (the strings should not be modified in the function). You should check they correct the code to pass the strings by const reference.
After this, you could show a constructor that uses assignment instead of initialization (for objects). Ask them to improve it.
Lastly, ask them simple questions about data structure selection.
e.g. When they should use a list rather than a vector.
If you feel they have a grasp of the fundamentals you could either ask how they approach optimization problems (discuss profilers etc), or ask them to optimize something less obvious.
Take a look into this C++ test. They have a questions about differences between pointers and references as you require.
Here is full list of topics:
Fundamentals: References & Pointers, Const Correctness, Explicit
Standard Library
Class Design, Overloading
Virtual Functions, Polymorphism, Inheritance
Memory Management, Exception Safety
Miscellaneous: Perfect Forwarding, Auto, Flow Control, Macros
These guys are really serious about their questions, they also made the great list of C++ interview question which you might ask your candidates:
https://tests4geeks.com/cpp-interview-questions/
I am involved in a contest, and in one event we have debugging questions. I have to design some really good debugging problems in C and C++.
How can I create some good problems on debugging? What aspects should I consider while designing the problems?
My brainstorming session:
Memory leaks of the subtle sort are always nice to have. Mess around with classes, constructors, copy-constructors and destructors, and you should be able to create a difficult-to-spot problem with ease.
One-off errors for array loops are also a classic.
Then you can simply mess with the minds of the readers by playing with names of things. Create variables with subtly different names, variables with randomized (AND subtly different) names, etc. and then let them try and spot the one place where you've mixed up length and lenght. Don't forget about casing differences.
Calling conventions can be abused to create subtle bugs too (like reversing the order of parameters).
Also let's not forget about endless hours of fun from tricky preprocessor defines and templates (did you know that C++ templates are supposedly Turing-complete?) Metaprogramming bugs should be entertaining.
Next idea that comes to mind is to provide a correct program, but flawed input data (subtly, of course). The program will then fail for the lack of error checking, but it will be some time until people realize that they are looking for problems in the wrong place.
Race conditions are often a difficult to reproduce and fix, try to play with multithreading.
Underflows/overflows can be easily missed by casual inspection.
And last, but not least - if you're a a programmer, try remembering what was the last big problem that you spent two weeks on solving. If you're not a computer programmer, try to find one and ask them. I'm a .NET programmer, so unfortunately my experiences will relate little to your requirement of C/C++.
For some simple "find the bug in this source code" excercises, check out PC-lint's bug of the month archive.
In addition to what's above, consider side effects. For example:
// this function adds two ints and returns the sum
int add_em(int &one, int &two)
{
two += one;
return two;
}
As you can see, this code modifies the two variable, although the comment doesn't mention that...
Debugging is a broad scope, and it may be wise to reflect that in your questions. Without going into details, I can see the following categories :
Source-level debugging - no hints
Questions in this category just have source code, without any further hints on what's wrong.
The actual bug can vary quite a lot here: from straightforward logic bugs like buffer overflows and counting errors to mistaken assumptions, via mathematical errors like rounding errors to just mistaken assumptions like assuming a particular endianness or padding.
Source-level debugging - problem stated
Questions in this category have source code, as well as desired versus actual output/behavior.
E.g. "This program should print 42, but instead prints Out of Memory. Why?"
Crashed code
Questions in this category come not just with source code, but also with a crash dump.
I'll add to the answers above that another form of bugs is the incorrect use of some library or API code. Superficially everything looks ok, but there is some caveat (e.g., a precondition or a limination) that one is not aware of. Interactive debuggers are not as effective by themselves in these situations because they don't expose that information to you (it's often hidden in the documentation).
For example, I did in the past a study of this stuff. I gave people code that used (a messaging API in Java), where the error was that the program was getting stuck as soon as you tried to receive a message. Debugging this interactively was almost impossible. They had to manually figure out what was going on, and realize that one of the queues wasn't set up correctly.
These sort of bugs are actually quite common.
Real world debugging would include find synchronization problems and problems between managed/unmanaged boundary, so please consider c/c++/c# as an option.
Or for real fun, consider using just c# and finding memory leaks.
Also, you will need to mention which tools are allowed to be used. On windows, there are literally dozens of debugging tools available.
How to test a result of a program that is basically a black box? For example one year ago I had to write a B tree as a homework and I really struggled with testing the correctness. What strategies do you use in such scenarios? Visualization? Robust input-->result sets of testing data? What do you do when it is hard to get such data because the only way how to get them is your proper working program?
EDIT: I think that my question was misunderstood. There was no problem with understanding how B tree works. That is trivial. But writing robust tests for validating its proper functionality is not so trivial. I think that this school problem is similar to many practical REAL word scenarios and test cases. And sometimes understanding the domain is quite different from delivering working and correct program...
EDIT2: And yes, with B tree it is possible to validate proper behavior with pen and paper. But this is really dirty and not fun :) This is not working well with problems that requires huge amount of data for their validation...
I'm not sure these answers really capture the problem at hand. A B-tree's input and output aren't any different from those of any other dictionary---but the algorithm performs better, if it's implemented correctly. It's only really got two functions to test (add, and find) so theoretically, "black-box" testing of this single component should be fine. Designing for testability isn't the issue, since no matter how you do it the whole algorithm will be one component.
So the question is: when you have to implement subtle algorithms, the kinds with complicated output that you can't always understand in your head so well, how do you test them? I think there are three different strategies you can use:
Black-box test basic functionality. For the B-tree case, this is things like cwash suggested, and also, things like making sure that when you add an item, you can then find it, etc.
Test certain invariants that your algorithm should maintain (the B-tree should be balanced, values within nodes should be sorted, etc.)
A few, small "pencil-and-paper" tests may be necessary -- work the algorithm out by hand and check that it matches what your code does. But the big-data tests can all be of type 2. These can also be brittle, so unless you need to be really sure about your algorithm, you may want to avoid them.
If you do not grasp the problem at hand, how can you develop a solution to it? My suggestion would be to understand the domain enough to be able to work out the problem on paper and ensure that your program matches.
Consult with an expert on the subject.
I know if I have a convoluted procedure I'm trying to fix, I have no idea what the output should be after my changes, so I need to consult a fellow developer with more knowledge of the business need, and they are able to verify what I've done is correct.
I would focus on constructing test cases that exercise the functionality of your B-tree algorithm. I haven't looked at it for years, but I'm fairly sure you'll be able to find a documented sequence of steps to insert a set of values in a specific order, then validate that the leaf nodes are as they should be. If you construct your testing along these lines, you should be able to prove your implementation is correct.
The key is to know there is a balance between testing something to death and doing tests that adequately cover what should be covered. Edge cases, e.g null inputs or checking inputs are numeric by testing an alphabet character or a punctuation character, are likely most of the tests you'd need. To complement this there may be one or two common cases to handle to show the program can handle a non-edge case as well. To cover all valid input in most programs is overkill and would result in an overwhelmingly large amount of tests.
I think the answer to the question you're asking boils down to designing for testability. Often you get a testable design for free when you test-drive the development of the solution. But let's face it, when you're implementing a highly mathematical algorithm, this just doesn't fall out.
To make sure you have a testable design, you need to understand what a seam is. Then you need to know a few rules of thumb, such as avoiding statics, using polymorphism, and properly decomposing problems and separating concerns.
Watch "The Clean Code Talks -- Unit Testing" by Misko Hevery, I think it will help you wrap your head around it.
Try looking at it from a requirements point of view, rather than an implementation point of view. Before you write code, you must understand exactly what you want it to do.
Testing and requirements should be a matching pair. If you're having trouble defining tests, maybe it's because the requirements are not well-defined. That in turn implies that you may have bugs that aren't so much implementation bugs, but "lack of clear requirements" bugs. The code writer in that case would be working to a mental list of requirements that he/she thinks is requirements, but can't be sure, and they're not written down for independent understanding and verification.
I've struggled with software where the requirements weren't clear, because the customer couldn't even tell us what they wanted. But when we delivered to them, they sure could tell us then what they didn't like about it! A big part of software engineering is getting the requirements right before the coding begins. This is true on the high-level (overall product, with requirement input from customer) and also the smaller level (modules, individual functions, where requirements are internally defined by software team or individuals). It is still true to some degree I think for iterative development, although the high-level requirements are more fluid.
#Bystrik Jurina,
I often get involved in projects which involve conversions between disparate data formats. Most answers have focused on testing a B-tree or similar algorithm, but it seems that you're looking for a more general answer.
Most of my work is based on the command line. It may sounds like a contradiction, but one of the first tools I use is visualization. I'll write some methods to write out my data structures in a format that's easy to consume. This can (and usually does) include something that's visually clear. But often it also means something that I could easily parse with a smaller test program, or even import into Excel.
I'll start by focusing on the basic outline, and write a program that does the bare minimum of what I need to accomplish. If it's a multi-step process, this might mean implementing one step at a time and validating the results of each step before moving on. Or writing something that works only in specific cases, and then expanding the set of cases where it's expected to work. At first you can validate that the code works in the limited set of cases, such as for known input data. As the project moves forward, you can start logging warnings for cases you might not have tested, or for unexpected types of input data. This has drawbacks, but is a nice approach when you're dealing with a known set of input data
Validation techniques can include formal test cases, or informal programs that work to challenge your assumptions. It could mean writing a basic driver program to exercise the "core" routines. A good example would be to add a record to a database, then read it back and compare the original object against the one loaded from the database.
If you have trouble wrapping your head around the way a program functions, think about what it needs to accomplish. It might be easier to writing code that tests the way different inputs produce different outputs. Producing visualizations is a good help, because the act of deciding how to display the data can make you think about different conditions and focus in on the most critical parts of your data structures.
Often I've found that building a visualization brings me to admit that the way the data is being stored just isn't very clear. For a B-tree, the representation isn't very flexible. But for other cases, you may be using parallel arrays when a nested tree of objects would be more natural.
Curious to get people's thoughts. I conduct frequent interviews, and have had enough in my career to reflect on them, and I've noticed a broad array of questions. I made this c++ specific, but it's worth noting that I have had people ask me algorithmic complexity questions over the phone, and I don't even mean what is the complexity of a hash lookup vs. a binary tree, I mean more like analytical problems, such as "imagine there are 4 bumble bees, each buzzing bla bla bla."
Now personally I prefer to keep phone screens a little more concrete, and leave the abstract questions for the white board. So when conducting c++ phone interviews, what kind of topics do you cover, especially for Sr. developers?
I know there is another thread similar to this, but frankly it seems to completely have missed the point that this is about phone screens, not interviews that are face to face. Plus this is more c++ specific.
I'd ask about resource/memory management, because it's an important subject in C++, and it doesn't require concrete code. Just sketch a simple hypothetical scenario, and ask how they'd ensure some vital resource gets freed even in the face of errors/exceptions. Say they're developing a network app, how do they ensure that we close our sockets properly? Of course the proper answer would be to wrap it in a RAII object, but don't ask them that directly (it's easy to google "RAII", while the above question "how would you ensure resources get released properly" actually shows you whether or not they know the appropriate techniques. If they answer "wrap everything in try/catch", they might have a problem. And this ties in nicely with questions about the differences between heap and stack.
You might be able to come up with some simple question about exception safety too, which doesn't require any real code. In general, I'd say a discussion of all the various C++ idioms might be a good idea, because many of them don't require much actual code, but are still vital language-specific concepts.
See if they know about smart pointers (again preferably by giving them a situation where smart pointers are called for, and see how they would solve the problem), and maybe templates/metaprogrammin (in the latter case, probably just find out if they're aware that it's possible ,rather than requiring them to code actual metaprograms on the phone)
You might also want to ask about some common areas of undefined behavior (what are the values of a and b after executing a = b++ + b++??), or allocate an array of 10 elements, and add 10 or 11 to the array pointer, and ask what the result is in each case (+=10 is legal, gives you a past-the-end pointer, +=11 is undefined). Or give them a scenario where they need to copy a lot of objects, and ask how they'd do that (plain for-loop copying each element at a time, memcpy or std::copy are obvious answers. Note the caveats with memcpy, that it's not safe for non-POD objects)
Or ask about their coding style in general. How do they feel about iterators? Do they prefer plain old for-loops? Do they know how to use std::for_each or std::transform?
Edit:
Seems the a = b++ + b++ (the answer is undefined behavior, btw) suggestion in particular generated a lot of comments. Perhaps people read too much into it. As the OP said he preferred to ask concrete (not abstract, and easy to explain/answer/discuss over the phone) questions, that'd reveal a bit about the interviewee's C++ skills, and this is a simple (and yes, perhaps nitpicky) example of that. The reasoning behind it is that 1) it has an intuitive meaning, which is wrong, and 2) you have to have a certain level of experience with C++ before you realize this. And of course 3), it's short and easy to ask over the phone. It doesn't require anyone to write code down. No, it won't reveal whether the candidate is a "great programmer", but as I understood the question, that wasn't the goal either. If someone gets it wrong, it doesn't mean much at all, but if they get it right, you can be fairly sure that they know a bit of C++. But if you read my answer again, you'll see that it was just a quick example of a category of questions I thought should be represented. C++ is full of undefined behavior, even in code that looks completely harmless and intuitive. Asking the candidate to recognize some instance of this may be useful, whether it's the "modify the same variable twice in the same expression" example above, or something different.
I like to find out if people actually know anything about the tools they're using. And I also find that "senior" developers can have serious gaps in their knowledge.
You could consider asking
What a vtable is.
How templates work
What the difference between the heap and the stack is. The depth of the reply to this one could be quite illuminating!
The "you really need to interview me" answer to the last question would cover
allocation - limits, use-cases, failure modes, efficiency, resource cleanup, destructors
stack call-frames - what happens when a function is called, parameters, backtraces
As part of a phone screen for C++ positions I've taken to asking 'Who is Bjarne Stroustrup".
I find it amazing that many many people claiming to be be senior C++ devs can't identify. Hint for English speakers: Go to his FAQ and listen to the correct pronunciation before you ask the question.
Design Patterns
Basics of C/C++
Virtual Functions
Poly morphism
Concepts from Scott Mayer's series
About templates.
(Not in particular order)
Last but not the least. I will give some sample problem I tell them to design the classes. Just the interfaces.