In C++, a function is annotated with inline to implore the compiler to inline that function’s code into its callers. The C++ standard says that injecting multiple differing definitions of an inline function into a program is illegal because it breaks the ODR.
An inline function shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case.
So, having multiple differing definitions is illegal but the compiler cannot really stop it. This is because two differing definitions of the same inline method might be inlined into their callers in two different compilation units that are linked together into a program.
Thus, if we use multiple different inline definitions, all bets are off and it is undefined behavior.
This example illustrates the final behavior differs based on just compilation flags:
If we want to control and want each caller to only use its local inline definition, then we can do that by placing the inline method inside an anonymous namespace. The practice of multiple differing inline definitions is still illegal, but at least we now have controlled what happens. This example illustrates this:
GLog is the logging library from Google for C++. It makes it real easy for you to add logging to any C++ application.
Install: Installing GLog header files and library files is easy:
$ sudo apt install libgoogle-glog-dev
Header file: The header file to include in your source file is glog/logging.h
Initialization: You will need to call google::InitGoogleLogging method with the name of your program as the input parameter to start logging.
Levels: There are 4 levels for logging messages in increasing order of severity: INFO, WARNING, ERROR and FATAL. These severity levels have values 0, 1, 2 and 3 respectively.
Log function: To log a message, use the LOG macro, similar to how you use cout. For example, this example shows logging messages of different severity:
int main(int argc, char* argv)
LOG(INFO) << "This is an info message";
LOG(WARNING) << "This is a warning message";
LOG(ERROR) << "This is an error message";
LOG(FATAL) << "This is a fatal message";
Library: To compile a source file using GLog, you will need to link using -lglog.
Log files: By default, when you run your program, 3 new log files will be created in /tmp directory. The filenames are of this format:
The file with INFO in its name has log messages of levels INFO and above. The file with WARNING int its name has log messages of levels WARNING and above. Similarly, for the file with ERROR in its name.
In addition, 3 symbolic links are created in the same logging directory pointing to the latest log files. These 3 filenames are of the format:
Log to display: By default, when you run your program, you will see log messages of ERROR and FATAL on the stderr, so they will appear on the console. Note that the first FATAL message will prompt the killing of your program.
If you want the program to log to stderr instead of writing to log files, set this environment variable GLOG_logtostderr=1
If you want to change the logging directory from /tmp to some other location, set this environment variable GLOG_log_dir=/some/path
Virtual functions are a key feature of C++ to enable runtime polymorphism. This post is my attempt in understanding how they are implemented and executed at runtime. The compiler used is GCC 5.4.0 on Ubuntu 16.04.
Here is a simple program that uses virtual functions that we will use as an example:
To aid us in understanding what this code is compiled into, we request GCC to add debugging information (using option -g) when we compile it:
$ g++ -g virtual_function_example.cpp
Almost all C++ compilers implement virtual functions by using virtual tables, more commonly called as vtables. This is a table of function addresses, one for each virtual function in the class. One virtual table is created for each class that has virtual functions.
We can see the existence of the methods and virtual tables of each class and their addresses by examining the binary:
Here we use the readelf program to extract the symbols from the binary. The symbols are in mangled form that is difficult to decipher for humans. So, we pipe it through a demangler.
Here is the output I got on my computer:
We can check which sections of virtual memory the class methods and virtual tables will be loaded into by examining the sections of the binary:
$ readelf --sections a.out
There are 37 section headers, starting at offset 0x6b78:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
 .text PROGBITS 00000000004007a0 0007a0 0002a2 00 AX 0 0 16
 .rodata PROGBITS 0000000000400a50 000a50 00008b 00 A 0 0 8
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
We can cross-examine the addresses of the class methods and virtual tables with the starting addresses and sizes of the sections. We see that the class methods will be loaded into the .text section and the virtual tables into the .rodata segment. The flags of these sections indicate that only the .text section is executable, as it should be.
Finally, let us examine how the virtual tables are used at runtime to determine which method to execute. To do this, we disassemble the binary instructions in the binary:
From the output of objdump, only the disassembly of the main function is shown above. In the above command, we have requested objdump to --disassemble the binary code to assembly code, to --demangle the symbol names to human readable form and to annotate the disassembly with the original C++ --source statements.
By examining the disassembled code, the runtime mystery is revealed. We need to note that every object of a class, that has virtual methods, stores a pointer to its class virtual table. On a 64-bit computer, this means that objects of such classes need extra space of 8 bytes. This pointer is placed at the beginning of the memory layout of the object, even before other members of the object.
When you call a virtual method in C++ code, the compiler generates these instructions:
Jump to the beginning of the object. This is a location on the heap or stack, depending on how the object was created. This is where a pointer to its class virtual table is stored.
Jump to the start of the class virtual table. This is a location in the .rodata section of the process virtual memory, as we noted earlier.
Depending on which virtual method is needed, jump to that entry in the virtual table. This entry has the address of that virtual method.
Finally, jump to the address of the virtual method and start executing its instructions. This is in the .text section of the process virtual memory.
Reference to pointer is a useful construct to be aware of in C++. As you might already know, the C++ language allows you to take a reference to any object that is not a temporary. You can take a const reference to any object, including a temporary. So what is special about reference to a pointer?
A common construct in C++ is to receive a pointer to a pointer as input argument to a function. This is typically used to allocate a primitive or an object inside the function. The allocated primitive or object will be available in the caller after the function is called and done with. This idiom is common with C programmers who have moved to C++. It is such code that becomes a lot cleaner and easier to write and read if a reference to pointer is used as input argument to function. As a bonus, since it is a reference it has to always refer to a pointer that actually exists.
The code example below shows the difference between pointer to pointer and reference to pointer:
Note how reference-to-pointer is cleaner to understand both at the caller location and inside callee.
If you are having a class hierarchy, with base class and derived classes, then try to always make the base class destructor as virtual.
I recently noticed an application having a serious memory leak after merging some code. Other than the leak, everything else about the code was executing fine! After debugging the code, the culprit turned out to be a base class destructor that was not virtual. If only the above rule had been followed diligently, the error would have been caught easily.
Why this rule? The reason for this rule is pretty simple. A derived class destructor might be deallocating objects or freeing memory that it had allocated earlier during its creation or execution. Now think about the scenario where this derived class object is held using a base class pointer and it is freed.
If base class destructor is not virtual: Only the base class destructor is called, thus causing a memory leak.
If base class destructor is virtual: The derived class destructor is called first (thus freeing its allocated objects correctly) before the trail of destruction heads up the chain of hierarchy, ending in the base class destructor. This is the intended correct behavior.
Here is a code example that illustrates this scenario:
override and final are two new qualifiers introduced in C++11. The use of these qualifiers is optional. They are meant to be used with virtual methods to show the intention of the method. The compiler uses these qualifiers to check if your intention matches the actual ground truth in your code and throws a compile error if it does not. Thus, it helps to catch bugs earlier at compile time.
When you specify override for a method, you are indicating to the compiler that this is a virtual method and it is overriding a virtual method with the same signature in one of the base classes that the current class inherits from. If the method is not inheriting from any virtual method with the same signature, the compiler throws an error. Thus if you made a mistake in the function signature while defining this method, you would not have caught it unless you used this qualifier.
When you specify final for a method, you are indicating that this is a virtual method and that no class that inherits from the current class can override this method. If any method tries to override this method in an inherited class, the compiler throws an error.
If override or final are used with non-virtual methods, the compiler throws an error.
These qualifiers are specified after the function input arguments and should be specified after const if the virtual method is a const method. If you put these qualifiers before a const, you will get a weird error with GCC that gives no hint that this is because of the order of qualifiers is wrong!
These qualifiers are to be specified only with the method declaration. If you try to use them with the method definition, the compiler will throw an error.
You can specify override final for a method, but it is the same as using final.
override is not allowed to be used with the base virtual method. This is for the obvious reason that the base virtual method is the first virtual method and it is not overriding any other method.
final can be used with the base virtual method. This can be used to specify that the first base virtual method cannot be overridden in any inherited class.
This code example shows how to use these qualifiers:
CPP Tools: The official extension for working with C++ code. Automatically indexes all code in the currently open directory, offers auto-completion and syntax highlighting.
Python by Don Jayamanne: There are many Python extensions, but this seems to be the most popular one. Syntax highlighting, indexing and code completion.
Vim: There are many Vim extensions, but this seems to be the most popular one. It has entire universes to traverse before it can be as good as Vrapper, the Vim extension for Eclipse. This VSCode extension offers very basic navigation and editing commands.
Git Blame: This extension does one little thing that I need everyday to work with code from other people: know who modified a line of code. This extension shows that for the current line in the status bar.
Matlab: I need to regularly browse through some MATLAB files. This extension offers syntax highlighting of Matlab files.
Tried with: Visual Studio Code 1.4 and Ubuntu 16.04
I recently came across a 2014 talk by Arvid Norberg about the new features in C++11. The video is here and slides are here.
C++ is huge and getting bigger every day. So, I keep discovering interesting new features that I like to note down for use in my own code. Below are my notes from this talk. I do not note aspects that I already know well. This talk has examples that are small but illustrative, so if you hit any of these features, you should see the video to look at the examples.
std::begin and std::end work on C arrays too. Note that this is only when the array size is known. So, the array must have been created in the same local scope.
decltype deduces the type of an expression. So its use is in type expressions. For example, as template arguments.
// vector of the return type of function f
Internally, it is used by auto to deduce type of expression
Lambda expression yields an unnamed function object. The tiny examples in the talk are good.
This is to help programmers find errors. For example, when virtual method in base class is not const and in derived it is. Programmer might miss this error. If virtual method in derived class is declared override and it is actually not, compiler will complain.
This smart pointer is not copyable, but movable. It is deleted when pointer goes out of scope.
Many functions create a heap-allocated object and return it. Traditionally, programmers had to worry about the ownership and lifetime of such a returned object. Return it as unique_ptr and forget about these worries.
Also great for storing such heap-allocated objects in containers.
This C++11 feature was something new to me! I did not understand how to apply it either. I might need to study this in future.
error_code represents an error. It has error_value integral value indicating what is the error. It has category indicating domain of error value.
category is an abstract base class implementing conversion of error_value to human readable message.
There are a whole bunch of old C, C++, Unix and POSIX time functions. They are not platform agnostic, have low time resolutions, have no type safety (milliseconds value can be passed to a function that takes in microseconds and so on) and are not monotonic. Monotonic in this context means that if you measure a time before DST is turned on and after it, the latter value should always be larger, though the wall clock may have been turned back by DST.
Chrono introduces a clock with its own epoch (start of life) and its own resolution.
time_point: A point in time relative to epoch. It has its resolution encoded inside it.
time_duration: Difference of two time points. It has its resolution encoded inside it.
Because these types have their resolutions embedded inside, two durations of different resolutions can be added together to produce a duration that has resolution that is highest or higher than both. They can be passed to function that accepts in a different resolution. The template machinery ensures that it all converts correctly.
Even after many years of working with C and C++, I continue to make new discoveries! All these years I had returned 0 from the main function on success and a non-zero value (almost always 1) on failure. Somewhere at the back of my head it had always troubled me that there was no standardization on these return values and that I was returning what were essentially magic numbers.
I recently discovered that though the C and C++ languages do not have anything to say about this issue (like they rightly should not), the C and C++ standard libraries do provide EXIT_SUCCESS and EXIT_FAILURE. These can be used to return from the main function. These are defined in the stdlib.h and cstdlib headers for C and C++ respectively.
Curious to see what values they represented, I looked up /usr/include/c++/5/cstdlib and found this: