Exception safety

Exception safety is about taking care of the resources that you are using, such as allocated memory or opened files, both in the happy path through your code and in the case of an exception being thrown.

The problem lies in code such as

What happens to the memory which is allocated dynamically for the array if the function bar throws an exception? What surprises many developers is that the execution will skip all code lines after the call to bar in the local function and continue with the first line in the appropriate exception handler. The line where the dynamically array is deleted (using delete[]) is not going to be called at all! This means that the code in the example above is not exception safe, when an exception is thrown then the code will leak memory.

Even though all code lines after the call to bar will be skipped, all the local variables (i.e. allocated on the stack) declared in the function foo are going to be cleaned up. And this is a feature which C++ uses to deal with exception safety. The technique is called RAII (Resource Acquisition is Initialization) and stores variables and data structures on the local stack instead of on the free store directly. Now you object that it is impossible to always store all the data on the stack, you may not know how much data you are going to need when you write the program. The size of a variable is something which you probably want to determine at run time, depending on the input data to your program.

The C++ solution to this is to store a handle to the memory that you have allocated on the free store on the local stack. When that handle goes out of scope its destructor will be called and you simply make sure that you delete all the memory used in the destructor. This is the basic concept of the standard library containers, such as vector, list or map, or the smart pointers shared_ptr and unique_ptr.  To convert the example above into exception safe code, replace the call to the dynamic memory allocation into using std::vector like this.

The instance of the vector is allocated on the local stack, but internally it will call new[] to allocate the memory for the 25 values on the free store. When the vector goes out of scope, either because the function ends or an exception is thrown, then the destructor of the vector is going to be called and this will delete all the memory that was allocated on the free store.

The C++ standard library containers and data structures all use the RAII technique for making sure that no resources are being leaked. When you create a new class you should also use the RAII technique to make sure your code is exception safe. Other developers who are using or modifying your code expect that you have taken care of releasing the resources that you are using and don’t want to worry about cleaning up the mess after you. Make the C++ code that you write developer friendly and clean up your own mess!

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Shared pointer – std::shared_ptr

With C++11 came a new smart pointer class, std::shared_ptr.

Copyable

The shared pointer is allocated on the stack, exactly like the std::unique_ptr, but unlike the unique pointer is it possible to copy the shared pointer. Just like the unique_ptr will the contents which the smart pointer manages be deleted once the smart pointer goes out of scope. The difference is that the resource will only be deleted when all copies of the shared_ptr have gone out of scope. That is to say that as long as there is at least one handle to the shared memory then the resource will not be cleared.

Performance

The behavior of the std::shared_ptr is in practice implemented through reference counting, creating a new copy of a std::shared_ptr increases a counter of the number of live instances and as each copy goes out of scope this counter is decreased one step. When the counter finally reaches zero then the memory is released. This reference counting means that there is a slight overhead with using a shared_ptr compared to using the unique_ptr which doesn’t have this overhead.

std::weak_ptr

There is also one danger with the shared_ptr, it is possible to create dependency chains where two objects references each other through a shared_ptr and none of the two objects will ever be cleared because there is always one reference to each (by the other object). This “bug” can be resolve by using the third smart pointer introduced in C++11, the std::weak_ptr. A weak pointer is like a copy of a shared_ptr but creating it does not increase the reference count. Therefore, when the object is only reference by weak pointers then the memory will be deleted. The weak pointer can be converted into a std::shared_ptr (which you should do before trying to use it) and if the memory referenced to by the weak_ptr has already been released then you will get an empty shared_ptr back.

Creating – std::make_shared

You create instances of shared_ptr using the helper method std::make_shared. You can of course fill in the contents of the shared pointer yourself but the make_shared method is more efficient (one memory operation less than if you do it yourself) and has the advantage that if the constructor of the object throws an exception then the std::make_shared will make sure that no memory leak occurs.

 

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Using RAII to raise the current thread priority temporarily (Windows specific)

In a recent project I had the need to raise the priority of the currently executing thread for a short period of time while an important I/O operation was being performed (the device which the data was read from didn’t have a buffer to store the data hence the reading thread had to be ready at all times to receive the data). I ended up creating a helper class which uses RAII to manage the priority of the current thread. When the object is created on the local scope the priority of the thread is raised and when the object goes out of scope then the destructor will make sure that the priority of the current thread will go back to its original value.

The class deletes the overload of the operator ‘new’ which makes sure that it is impossible to create the object on the free store.

The code to get the current thread and to change the thread priority is Windows specific but I guess this can quite easily be modified to fit other OS:s as well.

 

 

 

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Constructor – creating the object

The constructor is a special function used to setup an object to a valid state.

  • The constructor must have the same name as the class.
  • The constructor does not have a return type.
  • There can be many constructors for the same class, the difference between them is the number and type of the parameters.

A constructor without any parameters is called the default constructor. If you don’t write a default constructor then one will be created for you automatically and setup the object with default values (which may not be what you want).

The constructor of the object is together with the destructor an important part of the RAII paradigm.

Multiple constructors

A class can have many constructors, and one common problem is that these will usually perform the same initialization of the member variables thereby creating redundant code. With the interest of keeping your code DRY, you should consider to extract the common elements into separate functions. The, in many cases, most natural solution to this would be to make the constructors call each other – but there is a pitfall here. Calling one constructor from another will only work in C++11, so if you have a compiler which supports C++11 you can do the following (called delegating constructors):

If you don’t have a C++11 compiler then the common solution is to extract a separate function which does the initialization:

 

Private Constructor

Normally is the constructor of a class a public member function. If a class has a private constructor, then it is not possible to create instances of the class anywhere else but in the class itself. This can be handy if you want all instances to be created in exactly the same way or in the same location using a static factory method.

 

 

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RAII – Resource Acquisition Is Initialization

Resource Acquisition is Initialization (or RAII for short, usually pronounced by simply spelling out the letters R-A-I-I) is a programming technique where we make sure that resources are cleaned up when they are no longer needed by tying a resource to an object. The resource is kept as long as the object is alive and automatically released when the object is destroyed.

The resource that is being managed can be a segment of memory, a file handle, a network socket, a database handle or anything else which there is just a limited number of in the system and which we need to make sure we close properly after we are done with it.

The way RAII works is that we tie the resource to a class. When an instance of the class is created then the constructor of the object’s class is called to create the resource (or passed the resource to be managed). When the object is later destroyed (by going out of scope) then the destructor will automatically be called and here we write the code to release the resource again.

The name is somewhat difficult and controversial, some have suggested instead calling it Scope-Bound Resource Management but this has so far not been too successful.

I think an example of what I mean is necessary here. In the example below is a segment of memory allocated (using new[]) in the constructor of the File class and the memory is released again (using delete[]) in the destructor. The neat thing here is that the destructor will automatically be called when the object goes out of scope, as is done when the function main() in the example exits.

Notice: this is only done automatically for you if the File object is allocated as a local object, this is one of the reasons why you should never use new or delete.

The benefits of RAII is that it creates:

  • exception safety: a stack object will be destroyed at the end of the enclosing scope,  also if an exception is thrown from the code after the object is created.
  •  good locality: the code for allocating the resource is in the same class as the code for releasing the resource again.
  • encapsulation: the managed resource is located inside the owning class and not accessible outside of it.

Because of its benefits is RAII used extensively throughout the standard library classes.

As an example, the following code is using old C functions to write data to a text file NOT using RAII.

In the example above we didn’t use RAII to hold on to the resource (which in this case is the file handle FILE* f). This makes the code not exception safe, if anyone would later add a function call after the file has been opened and this method throws an exception then the call to fclose will not be executed at all, leaving the file opened and locked.

A better example is to instead use the standard library to open and write to the file:

in this example does the instance of the standard library class std::ofstream which we create on the first line hold on to the file resource and when the object goes out of scope when the function ends then the file will be closed automatically for us. Notice how much shorter and cleaner this code becomes. We don’t have to worry about closing the file since this is done for us.

References

  1. Stroustrup: Why doesn’t C++ provide a “finally” construct?

 

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Destructor

The destructor in a class is a special member function which takes care of cleaning up the contents of the object when it goes out of scope.

  • There can only be one destructor function per class.
  • The name of the destructor is the name of the class itself preceded with a tilde (~). It takes no arguments and does not have a return type.
  • The destructor is automatically called when an object goes out of scope.

A destructor is needed if the class holds or uses any resources which must be released when the object is destroyed, such as file handles or pointers to allocated memory. In a simple class which does not hold any resources then a destructor is not necessary since C++ will automatically clean up the memory that the object used. You also don’t need to write a destructor if the class holds resources through smart pointers (like unique_ptr) or standard library containers like std::vector as these will automatically delete themselves when they go out of scope. Notice that if you need to write a constructor in a class and intend to later on inherit from the class that you are writing now then you must make the destructor virtual (see the post on virtual destructor).

Example of a class where a destructor is necessary:

Example of a class using string from the standard library which makes it possible to avoid writing a destructor:

The destructor is an important piece in the automatic cleaning up of resources necessary for RAII.

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Scope

The scope of a variable determines who can see and use the variable and for how long the variable will be available until it is destroyed.

Local scope

Function parameters and variables declared inside a function (local variables) have local scope. That is to say that they can only be seen inside the function that declares them. The local variables are destroyed when the function exits and can no longer be accessed after the function exits.

Local scope prevents naming collisions. It is possible to have two variables with the same name, as long as they are in different scopes!

Be aware that it is possible to have nested scopes where two variables can exist with the same name but in different scopes, one inside another.

What happens in the example above is that within the nested block is a new variable ‘x’ created and this variable has its scope set to the local block which is smaller than the function main. Inside the nested scope is the outer variable ‘x’ not accessible it is hidden from view (this is called name hiding), only the variable ‘x’ declared in the inner scope can be seen and modified.  Notice that it is generally bad practice to declare a new variable with the same name is a smaller scope. The code quickly gets much more confusing when you have to remember which variable with the name ‘x’ you are seeing.

In the same way is it possible to declare a local variable with the same name as a function parameter.

This is also a bad practice and should always be avoided since it makes the code so much more difficult to read.

 

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