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!

Facebookgoogle_pluslinkedinmail

Free store vs Heap

There is much confusion regarding the usage of the words stack, heap and free store. Most developers are using the stack and heap wrongly for C++.

Stack

The stack (actually the call stack) is the memory where all your local variables in a function will be located. The stack is a memory structure where all the function calls, from the start of the current thread, up to the currently executed function are stored together with the memory that they use. The variables which reside on the stack are stored there until the function where they reside returns. Once the method returns then all local (stack) variables will be cleaned up.

Heap

The heap is a legacy from C which should not be used in C++. Objects on the heap are allocated using malloc or calloc and released again using free. You should never use malloc or calloc to allocate an object in C++. This since these calls does not call the constructor of the object, leaving you with allocated memory for an invalid object.

Free Store

The free store is in C++ what the heap was in C. Objects on the free store are allocated using new or new[] and released again using delete or delete[]. Objects allocated using new or new[] are allocated and their constructor is being called once the memory is allocated. Objects destroyed using delete or delete[] have their destructor called before the memory is released again.

Facebookgoogle_pluslinkedinmail

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?

 

Facebookgoogle_pluslinkedinmail

Stack Unwinding

Stack unwinding refers to the handling of exceptions in C++. When an exception occurs then the call stack is searched for the next matching catch statement and the execution of the program is continued at the beginning of the catch clause. No other code in between is ever evaluated.

This is to say that for the functions

Then the program will never output any of the output statements!

However, and this is important, if the function f3() above did instead look like this:

then the cout statement would not be called, but the destructor of Date would be called. All local objects will be destructed when an exception happens, in the reverse order they were constructed.

 

Facebookgoogle_pluslinkedinmail