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Test Driven Development in C/C++


Over the course of the last two years, our company has been promoting, conducting training, and consulting in XP (eXtreme Programming). We believe that XP is the best technique for delivering high-quality software because of its emphasis on communication, simplicity, testing, and rapid feedback. XP achieves its results through implementation of about a dozen proven practices. We are only going to focus on one those practices in this article — TDD (Test Driven Development).

Many companies fear they can’t be eXtreme for a variety of reasons, but we can’t imagine a company or process that would advocate not testing their software. The TDD technique can be, and should be, applied in any software development environment. You don’t even need your boss’ permission to do TDD because it is done as part of programming instead of a separate phase of the project. While testing, design, and programming are three distinct activities, we don’t separate these activities in time — they are done concurrently and continuously. The example presented in this article demonstrates how this works.

Nearly all of the publications concerning XP or TDD use a programming language that supports OO (object oriented) development. This is understandable since XP had its birth in the Smalltalk community and many — but not all — shops are in fact using an OO language.

A recent client of ours wanted to do TDD. In fact, they wanted to do all of the XP practices. The problem was that they were a C shop, and they were going to be using C for the foreseeable future, so we had to roll up our sleeves and figure out how to get the benefits of TDD in C. Unfortunately C doesn’t (directly) support polymorphism, a concept that is fundamental to the techniques used in TDD.

In TDD, one module is developed at a time. To properly test a given module, it must be isolated from most other modules with which it interacts. In OO languages like C++ and Java, we would use polymorphic interfaces to isolate the module under test. These interfaces also provide a testing environment for the module. We need these same capabilities in C.

We used our experience working in embedded systems projects where either the hardware wasn’t ready or wasn’t available to us. In these environments, we would have to stub out the function calls that provided access to the missing hardware. This stubbing technique is just what we need to get the benefits that polymorphism provides in languages that support it. In fact, stubbing is polymorphism, and we refer to it as link-time polymorphism.

TDD

TDD is a style of programming where we write a test before the code that it is supposed to test. The test shouldn’t even compile at first because the code to be tested hasn’t been written yet. We then write enough code to get everything to compile, but purposely code it so that our test fails. We then write code to make the test pass. Each of these steps is critical to the TDD process.

Finally, every time our tests pass, we refactor the code so that it is the best, résumé-quality code that we can produce. We strive to make our code have the best possible design for the functionality we have implemented so far, but for nothing more than that functionality. Only after we have refactored toward these goals do we go on to writing another test. We repeat this cycle of test-code-refactor in rapid succession throughout our development day.

How often have you spent an hour writing code, only to spend the entire day getting it to compile and debugged? We don’t enjoy doing that because most of the day is spent feeling like a failure for not getting the code correct the first time. Instead of programming a (possibly large) piece of functionality, TDD dictates that we work on a very small problem, solve it, and then move on to the next very small problem. After all, every large complex problem is just a bunch of little problems. We’ve found that by doing this, we can be successful throughout the entire day. Solving small problems, one after another, is sustainable, enjoyable, and highly productive.

The key to these series of successes is rapid feedback. We run the compiler just as soon as we have enough code that stands a chance of compiling. Compiling after every single line may not be possible in practice, but it’s not as far off as you might think. We also run our tests as often as possible. Our rule is that no more than 10 minutes may go by without having all of our tests running. Five minutes is better. One minute is better still. This obsession with running our tests frequently forces us to partition our work into a series of very small problems.

You might initially find this unnatural — we certainly did when we first learned about TDD. But after programming in this style for the last two years, there is no way we’ll ever go back to our old ways of writing code and either testing it afterwards or handing it off to QA for testing. Have you ever tried to test code after it was written? It is very difficult, so difficult that it’s often not done. By writing our tests first, we guarantee that there are tests and that the system is designed to be testable.

Since adopting TDD, we have almost never used a debugger on the code we have written. In fact, cranking up the debugger — or even adding a printf statement — is a sign of failure: we bit off a bigger problem than we could chew. Our response to the supposed need for debugging is to simply throw away the last few minutes worth of work. We then try again, taking even smaller steps.

Point-of-Sale System

As an example, we’ve chosen a point-of-sale system, like one might encounter in any retail store. A sales clerk passes items over a scanner that reads the barcode. A display monitor shows information on each item as it is scanned and also tracks the total for each sale of one or more items to a given shopper.

Our First User Story[1]

The total price of all items in the sale is available.

We have the concept of a Sale, and we know that it has a total. Let’s capture that in a test. We are essentially coding our requirements document so that it executes as a suite of unit tests! This is an important concept when practicing XP. We do not produce thick paper requirements or design documents that never seem to match the working code. Yet documenting requirements and design is essential to software development, so we do it in code instead.

Writing tests in code is essential for a second reason: executing tests manually is extremely time consuming. Ultimately it becomes impossible to keep up with executing all tests against even a modest body of code. We have to automate the tests; otherwise they don’t get run. As soon as tests aren’t being run, we’re shipping defects.

In TDD, a testing framework is an essential tool. It not only provides us with a means for automating the tests, but it is also an integral part of the minute-to-minute development process. Such a tool need not be an expensive investment. The tool we use is CppUnitLIte, which is a freely available macro-based tool.. CppUnitLite was developed for C++, but since C++ can call C, we use it for development of C applications as well. Also, its use of C++ is minimal so that C developers with little or no knowledge of C++ can quickly learn how to use it.

For our user story, we first create the file SaleTest.cpp to contain the tests for Sale. CppUnitLite requires a little boilerplate code:

#include "../TestHarness/TestHarness.h"

int main()
{
   TestResult tr;
   TestRegistry::runAllTests( tr );
   return 0;
}

The main function acts as the test driver. The basic idea is that the runAllTests method is responsible for extracting and executing all of the tests we have defined. It isn’t necessary to know how the testing framework works to use it — in fact we use it in our C++ course starting on the first day of class, long before students know enough C++ to understand how it works.

Now that the boilerplate code is in place, we can write our first test. Each test is constructed with the TEST macro that is part of CppUnitLite. Anything defined with this macro is automatically picked up by runAllTests. The macro takes two parameters: an arbitrary name used to group the tests and a descriptive name for the test.

#include "../TestHarness/TestHarness.h"

int main()
{
   TestResult tr;
   TestRegistry::runAllTests( tr );
   return 0;
}

TEST( Sale, totalNewSale )
{
   CHECK( 0 == GetTotal( ) );
}

The name of our first test is totalNewSale. It represents the state of a new Sale before any Items are added to it. It seems reasonable that before anything is purchased the total of the Sale should be zero, so we express this in our test.

Note that all tests contain one or more CHECK macros. We use the CHECK macro to ensure that a Boolean condition holds true. A test may have more than one CHECK statement; each of these is executed regardless of whether a prior CHECK passed or failed.

SaleTest will of course not compile, since the function GetTotal doesn’t exist yet. We compile anyway as our first level of feedback. If SaleTest compiles, we know we have a serious configuration problem.

g++ -c SaleTest.cpp
SaleTest.cpp:
   In method 'void totalNewSaleSaleTest::run(TestResult &)':
SaleTest.cpp:12:
   implicit declaration of function 'int GetTotal(...)'

Once we get our compile errors, we create just enough of Sale.h:

#ifdef __cplusplus
extern "C"
{
#endif

int GetTotal();

#ifdef __cplusplus
}
#endif

We include Sale.h in SaleTest.cpp and create just enough of Sale.c.

#include "Sale.h"

int GetTotal()
{
   return -1;
}

Note the use of the linkage specifier extern "C" in Sale.h. This is necessary because our testing framework is written in C++ and #includes this file; Sale.c is written in C and also includes this file. This is the mechanism needed for C++ code to call C code.

Our purpose is to get to compilation so that we can link and run our tests. To run the test, we simply execute SaleTest:

Failure: "0 == GetTotal( )" line 13 in SaleTest.cpp

There was one failure.

This is as expected: we have written a test but no production code yet. We want to see a failure, though, as part of our feedback-driven, test-first design cycle. A reminder of the cycle:

  • Write a test for nonexistent code.
  • Expect that the test will fail.
  • Write just enough code to make the test pass.
  • Refactor.

Every once in a while, the test that we expect to fail passes. This should be a shock!

For our Sale application, we can make the test pass by simply returning 0 from GetTotal.

int GetTotal( )
{
   return 0;
}

As ridiculous as it looks to just return a hard-coded value to make the test pass, this is part of TDD. We have a failing test case, and we want to get it to pass as quickly as possible. Once the test is passing, we’ll worry about making it “right.” Returning 0 seems to be “wrong” in the sense that the hard-coded zero will have to be replaced very shortly. But the fact that we provided just enough code to make the current test pass means that we will have to write more tests. These tests will fail, proving that a more complicated algorithm is necessary. We increase our test coverage this way at a steady pace. More tests are a good thing.

There were no test failures.

Our first test is complete. We know the state a Sale should be in when nothing has been done to it. Now we want the capability to sell at least a single item, so we write the test sellOneItem. For our test, we make up an item whose barcode is "a123" and cost is 199 cents. (It’s easiest to count pennies to avoid any floating-point rounding problems later.)

TEST( Sale, sellOneItem )
{
   BuyItem( "a123" );
   CHECK( 199 == GetTotal( ) );
}

To sell an item, we need to pass in its barcode string to the Sale. We devise the function BuyItem to accomplish this. Buying item "a123" should increase the total of the Sale to 199. We compile, expecting failure:

SaleTest.cpp:18:
   implicit declaration of function 'int BuyItem(...)'

To pass compilation, we must modify the header:

int GetTotal( );
void BuyItem( char* barCode );
as well as Sale.c:
#include "Sale.h"

int GetTotal( )
{
   return 0;
}

void BuyItem( char* barCode )
{
}

We can now run all the tests, expecting sellOneItem to fail.

Failure: "199 == GetTotal( )" line 19 in SaleTest.cpp

There was one failure.

Time to add the code to make this test pass. Each shopper’s purchases must start with a total of 0. At this point, the simplest thing that will work is to introduce a variable to track the total. We declare it in Sale.c as a static. This gives the variable visibility only at file scope.

In Sale.c, we return total from GetTotal and update it in BuyItem.

#include "Sale.h"

static int total = 0;

int GetTotal()
{
   return total;
}

void BuyItem( char* barCode )

{
   total = 199;
}

Both tests now pass [2].

After each new test passes, we always pause and look over the code to be sure that it communicates. We see the string literal and magic number 199 in sellOneItem and feel that the code isn’t as clear as it could be. Since the tests will evolve and be maintained along with the code that is being tested, we want them to serve as excellent documentation on how to manipulate the Sale functions. This obsession we have with clearly communicating code is far better than placing comments in the code, which quickly become outdated. Introducing local variables in the test yields:

TEST( Sale, sellOneItem )
{
   char* milkBarcode = "a123";
   int milkPrice = 199;

   BuyItem( milkBarcode );
   CHECK( milkPrice == GetTotal( ) );
}

We can now move on and write a third test, which we expect to fail. Note that we already have a need to reuse the constant declarations of milkBarcode and milkPrice, so we move them out of sellOneItem and declare them as static.

TEST( Sale, sellTwoItems )
{
   BuyItem( milkBarcode );
   BuyItem( milkBarcode );
   CHECK( 2 * milkPrice == GetTotal( ) );
}

It fails, as expected:

Failure: "2 * milkPrice == GetTotal( )" line 29 in SaleTest.cpp

There was one failure.

Building the correct production code in Sale.c appears to be straightforward. Instead of just setting the total to the price of milk, we add the price into the total:

void BuyItem( char* barCode )
{
   total = total + 199;
}

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