Replacing with Function Pointers

There are cases where you want to alternate between product code and test doubles on a per-test-case basis within a single test suite executable. In such cases, you can use "replacing with function pointers."

caution

Since this method can make the code harder to read, use "replacing with the linker" when you don't need to switch between product code and test doubles.

How to Switch with Function Pointers

Consider the following function for registering a file write queue to USB drive:

original function declaration

void FileCtrl_EnQueue(ST_QUEUE_ELEMENT* el)

To alternate between the product code FileCtrl_EnQueue_Real and the test double FileCtrl_EnQueue_Mock, change the function declaration to a function pointer declaration.

function declaration using function pointer

extern void (*FileCtrl_EnQueue)(ST_QUEUE_ELEMENT* el);

By assigning the function pointer FileCtrl_EnQueue_Real of the product code to FileCtrl_EnQueue, calls to FileCtrl_EnQueue() are routed to FileCtrl_EnQueue_Real().

Product Code FileManager.c

void FileCtrl_EnQueue_Real(ST_QUEUE_ELEMENT* el) {
    // Processing that runs on the actual microcontroller
}
void (*FileCtrl_EnQueue)(ST_QUEUE_ELEMENT* el) = FileCtrl_EnQueue_Real;

In test cases where you want to use the test double FileCtrl_EnQueue_Mock, assign the function pointer FileCtrl_EnQueue_Mock to FileCtrl_EnQueue. After the test case finishes, restore the original reference so that only that test case uses FileCtrl_EnQueue_Mock.

Test Code test case using real

void FileCtrl_EnQueue_Mock(ST_QUEUE_ELEMENT* el) {
    // Test double processing
}

TEST_F(TestLogSave, TestCaseUsingFileCtrl_EnQueue) {

void (*FileCtrl_EnQueue_Saved)(ST_QUEUE_ELEMENT* el);
    // Pre-test setup
    FileCtrl_EnQueue_Saved = FileCtrl_EnQueue;
    FileCtrl_EnQueue = FileCtrl_EnQueue_Mock;

    // Perform assertions using FileCtrl_EnQueue_Mock
    // ..

    // Post-test cleanup
    FileCtrl_EnQueue = FileCtrl_EnQueue_Saved;
}

Tips for GoogleTest

When you want to use FileCtrl_EnQueue_Mock throughout a test file in GoogleTest, defining SetUp() like below will eliminate the need to assign the pointer in each test case. This makes the tests easier to read.

Test Code tetst class

class TestLogSave: public ::testing::Test {
  protected:
    void (*FileCtrl_EnQueue_Saved)(ST_QUEUE_ELEMENT* el);

    void SetUp() override {
        // Use mock (FileCtrl_EnQueue_Mock) for FileCtrl_EnQueue
        FileCtrl_EnQueue_Saved = FileCtrl_EnQueue;
        FileCtrl_EnQueue = FileCtrl_EnQueue_Mock;
    }

    void TearDown() override {
        // Restore FileCtrl_EnQueue to reference the product code (FileCtrl_EnQueue_Real)
        FileCtrl_EnQueue = FileCtrl_EnQueue_Saved;
    }
};

Test Code test case using mock

TEST_F(TestLogSave, TestCaseUsingFileCtrl_EnQueue) {
  // ..
}