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Piotr's Less Obvious Advice on Google Mock: Assigning values through method calls

On many occasions, we may want to test a real object by passing to it prepared buffers of data (characters, bytes, etc.) that are filled in when the object calls a mock method. This can be achieved by using SetArrayArg.

Suppose we have a Client object that implements a find method, which takes an array of characters as input argument. The method queries a Driver object for chunks of data (other arrays of characters of the same size as the input argument) and compares each chunk with the input array:

void Client::find(char * buf)
    char tmp_buf[buflen];
    bool found = false;

    while(!found && _drv.getDataChunk(tmp_buf))
        int i = 0;
        while(i < buflen && buf[i] == tmp_buf[i])

        if(i == buflen)
            found = true;

        cout << "Found matching chunk of data." << endl;
        cout << "No matching chunk of data found." << endl;

Client assumes that Diver implements the following contract:
  • if there is no more data chunks, getDataChunk will return false,
  • otherwise, it will return true and will fill in the buffer passed to it through pointer as input, with next chunk of data.
class Driver

    virtual ~Driver() {}

    virtual bool getDataChunk(char *) = 0;

class MockDriver : public Driver

    MOCK_METHOD1(getDataChunk, bool(char *));

Let's now consider a test in which we want to simulate that a matching chunk of data is found on the first call of getDataChunk.

TEST(ClientTest, FindsAMatchOnFirstCall)
    MockDriver drv;
    Client client(drv);

        //this is the buffer that will be searched for
    char buf [] = {'a','a','a','a','a','a','a','a'};

        //this is the matching buffer that will be returned by MockDriver
    char bufA [] = {'a','a','a','a','a','a','a','a'};

    EXPECT_CALL(drv, getDataChunk(_))
        .Times(1) //we are expecting just one call, since the first call will return the matching buffer

                //the mock will return true and before doing so it will copy the contents of bufA into the table
                //pointed to by the pointer passed to getDataChunk method as the input parameter
        .WillOnce(DoAll(SetArrayArgument<0>(bufA, bufA+8), Return(true)));

Exercise: try to modify the code above to validate your expectations on Client::find method when there is no matching chunk of data.


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Piotr's Less Obvious Advice on Google Mock: State maintenance

Google Mock provides several ways to maintain state inside mock objects. One way of implementing state maintenance is with SaveArg. Consider the following example.

We have a class Configurator, which allows a caller to set and get values of a parameter:

class Configurator

    virtual ~Configurator() {}

    virtual void setParamX(int n) = 0;
    virtual int getParamX() = 0;

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class Client

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    void setParamX(int n);
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    int getParamX();


    Configurator & _cfg;

incParamXBy internally calls setParamX and getParamX on Configurator:

void Client::incParamXBy(int n)
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Piotr's Less Obvious Advice on Google Mock: Returning new objects from a mock

Google Mock provides a way to return newly created objects from a mock method. Suppose we have a  Generator class that is supposed to generate new objects when createNewRecord method is called:

class Generator
    virtual ~Generator() {}
    virtual Record * createNewRecord() = 0;

...and suppose we want to mock this class:

class MockGenerator : public Generator
    MOCK_METHOD0(createNewRecord, Record * ());

Suppose the caller class Client has run method defined as follows:

void Client::run()
    for(int i = 0; i < 3; i++)
        rec_tab[i] = gen.createNewRecord();

We want the mock to return a pointer to a new object each time createNewRecord is called. This is how we can express this in the test code:

TEST(ClientTest, CanRun)
    MockGenerator gen;
    Client c(gen);

    EXPECT_CALL(gen, createNewRecord())
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