Functions calling order in unit tests in C++

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To ensure a certain order of function calls, GTest provides 2 ways to do that:

• InSequence object
• Sequence object

## InSequence object

The simplest way to expect specific function call order is to create a InSequence object in dedicated scope:

#include <gmock/gmock.h>
#include <gtest/gtest.h>

using namespace ::testing;

struct SomeInterface
{
    virtual ~SomeInterface() = default;

    virtual void FirstMethod() = 0;
    virtual void SecondMethod() = 0;
    virtual void ThirdMethod() = 0;
    virtual void FourthMethod() = 0;
};

struct SomeMock : public SomeInterface
{
    MOCK_METHOD(void, FirstMethod, (), (override));
    MOCK_METHOD(void, SecondMethod, (), (override));
    MOCK_METHOD(void, ThirdMethod, (), (override));
    MOCK_METHOD(void, FourthMethod, (), (override));
};

void SomeFunctionToTest(SomeInterface &obj)
{
    obj.FirstMethod();
    obj.SecondMethod();
    obj.ThirdMethod();
    obj.FourthMethod();
}

TEST(TestSomeFunction, CallingOrderIsCorrect)
{
    SomeMock obj;

    {
        // in this scope the call order matters...
        InSequence seq;

        EXPECT_CALL(obj, FirstMethod);
        EXPECT_CALL(obj, SecondMethod);
    }

    // ...and outside of the scope it doesn't
    EXPECT_CALL(obj, FourthMethod);
    EXPECT_CALL(obj, ThirdMethod);

    SomeFunctionToTest(obj);
}

Sequence object

To gain some flexibility, you can create a dedicated Sequence object which then can be passed when calling EXPECT_CALL macro or to any other function as an input argument:

TEST(TestSomeFunction, CallingOrderIsCorrect)
{
    SomeMock obj;
    Sequence seq;

    // these functions must be called in order...
    EXPECT_CALL(obj, FirstMethod).InSequence(seq);
    EXPECT_CALL(obj, SecondMethod).InSequence(seq);

    // ...and these not necessarly
    EXPECT_CALL(obj, FourthMethod);
    EXPECT_CALL(obj, ThirdMethod);

    SomeFunctionToTest(obj);
}