Specify constructor arguments for a Google test Fixture

Use the current class as a base class for your fixtures:

class TheClassTestBase : public ::testing::Test {
 protected:
  TheClassTestBase(std::string filename) : datafile(filename) {}
  ...
 };

For every specific filename - use derived fixture:

class TheClassTestForFooTxt : public TheClassTestBase {
protected:
    TheClassTestForFooTxt() : TheClassTestBase ("foo.txt") {}
};

However this is extra step needed for every set of parameters - so you can try to use templates or macros to get it done with less effort. Like:

template <typename ClassTestTag>
struct ClassTestParams
{
    static std::string filename;
};

template<typename  ClassTestTag>
class TheClassTest : public TheClassTestBase {
protected:
    TheClassTest() : TheClassTestBase (ClassTestParams<ClassTestTag>::filename) {}
};

Then - for every set of parameters - do that:

class FooTxtTag {};
template <> std::string ClassTestParams<FooTxtTag>::value = "foo.txt";
using TheClassTestForFooTxt = TheClassTest<FooTxtTag>;
TEST_F(TheClassTestForFooTxt, xxxx) {}

However - in your specific case - I would also try GoogleTest:type-parameterized-tests.

As suggested by another user, you cannot achieve what you want by instantiating a fixture using a non-default constructor. However, there are other ways. Simply overload the SetUp function and call that version explicitly in the tests:

class TheClassTest : public ::testing::Test {
protected:
    TheClassTest() {}
    virtual ~TheClassTest() {}
    void SetUp(const std::string &filename) {
        data = new TheClassData(filename);
        tc = new TheClass(data);
    }
    virtual void TearDown() {
        delete tc;
        delete data;
    }

    TheClassData* data;
    TheClass* tc;
};

Now in the test simply use this overload to set up filename:

TEST_F(TheClassTest, MyTestCaseName)
{
    SetUp("my_filename_for_this_test_case");

    ...
}

The parameterless TearDown will automatically clean up when the test is complete.