C++ Set Template Bool Parameter from Class Constructor Argument

Question

I've simplified the scenario as much as possible below. Imagine a class that has template specialization using a bool parameter.

template <bool hasExtraParam>
class TestClass {};

template <>
class TestClass<true> {
   public:
    int param1;
    int param2;
};

template <>
class TestClass<false> {
   public:
    int param1;
};

Now, I would like to have a container class that holds a large amount of these TestClasses as member variables. I want to be able to set the template parameter of each member variable based on the constructor arguments as below:

constexpr bool ep1, ep2, ep3, ep4;
class Container
{
   public:
    constexpr Container(bool extraParam1, bool extraParam2, bool extraParam3,
                        bool extraParam4)
    {
        ep1 = extraParam1;
        ep2 = extraParam2;
        ep3 = extraParam3;
        ep4 = extraParam4;
    }
    TestClass<ep1> testClass1;
    TestClass<ep2> testClass2;
    TestClass<ep3> testClass3;
    TestClass<ep4> testClass4;
};

How can I achieve this pattern? I want for my actual use-case to pass in a large config struct that has a boolean that will link to each member variable setting its respective template parameter. I cannot wrap my head around how to achieve this and feel like I'm missing some alternate fundamental approach to the problem. Also, I don't this its feasible for Container to have a bunch of templated arguments for scalability purposes since the config struct can be large.

Answer 1

What you are attempting is not possible in C++. Template aarguments can only be specified at compile-time. You will have to instantiate the objects dynamically at runtime instead, eg:

class TestClassBase {
public:
    virtual ~TestClassBase() = default;

    // common members of both classes...

    // virtual methods for both classes to override...
};

template <bool hasExtraParam>
class TestClass {};

template <>
class TestClass<true> : public TestClassBase {
public:
    int param1;
    int param2;

    // override virtual methods as needed...
};

template <>
class TestClass<false> : public TestClassBase {
public:
    int param1;

    // override virtual methods as needed...
};

class Container
{
public:
    Container(bool extraParam1, bool extraParam2, bool extraParam3, bool extraParam4)
    {
        testClass1 = extraParam1 ? new TestClass<true> : new TestClass<false>;
        testClass2 = extraParam2 ? new TestClass<true> : new TestClass<false>;
        testClass3 = extraParam3 ? new TestClass<true> : new TestClass<false>;
        testClass4 = extraParam4 ? new TestClass<true> : new TestClass<false>;
    }

    ~Container()
    {
        delete testClass1;
        delete testClass2;
        delete testClass3;
        delete testClass4;
    }

    TestClassBase* testClass1;
    TestClassBase* testClass2;
    TestClassBase* testClass3;
    TestClassBase* testClass4;
};

Alternatively:

template <bool hasExtraParam>
class TestClass {};

template <>
class TestClass<true> {
public:
    int param1;
    int param2;
};

template <>
class TestClass<false> {
public:
    int param1;
};

class Container
{
public:
    Container(bool extraParam1, bool extraParam2, bool extraParam3, bool extraParam4)
    {
        if (extraParam1)
            testClass1.emplace<TestClass<true>>();
        else
            testClass1.emplace<TestClass<false>>();

        if (extraParam2)
            testClass2.emplace<TestClass<true>>();
        else
            testClass2.emplace<TestClass<false>>();

        if (extraParam3)
            testClass3.emplace<TestClass<true>>();
        else
            testClass3.emplace<TestClass<false>>();

        if (extraParam4)
            testClass4.emplace<TestClass<true>>();
        else
            testClass4.emplace<TestClass<false>>();
    }

    std::variant<TestClass<true>, TestClass<false>> testClass1;
    std::variant<TestClass<true>, TestClass<false>> testClass2;
    std::variant<TestClass<true>, TestClass<false>> testClass3;
    std::variant<TestClass<true>, TestClass<false>> testClass4;
};