C++ - Pass Pointer of a Template Class to A Function

Question

I am trying to pass a pointer to a templated object to another class.

template <int size>
class A {
public:
    int a[size] = {0};

    int getA(int n) {
       return a[n];
    }
};


class B {
public:
    A<>* b;

    void setB(A<>* n) {
        b = n;
    }
};


int main()
{
    const int size1 = 10;

    A<size1> data1;
    B b1;
    
    b1.setB(&data1);
}

Which doesn't work.

As a solution, I can create the B class as a template class and create B object as B<A<size1>> b1; but this will create multiple objects if I multiply A<sizeX>, which I don't want since this code is for an embedded project which has finite resources.

All I want is to pass the pointer of data1 object to another class function and store it inside. The code I'm looking for is for C++03, I cannot use C++11 features such as shared pointers.

Is there a way to do this?

Appreciate any help,

And there's no reasonable way to make class B a template as well? — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Mar 3, 2022 at 17:27
What about a base class for A<N> types? What about registering pointers to base type? — max66
– max66, Commented Mar 3, 2022 at 17:32
Depending on what exactly you are doing, you could have A inherit from a base class, and then in B you store a pointer to that base class. — NathanOliver
– NathanOliver, Commented Mar 3, 2022 at 17:32
@MeCe That is a difficult problem to solve. Mixing run-time and compile-time constructs is not very easy. — NathanOliver
– NathanOliver, Commented Mar 3, 2022 at 17:47

Remy Lebeau · Accepted Answer · 2022-03-04 00:29:18Z

You have gotten yourself into a bit of a catch-22 situation.

You can't hold a templated A inside of B without making B templated as well, eg:

template <int size>
class A {
public:
    int a[size];

    int getA(int n) {
       return a[n];
    }
};

template <int size>
class B {
public:
    A<size>* b;

    int getA(int n) {
        return b->getA(n);
    }

    void setB(A<size>* n) {
        b = n;
    }
};

int main()
{
    const int size1 = 10;

    A<size1> data1;
    B<size1> b1;
    
    b1.setB(&data1);
    int a = b1.getA(0);
}

Or by giving A a non-templated base class for B to hold, eg:

class A_base
{
    virtual ~A_base() {}
    virtual int getA(int n) = 0;
};

template <int size>
class A : public A_base {
public:
    int a[size];

    int getA(int n) {
       return a[n];
    }
};

class B {
public:
    A_base* b;

    int getA(int n) {
       return b->getA(n);
    }

    void setB(A_base* n) {
        b = n;
    }
};

int main()
{
    const int size1 = 10;

    A<size1> data1;
    B b1;
    
    b1.setB(&data1);
    int a = b1.getA(0);
}

Otherwise, you will have to make B just hold a void* pointer, and then require the caller to extract that void* and decide what to cast it to, eg:

template <int size>
class A {
public:
    int a[size];

    int getA(int n) {
       return a[n];
    }
};

class B {
public:
    void* b;

    void* getB() {
        return b;
    }

    void setB(void* n) {
        b = n;
    }
};

int main()
{
    const int size1 = 10;

    A<size1> data1;
    B b1;
    
    b1.setB(&data1);
    int a = static_cast< A<size1>* >(b1.getB())->getA(0);
}

Blank · Accepted Answer · 2022-03-03 17:28:29Z

1

I think you will want to pass the size from a constructor. Templates don't fit for this use case.

answered Mar 3, 2022 at 17:28

Blank

518 bronze badges

1 Comment

MeCe Over a year ago

Thanks for the answer. However I cannot pass size from the constructor since I don't want dynamic memory allocation on an embedded system.

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