Use hiding members in inherited class constructor

Question

In an inherited class constructor, I want to use a class constant member that hides the base class one from the base class constructor:

#include <string>
#include <iostream>

class BaseClass {
    private:
    const int constant_variable { 21 };

    public:
    int mutable_variable;
    BaseClass(): mutable_variable(constant_variable) {};
};

class DerivedClass: public BaseClass {
    private:
    const int constant_variable { 42 };

    public:
    using BaseClass::BaseClass;
};

int main () {
    DerivedClass dc;
    std::cout << dc.mutable_variable << std::endl; // 21, but I want it to be 42
    return 0;
}

In the example code, for instance, BaseClass' constructor uses its own value of constant_variable while I'd like it to use the DerivedClass' constant_variable.

How to do this in c++?

You aren't overriding anything. There are two const integers in every DerivedClass object. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Jan 11, 2018 at 8:28

StoryTeller - Unslander Monica · Accepted Answer · 2018-01-11 08:33:07Z

4

I don't know if you need the constant member in the base, but if you want to specify its value from a derived class constructor, as well as that of the mutable member, you can just use a protected c'tor:

class BaseClass {
    private:
    const int constant_variable { 21 };

    protected:
    BaseClass(int init) : constant_variable{init}, mutable_variable{init}
    {}

    public:
    int mutable_variable;
    BaseClass(): mutable_variable(constant_variable) {}
};

class DerivedClass: public BaseClass {
    public:
    DerivedClass() : BaseClass(42) {}
};

You should know that constant_variable is not a compile time constant, it occupies space in each and every object.

answered Jan 11, 2018 at 8:33

StoryTeller - Unslander Monica

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Comments

acraig5075 · Accepted Answer · 2018-01-11 08:32:46Z

1

Make the base class constructor take the value as a defaulted parameter.

BaseClass::BaseClass(int constant = 21)
  : mutable_variable(constant)
{}

Then the derived class can provide something different

DerivedClass::DerivedClass()
 : BaseClass(41)
{}

answered Jan 11, 2018 at 8:32

acraig5075

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Comments

SoronelHaetir · Accepted Answer · 2018-01-11 08:32:43Z

0

The base class constructor will have to be told about the value, it has no idea the derivation even occurs. The easiest would be to alter your base class ctor to take a parameter with the value of interest.

answered Jan 11, 2018 at 8:32

SoronelHaetir

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Comments

Gonen I · Accepted Answer · 2018-01-11 08:44:22Z

0

Just override the non mutable variable in the derived class constructor

#include <string>
#include <iostream>

class BaseClass {
    private:
    const int constant_variable { 21 };

    public:
    int mutable_variable;
    BaseClass(): mutable_variable(constant_variable) {};
};

class DerivedClass: public BaseClass {
    private:

    public:
        DerivedClass()
        {
            mutable_variable = 42;
        }
};

int main () {
    DerivedClass dc;
    std::cout << dc.mutable_variable << std::endl; // returns 42
    return 0;
}

answered Jan 11, 2018 at 8:44

Gonen I

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