Question

I’ve got a class that has a couple of data members of class type. I don’t want the constructors for these members to be called when declared, so I’m trying to hang onto a pointer to the object explicitly.

I thought maybe I could do the following, where the constructor is called immediately when initializing the data members:

class MyClass {
    public:
        MyClass(int n);
    private:
        AnotherClass another(100); // Construct AnotherClass right away!
};

But I want the MyClass constructor to call the AnotherClass constructor. Here’s what my code looks like:

BigMommaClass.h

#include "ThingOne.h"
#include "ThingTwo.h"

class BigMommaClass {
    public:
        BigMommaClass(int numba1, int numba2);

    private:
        ThingOne* ThingOne;
        ThingTwo* ThingTwo;
};

BigMommaClass.cpp

#include "BigMommaClass.h"

BigMommaClass::BigMommaClass(int numba1, int numba2) {
    this->ThingOne = ThingOne(100);
    this->ThingTwo = ThingTwo(numba1, numba2);
}

Here’s the error I’m getting when I try to compile:

g++ -Wall -c -Iclasses -o objects/BigMommaClass.o classes/BigMommaClass.cpp
In file included from classes/BigMommaClass.cpp:1:0:
classes/BigMommaClass.h:12:8: error: declaration of âThingTwo* BigMommaClass::ThingTwoâ
classes/ThingTwo.h:1:11: error: changes meaning of âThingTwoâ from âclass ThingTwoâ
classes/BigMommaClass.cpp: In constructor âBigMommaClass::BigMommaClass(int, int)â:
classes/BigMommaClass.cpp:4:30: error: cannot convert âThingOneâ to âThingOne*â in assignment
classes/BigMommaClass.cpp:5:37: error: â((BigMommaClass*)this)->BigMommaClass::ThingTwoâ cannot be used as a function
make: *** [BigMommaClass.o] Error 1

Am I using the right approach, but the wrong syntax? Or should I be coming at this from a different direction?

Answer 1

You can specify how to initialize members in the member initializer list:

BigMommaClass {
    BigMommaClass(int, int);

private:
    ThingOne thingOne;
    ThingTwo thingTwo;
};

BigMommaClass::BigMommaClass(int numba1, int numba2)
    : thingOne(numba1 + numba2), thingTwo(numba1, numba2) {}