How can I initialize C++ object member variables in the constructor?

Question

I've got a class that has a couple of objects as member variables. 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 have no idea what I'm doing.

I thought maybe I could do the following, where the constructor is called immediately when initializing the object member variable:

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:

FIle BigMommaClass.h

#include "ThingOne.h" #include "ThingTwo.h"  class BigMommaClass {          public:                 BigMommaClass(int numba1, int numba2);          private:                 ThingOne* ThingOne;                 ThingTwo* ThingTwo; }; 

FIle 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) {}