Question

I have two classes: OuterClass and InnerClass. InnerClass is a private member of OuterClass and should be created in OuterClass constructor with an InnerClass(int) constructor, however the default InnerClass constructor is still called.

InnerClass.hpp:

#ifndef INNERCLASS_HPP_
#define INNERCLASS_HPP_

class InnerClass {
public:
    int a;
    InnerClass();
    InnerClass(int);
    ~InnerClass();
};

#endif /* INNERCLASS_HPP_ */

InnerClass.cpp:

#include "InnerClass.hpp"
#include <iostream>

InnerClass::InnerClass() {
    a = 1;
    std::cout << "inner class constructed, a = " << a << std::endl;
}
InnerClass::InnerClass(int x) {
    a = x;
    std::cout << "inner class constructed, a = " << a << std::endl;
    //automatically: object InnerClass (a=3) is destroyed here...
}
InnerClass::~InnerClass() {
    std::cout << "inner class destructed, a = " << a << std::endl;
}

OuterClass.hpp:

#ifndef OUTERCLASS_HPP_
#define OUTERCLASS_HPP_

#include "InnerClass.hpp"

class OuterClass {
private:
    InnerClass blah;
public:
    OuterClass();
    ~OuterClass();
    void doSth();
};

#endif /* OUTERCLASS_HPP_ */

OuterClass.cpp:

#include "OuterClass.hpp"
#include <iostream>

OuterClass::OuterClass() {
    // automatically: blah = InnerClass();
    std::cout << "outer class constructing started, blah.a = " << blah.a << std::endl;
    blah = InnerClass(3);
    std::cout << "outer class constructed" << std::endl;
}

OuterClass::~OuterClass() {
    std::cout << "outer class destructed" << std::endl;
}

void OuterClass::doSth() {
    std::cout << "doSth: " << blah.a << std::endl;
}

main:

#include "OuterClass.hpp"
#include <iostream>

int main(int argc, char** argv) {
    std::cout << "Compiled at " << __TIME__ << std::endl;

    OuterClass x = OuterClass();
    x.doSth();

    std::cout << "done" << std::endl;
}

output:

Compiled at 12:11:12
inner class constructed, a = 1 //this is unexpected
outer class constructing started, blah.a = 1 //this should be random data
inner class constructed, a = 3
inner class destructed, a = 3 //this is unexpected
outer class constructed
doSth: 3
done
outer class destructed
inner class destructed, a = 3

Questions:

1. Why is the default constructor of InnerClass called at the start of OuterClass constructor?
2. What and why is destructed in OuterClass constructor (“inner class destructed, a = 3 //this is unexpected”)?
3. It seems that InnerClass object with a = 3 was destructed in the OuterClass constructor, than why does method doSth() return 3 instead of random data?
4. Why does removing an InnerClass() constructor (from both InnerClass.hpp and InnerClass.cpp files) result in compile-time error at the OuterClass constructor in OuterClass.cpp file? The error says that no InnerClass() definition found.

Answer 1

Use initializer-list in constructor.

OuterClass::OuterClass() : blah(3) {
    // automatically: blah = InnerClass();
    std::cout << "outer class constructing started, blah.a = " << blah.a << std::endl;
    std::cout << "outer class constructed" << std::endl;
}

Since when you use

OuterClass::OuterClass() {
    // automatically: blah = InnerClass();
    std::cout << "outer class constructing started, blah.a = " << blah.a << std::endl;
    blah = InnerClass(3);
    std::cout << "outer class constructed" << std::endl;
}

firstly for initialize blah will be called default c-tor and in blah = InnerClass(3);, that creates temporary object and copy it to blah, after this string will be called destructor of temporary object.