ich habe mir gerade einen kleinen Test geschrieben und bin etwas verwundert, warum das temporaere Objekt bei return _a zwar seinen Destruktor abarbeitet, aber nie einen Konstruktor. Kann mir das einer bitte erklaeren?

#include <iostream>

using namespace std;

class A
{
private:
    int foo;
public:
    A() { cout << "A(" << this << "):ctor" << endl; }
    ~A() { cout << "A(" << this << "):dtor" << endl; }
};

class B
{
private:
    int foo;
    A _a;
public:
    B() {  cout << "B(" << this << "):ctor" << endl; }
    ~B() { cout << "B(" << this << "):dtor" << endl; }
    A get() { cout << "B(" << this << "):get()" << endl; return _a; }

};

int main()
{
    {
        cout << "B b;" << endl;
        B b;
        cout << "A a2;" << endl;
        A a2;
        cout << "a2 = b.get();" << endl;
        a2 = b.get();
        cout << "leave scope" << endl;
    }

    return 0;
}

Ausgabe:

B b;
A(0x7fff5b8fcb34):ctor
B(0x7fff5b8fcb30):ctor
A a2;
A(0x7fff5b8fcb20):ctor
a2 = b.get();
B(0x7fff5b8fcb30):get()
A(0x7fff5b8fcb18):dtor  <-- wo ist dazu der ctor?
leave scope
A(0x7fff5b8fcb20):dtor
B(0x7fff5b8fcb30):dtor
A(0x7fff5b8fcb34):dtor

#include <iostream>

using namespace std;

class A
{
private:
    int foo;
public:
    A() { cout << "A(" << this << "):ctor" << endl; }
    A(const A& a) {  cout << "A(" << this << "):copy ctor" << endl; }
    ~A() { cout << "A(" << this << "):dtor" << endl; }
};

class B
{
private:
    int foo;
    A _a;
public:
    B() {  cout << "B(" << this << "):ctor" << endl; }
    B(const B& b) {  cout << "B(" << this << "):copy ctor" << endl; }
    ~B() { cout << "B(" << this << "):dtor" << endl; }
    A get() { cout << "B(" << this << "):get()" << endl; return _a; }

};

int main()
{
    {
        cout << "B b;" << endl;
        B b;
        cout << "A a2;" << endl;
        A a2;
        cout << "a2 = b.get();" << endl;
        a2 = b.get();
        cout << "leave scope" << endl;
    }

    return 0;
}

B b;
A(0x7fff5478eb34):ctor
B(0x7fff5478eb30):ctor
A a2;
A(0x7fff5478eb20):ctor
a2 = b.get();
B(0x7fff5478eb30):get()
A(0x7fff5478eb18):copy ctor <-- Da ist die Loesung
A(0x7fff5478eb18):dtor
leave scope
A(0x7fff5478eb20):dtor
B(0x7fff5478eb30):dtor
A(0x7fff5478eb34):dtor