コピーコンストラクターをよく理解しようとしていますが、コードのこの部分を見つけました。
#include<iostream>
using namespace std;
class A1 {
int data;
public:
A1(int i = 10) :
data(i) {
cout << "I am constructing an A1 with: " << i << endl;
}
A1(const A1& a1) :
data(a1.data) {
cout << "I am copy constructing an A1" << endl;
}
~A1() {
cout << "I am destroying an A1 with: " << data << endl;
}
void change() {
data = data * 10;
}
};
class A2 {
int data;
public:
A2(int i = 20) :
data(i) {
cout << "I am constructing an A2 with: " << i << endl;
}
A2(const A2& a2) :
data(a2.data) {
cout << "I am copy constructing an A2" << endl;
}
~A2() {
cout << "I am destroying an A2 with: " << data << endl;
}
void change() {
data = data * 20;
}
};
class A3 {
public:
A3() {
cout << "I am constructing an A3" << endl;
}
A3(const A3& a3) {
cout << "I am copy constructing an A3" << endl;
}
~A3() {
cout << "I am destroying an A3" << endl;
}
void change() {
cout << "Nothing to change" << endl;
}
};
class A {
A1 a1;
A2 a2;
A3 a3;
public:
A() {
cout << "I am constructing an A" << endl;
}
A(const A& a) :
a1(a.a1) {
cout << "I am copy constructing an A" << endl;
}
~A() {
cout << "I am destroying an A" << endl;
}
A& operator=(const A& a) {
cout << "I am performing a stupid assignment between As" << endl;
if (this != &a)
a1 = a.a1;
return *this;
}
void change() {
a1.change();
a2.change();
a3.change();
}
};
class BigA {
A data1;
A& data2;
public:
BigA(A& a) :
data1(a), data2(a) {
cout << "I just constructed a BigA" << endl;
}
~BigA() {
cout << "I am destroying a BigA" << endl;
}
A get(int index) {
if (index == 1)
return data1;
else
return data2;
}
};
BigA volta(BigA& biga)
//BigA& volta(BigA& biga)
{
cout << "Volta ta data?" << endl;
return biga;
}
int main() {
A first;
BigA biga(first);
volta(biga).get(2).change();
return 0;
}
しかし、なぜこれらの結果が得られるのか理解できません。特に、コンストラクターではなく A1 と A のコピー コンストラクターが呼び出され、volta 関数が呼び出されたときにまったく得られない理由 (**** で囲まれた結果) :
I am constructing an A1 with: 10
I am constructing an A2 with: 20
I am constructing an A3
I am constructing an A
I am copy constructing an A1
I am constructing an A2 with: 20
I am constructing an A3
I am copy constructing an A
I just constructed a BigA
****
Volta ta data?
I am copy constructing an A1
I am constructing an A2 with: 20
I am constructing an A3
I am copy constructing an A
I am copy constructing an A1
I am constructing an A2 with: 20
I am constructing an A3
I am copy constructing an A
Nothing to change
I am destroying an A
I am destroying an A3
I am destroying an A2 with: 400
I am destroying an A1 with: 100
I am destroying a BigA
I am destroying an A
I am destroying an A3
I am destroying an A2 with: 20
I am destroying an A1 with: 10
****
I am destroying a BigA
I am destroying an A
I am destroying an A3
I am destroying an A2 with: 20
I am destroying an A1 with: 10
I am destroying an A
I am destroying an A3
I am destroying an A2 with: 20
I am destroying an A1 with: 10
EDIT_AssignmentOperatorQuery : この関数を BigA に追加すると
void change() {
A& rdata1 = data1;
A cdata2 = data2;
}
main から呼び出します:biga.change();デフォルトの代入演算子の代わりに、copy-constructor と constructor が呼び出されているのはなぜですか?
I am copy constructing an A1
I am constructing an A2 with: 20
I am constructing an A3
I am copy constructing an A
EDIT_AnsweringMyOwnQuery : これはコピー コンストラクターによる初期化であり、代入演算子による代入ではないことがわかりました。