The construction operation of the default constructor
1 member object with default constructor
1.1 Behavior
When the external object contains a member object, the compiler will automatically add the call when the member object constructor is not explicitly called;
1.2 Sample code
#include <iostream>
#include <string>
using namespace std;
class A
{
public:
A() {
cout << "call A()" << endl; }
A(int value) {
cout << "call A(value)" << endl; }
};
class B
{
private:
A a;
int value;
string name;
public:
B() {
name = "aloha";
}
void show()
{
cout << "B value=" << value << ",name=" << name <<endl;
};
};
int main()
{
B b;
b.show();
}
1.3 Output
call A()
B value=-858993460,name=aloha
Note:
If the default constructor of A in the comment code, the compilation will not pass.
Need to explicitly call the non-default constructor of A in the constructor of B through the initialization list
B():a(0) {
name = "aloha";
}
2 The parent type has a default constructor
2.1 Behavior
In the constructor of the subtype, the constructor of the parent type is automatically called.
2.2 Sample code 1
#include <iostream>
#include <string>
using namespace std;
class A
{
public:
A() {
cout << "call A()" << endl; }
A(int value) {
cout << "call A(value)" << endl; }
};
class AA:public A
{
int value;
string name;
public:
AA() {
value = 0;
name = "aloha";
}
void show()
{
cout << "AA value=" << value << ",name=" << name << endl;
};
};
int main()
{
AA aa;
aa.show();
}
2.3 Output 1
call A()
AA value=0,name=aloha
2.4 Sample code 2
#include <iostream>
#include <string>
using namespace std;
class A
{
public:
A() {
cout << "call A()" << endl; }
A(int value) {
cout << "call A(value)" << endl; }
};
class AA:public A
{
int value;
string name;
public:
AA()
{
value = 0;
name = "aloha";
}
AA(int value)
{
this->value = value;
name = "aloha";
}
void show()
{
cout << "AA value=" << value << ",name=" << name << endl;
};
};
int main()
{
AA aa(5);
aa.show();
}
2.5 Output 2
call A()
AA value=5,name=aloha
3 The case of both
3.1 Behavior
First call the default construction method of the parent class, and then call the default construction method of the member object
3.2 Sample code
#include <iostream>
#include <string>
using namespace std;
class A
{
public:
A() {
cout << "call A()" << endl; }
A(int value) {
cout << "call A(value)" << endl; }
};
class B
{
public:
B() {
cout << "call B()" << endl; }
B(int value) {
cout << "call B(value)" << endl; }
};
class AAB:public A
{
int value;
string name;
B b;
public:
AAB()
{
value = 0;
name = "aloha";
}
AAB(int value)
{
cout << "AAB start" << endl;
this->value = value;
name = "aloha";
cout << "AAB end" << endl;
}
void show()
{
cout << "AAB value=" << value << ",name=" << name << endl;
};
};
int main()
{
AAB aab(5);
aab.show();
}
3.3 Output
call A()
call B()
AAB start
AAB end
AAB value=5,name=aloha
4 references
"In-depth exploration of the C++ object model", by Stanley B.Lippman, translated by Hou Jie, Electronic Industry Press, 2012;