C++‘s constructor is different than java or C#.
A C++ constructor like any other function, has a name, a parameter list, and a function body. Unlike other functions, a C++ constructor may also contain a constructor initializer list:
class ClassC{
public:
ClassC():m_a(1),m_b(1.5){ // the constructor initializer list
}
private:
int m_a;
float m_b;
};
The constructor initializer starts with a colon, which is followed by a comma-separated list of data members each of which is followed by an initializer inside parentheses. The constructor initializes the m_a to 1, and initializes m_b to 1.5
What's the different between the two ways.
class ClassC{
public:
ClassC(){
m_a = 1;
m_b = 1.5;
}
private:
int m_a;
float m_b;
};
Look at the example below:
#ifndef _TESTCLASS_
#define _TESTCLASS_
#include <iostream>
using namespace std;
class ClassA{
public:
ClassA(){
cout << "call ClassA's default constructor" << endl;
}
ClassA(int a){
m_a = a;
cout << "call ClassA's \"ClassA(int a)\" constructor" << endl;
}
private:
int m_a;
};
class ClassB{
public:
ClassB(){
classa = ClassA(1);
cout << "call ClassB's constructor" << endl;
}
private:
ClassA classa;
};
#endif
When we call
ClassB b;
The classa is initialize before call classa = ClassA(1)!. The result is not we expected.
When we use the constructor initializer, the result is OK now.
#ifndef _TESTCLASS_
#define _TESTCLASS_
#include <iostream>
using namespace std;
class ClassA{
public:
ClassA(){
cout << "call ClassA's default constructor" << endl;
}
ClassA(int a){
m_a = a;
cout << "call ClassA's \"ClassA(int a)\" constructor" << endl;
}
private:
int m_a;
};
class ClassB{
public:
ClassB():classa(1){ //use constructor initializer list
cout << "call ClassB's constructor" << endl;
}
private:
ClassA classa;
};
#endif
ClassB b;
In many classes, the distinction between initialization and assignment is strictly a matter of low-level efficiency: A data member is initialized and assigned when it could have been initialized directly.
