In this specific case, is there a difference between using a member initializer list and assigning values in a constructor?

Question

Internally and about the generated code, is there a really difference between :

MyClass::MyClass(): _capacity(15), _data(NULL), _len(0)
{
}

Answer 1

There is a difference between initialization list and initialization statement in a constructor. Let's consider below code:

#include 
#include 
#include 
#include 

class MyBase {
public:
    MyBase() {
        std::cout << __FUNCTION__ << std::endl;
    }
};

class MyClass : public MyBase {
public:
    MyClass::MyClass() : _capacity( 15 ), _data( NULL ), _len( 0 ) {
        std::cout << __FUNCTION__ << std::endl;
    }
private:
    int _capacity;
    int* _data;
    int _len;
};

class MyClass2 : public MyBase {
public:
    MyClass2::MyClass2() {
        std::cout << __FUNCTION__ << std::endl;
        _capacity = 15;
        _data = NULL;
        _len = 0;
    }
private:
    int _capacity;
    int* _data;
    int _len;
};

int main() {
    MyClass c;
    MyClass2 d;

    return 0;
}

When MyClass is used, all the members will be initialized before the first statement in a constructor executed.

But, when MyClass2 is used, all the members are not initialized when the first statement in a constructor executed.

In later case, there may be regression problem when someone added some code in a constructor before a certain member is initialized.