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) { } 

and

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

thanks...

Answer 1

You need to use initialization list to initialize constant members,references and base class

When you need to initialize constant member, references and pass parameters to base class constructors, as mentioned in comments, you need to use initialization list.

struct aa {     int i;     const int ci;       // constant member      aa() : i(0) {} // will fail, constant member not initialized };  struct aa {     int i;     const int ci;      aa() : i(0) { ci = 3;} // will fail, ci is constant };  struct aa {     int i;     const int ci;      aa() : i(0), ci(3) {} // works }; 

Example (non exhaustive) class/struct contains reference:

struct bb {};  struct aa {     bb& rb;     aa(bb& b ) : rb(b) {} };  // usage:  bb b; aa a(b); 

And example of initializing base class that requires a parameter (e.g. no default constructor):

struct bb {};  struct dd {     char c;     dd(char x) : c(x) {} };  struct aa : dd {     bb& rb;     aa(bb& b ) : dd('a'), rb(b) {} }; 

Answer 2

Assuming that those values are primitive types, then no, there's no difference. Initialization lists only make a difference when you have objects as members, since instead of using default initialization followed by assignment, the initialization list lets you initialize the object to its final value. This can actually be noticeably faster.