C++ - initializing variables in header vs with constructor

Question

Regarding the following, are there any reasons to do one over the other or are they roughly equivalent?

class Something {     int m_a = 0; }; 

vs

class Something {     int m_a;     Something(int p_a); };  Something::Something(int p_a):m_a(p_a){ ... }; 

Answer 1

The two code snippets you posted are not quite equal.

class Something {     int m_a = 0; }; 

Here you specify the value with which to initialise, i.e. 0, at compile time.

class Something {     int m_a;     Something(int p_a); };  Something::Something(int p_a):m_a(p_a){ ... }; 

And here you do it at run time (or possibly at run time), with the value p_a not known until the constructor is called.

The following piece of code comes closer to your first example:

class Something {     int m_a;     Something(); };  Something::Something() : m_a(0) { /* ... */ }; 

What you have to consider here is that in the first case, the value appears directly in the class definition. This may create an unnecessary dependency. What happens if you need to change your 0 to 1 later on? Exposing the value directly in the class definition (and thus, usually, in a header file) may cause recompilation of a lot of code in situations where the other form of initialisation would avoid it, because the Something::Something() : m_a(0) part will be neatly encapsulated in a source file and not appear in a header file:

// Something.h - stable header file, never changed class Something {     int m_a;     Something(); };  // Something.cpp - can change easily Something::Something() : m_a(0) { /* ... */ }; 

Of course, the benefits of in-class initialisation may vastly outweigh this drawback. It depends. You just have to keep it in mind.

Answer 2

The first form is more convenient if you have more than one constructor (and want them all to initialise the member in the same way), or if you don't otherwise need to write a constructor.

The second is required if the initialiser depends on constructor arguments, or is otherwise too complicated for in-class initialisation; and might be better if the constructor is complicated, to keep all the initialisation in one place. (And it's also needed if you have to support pre-C++11 compilers.)