C++ Initialising fields directly vs initialisation list in default constructor

Question

I\'d like to know if there is a difference between this code:

class Foo{
 private:
    int a = 0;
 public:
    Foo(){}
}

And:

Answer 1

The two are identical.

One rule of software engineering is DRY -- don't repeat yourself. DRY states that if you can avoid repeating the same token twice, or having two identical lists, you should.

This is for a few reasons. Maintaining two identical lists is surprisingly error prone; one gets modified, or has a typo, and the other does not. It makes code longer, which can make it harder to read. And avoiding copy-paste coding encourages using some very powerful and expressive techniques that can make what you are doing clearer than doing it manually 17 times.

struct foo {
  int a;
  foo():a(7) {}
};

here we have repeated ourselves -- the list of member variables, in particular, is listed twice. Once in the definition of foo, and again in the initializer list of foo::foo. If it is missing somewhere, you get uninitialized data.

struct foo {
  int a = 7;
  foo() {}
};

Here we do not repeat ourselves.

struct foo {
  int a = 7;
  foo() {}
  foo(int i):a(i) {}
};

Here there is some repetition, but the repetition is unavoidable. It is, however, minimized.

There is some cost here, because someone might interpret a=7 to mean "it always starts at 7", and not "the default is 7".

struct foo {
  int a = 7;
  foo():a(3) {}
  foo(int i):a(i) {}
};

And the above is a horrible anti-pattern.