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;
};

v

Answer 1

Elaborating on Christian Hackl's answer.

The first form allows to initialize m_a and have a default c'tor at the same time. Or you can even be explicit in your code and define a constructor with the default keyword:

class Something
{       
    int m_a = 0;

    // explicitly tell the compiler to generate a default c'tor
    Something() = default;
};

With the second form, an auto-generated default c'tor would leave m_a uninitialized, so if you want to initialize to a hard-coded value, you have to write your own default c'tor:

class Something
{
    int m_a;

    // implement your own default c'tor
    Something() : m_a(0) {}
};