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

Even though it's supported, this type of initialization will create bugs that will be pretty hard to track down. It's the type of "aesthetic optimization" that you will regret a couple months down the road.

See the example below:

class_x_1.h:

class X
{
private:
    int x = 10;
public:
     int GetX();
};

class_x_2.h:

class X
{
private:
    int x = 20;
public:
    int GetX();
};

class_x.cpp:

#include "class_x_1.h" // implementation uses the version that initializes x with 10

int X::GetX()
{
    return x;
}

main.cpp:

#include "class_x_2.h" // main includes definition that initializes x with 20

#include 

int main()
{
    X x;
    std::cout << x.GetX() << std::endl;
    return 0;
}

Output:

20

As expected, it will return 20 because this is the version that initializes x with 20. However, if you go to the implementation of X, you might expected it to be 10.