Which way is better for declaring class variable with constants initial value?

I have a class Foo that contain a variable int bar and I want to make 255 as an initial value for bar like this.

Foo foo1, foo2;
cout << foo1.bar << "," << foo2.bar; //255,255
foo1.bar = 254;
cout << foo1.bar << "," << foo2.bar; //254,255

As I know, there is lot of way to do this! For example:

//Use int bar = 255
class Foo {
public:
    int bar = 255;
    Foo() {}
};

//Use bar(255) or bar = 255 on constructor
class Foo {
public:
    int bar;
    Foo() : bar(255) {}
    //Or Foo() { bar = 255; }
};

So my question is, which way is better in my case? (Or it was just the same?)
I need this for my project because there is a lot of very big class that contain over 100 variables (some are over 300) so I worry about the performance. Thanks!

>Solution :

If you declare one constructor, then int bar = 255; is equivalent to : bar(255).

The only difference between them is that the former applies to any constructor you provide that doesn’t have an initialiser for bar.

There is a variation on your first case:

class Foo {
public:
    int bar = 255;
    // among other things, implicit default constructor is defined
};

C++ is defined by observable behaviour, so all of them, even { bar = 255; } can have the same implementation in object code.