I thought I had understood it. But now this site is confusing me... http://www.informit.com/articles/article.aspx?p=1852519

Class Member Initialization

C++11 pulls another rabbit out of its hat with class member initializers. Perhaps an example will best illustrate these:

class C
{
int x=7; //class member initializer
public:
C();
};

The data member x is automatically initialized to 7 in every instance of class C. In former dialects of C++, you would use the more cumbersome mem-init notation for the same purpose:

class C
{
int x;
public:
C() : x(7) {}
};

(...)

Notice that a class member initializer can consist of any valid initialization expression, whether that's the traditional equal sign, a pair of parentheses, or the new brace-init:

class C
{
string s("abc");
double d=0;
char * p {nullptr};
int y[5] {1,2,3,4};
public:
C();
};

Regardless of the initialization form used, the compiler conceptually transforms every class member initializer into a corresponding mem-init. Thus, class C above is semantically equivalent to the following class:

class C2
{
string s;
double d;
char * p;
int y[5];
public:
C() : s("abc"), d(0.0), p(nullptr), y{1,2,3,4} {}
};

class C
{
int x=7; //class member initializer

The way I understood it, this shouldn't be a class member initializer. It should be an assignment instead....

And how can these two be similar? That's exactly what I was trying in the first place!

class C
{
string s("abc");

C() : s("abc")

Extended support for this kind of initialization was introduced with C++11, so if you're not using C++11 then you cannot do that and you have to use the initializer list. But even if you do use a compiler that claims C++11 support, you may only have partial C++11 support. Visual Studio up to 2012 for example, without the very latest test patches at least, does not support this.

As far as my quick tests show, and I am not able to look into the language specification right now, you can only initialize with the assignment operator so your multi-parameter constructor call appears to be invalid anyway.

In C++11, if you have it enabled, and if your compiler supports it, it'd look like this:

Button bSstart = Button("StartGame",   50, 20, 50, 5);
//OR:
Button bSstart = {"StartGame",   50, 20, 50, 5};
//OR:
Button bSstart{"StartGame",   50, 20, 50, 5};
 
//But not:
Button bSstart("StartGame",   50, 20, 50, 5); //<--- Syntax error

(I compiled a quick snippet to be sure)

The middle version (blah = {...}) can cause confusion if your constructor takes a std::initializer_list as a single parameter, like the standard library containers do.