Declaring a probability distribution within a class

[lastchance]

I would like to declare a probability distribution within a class. Why does the desired form (2) in the below code fail, but any of positions (1), (3) or (4) work?

#include <iostream>
#include <random>
#include <ctime>
using namespace std;

mt19937 gen( time( 0 ) );
// uniform_real_distribution<double> uni( 0.0, 1.0 );                                       // (1) (OK)


class X
{
   uniform_real_distribution<double> uni( 0.0, 1.0 );                                       // (2) (fails)
// uniform_real_distribution<double> uni = uniform_real_distribution<double>( 0.0, 1.0 );   // (3) (OK)
public:
   void f()
   {
//    uniform_real_distribution<double> uni( 0.0, 1.0 );                                    // (4) (OK)
      cout << uni( gen );
   }
};


int main()
{
   X x;
   x.f();
}

[coder777]

It's the difference between parentheses and curly braces. The latter works.

[lastchance]

Thank-you, @coder777