Re: Singletons / classes with only one member

Immortal Nephi <>
Fri, 27 Feb 2009 13:36:20 -0800 (PST)
On Feb 27, 2:30 pm, W Karas <> wrote:

Suppose you have a class that has only one instance, and there is
little reason to think it would ever make sense for the class to have
more than one instance. What are some criteria for deciding whether
it should even be a class? For example, does:

I ask myself the same question. You refer one instance. You mean one
variable per running program in memory. For example like procedural

// Header.h

int g_variable = 0; // Global Variable

void foo(int &g_v)

void foo2()

// Main.cpp
#include "Header.h"

int main()
int var = 20;
foo(var); 1% slower

g_variable = var;
foo2(); // Faster

return 0;

According to my performance test. foo2() is faster than foo() because
it has fewer x86 instructions than foo() does. It looks like that 200
global functions do not have reference are able to modify g_variable
directly. One problem is that you don't know which global function
modify g_variable. It is a good design unless you wrap 200 global
functions and 10 global variables inside file scope. I am sure that
everyone agree to be a bad design, but critical performance is

class A
    int a;

    void x();
    void y();

You define A a; data member and function member are wrapped inside
class. It uses a pointer to access data member or function member
indirectly. It seems to be good performance. I don't recommend it
unless you create A array like A a[5] or A a1 and A a2.

A only_instance_of_a;


class A
    static int a;

    static void x();
    static void y();

Class A with static on both data member and function member are better
because static causes them to become global variable and global
function like I described above. You should always use class because
you want encapsulation.

have major advantages over:

// Header file

namespace A

void x();
void y();



// Implementation file.

namespace A

namespace Impl
int a;


using namespace Impl;

void x() { ... }
void y() { ... }


I do not recommend namespace because it does not have encapsulation.
It has opened code, but class has closed code with encapsulation.

The one obvious advantage of a class is that privacy is strictly
enforced by the compiler (as opposed to putting private stuff in a sub-
namespace that is private by convention). The disadvantage of a class
is the classic problem of trying to minimize implementation details in
the header file for external code. I'm aware of the technique of
using a pointer or reference to class private data, but is it more
trouble than it's worth? It seem generally unintuitive to have a
class of objects with only one object in it.


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