Exercise: Singleton (Flexible And Strict)¶

Complications ¶

In Exercise: Singleton (Flexible), we showed a more flexible way to instantiate the instance at runtime.

Unfortunately, that implementation has the its own shortcomings:

Overwriting the instance can only result in a runtime error
Program startup has to be crafted carefully
Users are now explicitly enabled to instantiate either of the two implementation (alas, it’s the user who writes startup code)
All in all: users see that there are concrete implementations, and must be told rules:
- Only instantiate concrete implementation at startup
- After startup, only access interface via SocialInsurance::instance()

Quick fix: re-instantiate the requirements from Exercise: Singleton (Inflexible):

Eliminate the offending SocialInsurance::set_instance() method.
Introduce an environment variable SOCINSUR that is set to either name() of the available implementations (i.e., SVS or OEGK).
Modify SocialInsurance::instance() to read the SOCINSUR environment variable, and to instantiate the respective inisurance implementation.

Reimplement the class hierarchy such that the following program can run. Take special care:

Uncomment the commented-out lines and make sure that they emit compiler errors.
There must not be a memory leak at program end.

/trainings/material/soup/cxx-code/design-patterns-singleton/singleton-social-insurance-flexible-strict-main.cpp¶

#include "social-insurance-flexible-strict.h"

// only there to deny compiability below
// #include "social-insurance-svs-strict.h"
// #include "social-insurance-oegk-strict.h"

#include <iostream>
#include <string>


int main()
{
    std::string id("1037190666");

    SocialInsurance::instance().charge(id, 1254.60);
    SocialInsurance::instance().charge(id, 231.34);

    std::cout << id << " owes \"" << SocialInsurance::instance().name() << "\" " << SocialInsurance::instance().debt(id) << " Euros" << std::endl;

    // MUST NOT COMPILE
    // ================

    // explicit instantiation (neither iface nor implementations)
    // ----------------------------------------------------------

    // SocialInsurance iface_instance("Another Insurance");
    // SVS svs_instance;
    // OEGK oegk_instance;

    // copy initialization
    // -------------------

    // SocialInsurance another_instance = SocialInsurance::instance();

    // copy assignment
    // ---------------

    // another_instance = SocialInsurance::instance();

    // and dynamic_cast?
    // -----------------

    // STOP HERE!!!

    return 0;
}

Tests ¶

SOCINSUR=OEGK

$ SOCINSUR=OEGK ./singleton-social-insurance-flexible-strict-main
1037190666 owes "OEGK" 1485.94 Euros

SOCINSUR=SVS

$ SOCINSUR=SVS ./singleton-social-insurance-flexible-strict-main
1037190666 owes "SVS" 1634.53 Euros

SOCINSUR unset

$ ./singleton-social-insurance-flexible-strict-main
terminate called after throwing an instance of 'std::runtime_error'
  what():  Environment variable "SOCINSUR" is not set
Aborted (core dumped)

SOCINSUR invalid

$ SOCINSUR=xxx ./singleton-social-insurance-flexible-strict-main
terminate called after throwing an instance of 'std::runtime_error'
  what():  Environment variable "SOCINSUR" has invalid value
Aborted (core dumped)

Exercise: Singleton (Flexible And Strict)¶

Complications¶

Tests¶

Complications ¶

Tests ¶