CODEWITHSUNDEEP
c++singleton
menawi
menawi

Reputation: 399

C++ Why is my singleton not maintaining member value

I'm learning C++ and I'm trying to implement a Singleton. Here's the code:

#include <string>
#include <iostream>

class Configuration{

    Configuration(){
        languageName = "English";
    };

    public:
    std::string languageName;

    static Configuration& instance(){
        static Configuration _instance;
        return _instance;
    }
};

int main(){
    auto config = Configuration::instance();
    config.languageName = "Deutsch";
    std::cout<<config.languageName<<std::endl;//Deutsch

    auto config2 = Configuration::instance();
    std::cout<<config.languageName<<std::endl;//Deutsch
    std::cout<<config2.languageName<<std::endl;//English
    std::cout<<(&config == &config2)<<std::endl;//0
}

I expected that if I set the value of language on one instance, the same value will be held by a second instance since both instances are in fact the same.

But in the above code config2 and config have different values for languageName. What is the reason for this? I'm following the code from a tutorial.

Upvotes: 1

Views: 174

Answers (1)

songyuanyao
songyuanyao

Reputation: 172994

both instances are in fact the same

No, they're not the same instance. Even Configuration::instance() returns reference, the variables in main() are not declared as reference too, so the returned reference will be copied. You need to declare them to be reference to avoid copying. Such as,

auto& config = Configuration::instance();
    ~
auto& config2 = Configuration::instance();
    ~

LIVE

BTW: To ensure there's only one instance, you might want to delete the copy/move constructor and assignment operator.

Upvotes: 2

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