Linking error for a naive singleton class in C++

Question

My code is:

class cMySingleton{
private:
    static bool bInstantiated;
    int mInt;
    cMySingleton(){
        mInt=0;
    }
public:
    cMySingleton(int c){
        if (bInstantiated){
            cout << "you can only instantiated once";
        }
        else {
            cMySingleton();
            mInt=c;
        }
    }
};

int main () {

    cMySingleton s(5);
    cMySingleton t(6);
}

The linker keeps complaining:

Undefined symbols for architecture x86_64:
  "cMySingleton::bInstantiated", referenced from:
      cMySingleton::cMySingleton(int) in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

What is going on? C++ novice here~~

have a look at this http://stackoverflow.com/questions/8906817/singleton-class-in-c — Rohit Vipin Mathews, Sep 11 '12 at 07:01

score 3 · Accepted Answer · edited May 23 '17 at 11:49

3

you should initialize static field.

http://ideone.com/Y1huV

#include <iostream>
class cMySingleton{
private:
    static bool bInstantiated;
    int mInt;
    cMySingleton(){
        mInt=0;
    }
public:
    cMySingleton(int c){
        if (bInstantiated){
            std::cout << "you can only instantiated once";
        }
        else {
            cMySingleton();
            mInt=c;
        }
    }
};
bool cMySingleton::bInstantiated = true;
int main () {

    cMySingleton s(5);
    cMySingleton t(6);
}

More information you can be find here:

Static Data Member Initialization

there was also missing include and std:: around cout.

edited May 23 '17 at 11:49

Community

1
1

answered Sep 11 '12 at 06:39

CyberGuy

2,783
1
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`bool cMySingleton::bInstantiated = 3;` really confused. Use true or false for bool initializers. – ForEveR Sep 11 '12 at 06:41
bool cMySingleton::bInstantiated = 3; nice – Tutankhamen Sep 11 '12 at 06:41

score 2 · Answer 2 · answered Sep 11 '12 at 06:37

2

Initialize

static bool bInstantiated;

outside of cMySingleton

bool CMySingleton::bInstantiated;

answered Sep 11 '12 at 06:37

ForEveR

55,233
2
119
133

score 2 · Answer 3 · answered Sep 11 '12 at 06:40

2

Dont forget to initialize your static member outside of your class declaration in .cpp file:

bool cMySingleton::bInstantiated = false;

answered Sep 11 '12 at 06:40

Tutankhamen

3,532
1
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38

Linking error for a naive singleton class in C++

3 Answers3