Why doesn't my short singleton code in c++ compile?

Question

I know the code at https://www.tutorialspoint.com/Explain-Cplusplus-Singleton-design-pattern compiles and works but I want to write something shorter.

The following gives me this compilation error.

../Playground/: /tmp/ccwamx3r.o: in function `main':
file0.cpp:(.text+0x14): undefined reference to `Example::singleton'
collect2: error: ld returned 1 exit status

Why is "Example::singleton" undefined in main here?

#include <iostream>
using namespace std;

class Example
{
public:
int getValue();
static Example singleton;
};

int Example::getValue()
{
return 1;
}

int main() {
cout << Example::singleton.getValue() << endl;
return 0;
}

Answer 1

You are only 'declaring' the static member 'singleton'. You are not initializing it (in fact, in C++ you cannot initialize static members from inside the class unless they're declared constexpr). That is why the compilation succeeds because the compiler knows that Example::singleton exists. But the linker fails because it does not know where Example::singleton is.

You need to initialize Example::singleton from outside the class. Adding this line anywhere outside the 'Example' class will do it
`Example Example::singleton;`

Answer 2

Thanks, XXX. That works.

Here is the working code:
#include <iostream>
using namespace std;

class Example
{
public:
int getValue();
static Example singleton;
};

Example Example::singleton;

int Example::getValue()
{
return 1;
}

int main() {
cout << Example::singleton.getValue() << endl;
return 0;
}

Answer 3

XXX Nice explanation.

Josh Greig It's been over 2 decades since I spent much time with C++. Seeing this example has me wondering how a static types in C# are actually implemented when compiled to IL.

Perhaps, it's not so different from how singletons are implemented in C++.

It'll certainly be something I'll add to my short list of things to explore when I ever manage to find some free time. 😉