Undefined reference to a static member of the class [duplicate]

Question

I am working on a homework assignment, and I have almost everything done except for this obnoxious static value that our professor wishes us to use: value

The header file contains:

private: static int value;

And we have to have a function calculate the value, like so:

static void calculate() { long a = 1L; int count = 0; while( a != 0 ) { a = a << 1; count++; } value = count; }

This is essentially calculating the number of bits in a long, using bit shifting.

However, I am getting the error " undefined reference to `Class1::value'

I've spent the last hour and a half figuring this out, and it's killing me. Any help would be great, all searches have come up dead.

Thanks!

Update:

I included

int Class1::value = 0;

However, now I am getting an error saying "error: int Class1::value is private

Roman Byshko · Accepted Answer · 2011-12-05 02:17:39Z

In your *.cpp file add

int ClassName::value = 0;

This will allocate storage for a value.

The piece of code that you actually have in a class declaration just declares this variable (makes the compiler aware that such a variable exists). However, each variable must be declared and defined. A definition will make sure the storage is put aside for this variable and create a symbol your compiler was unable to find before.

KV Prajapati · Accepted Answer · 2011-12-05 02:12:26Z

You need to define a static data member in (.cpp) source file with following syntax:

datatype Your_ClassName::variable;

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