C++ header and class file - undefined reference

Question

Possible Duplicate:
What is an undefined reference/unresolved external symbol error and how do I fix it?

.h:

class ArithmeticCoding
{
public:
    ArithmeticCoding();
    static void test(QString text);

    static QMap<QChar,int> letters_freq;
}

.cpp:

QMap<QChar, int> letters_freq;

ArithmeticCoding::ArithmeticCoding()
{
}

void ArithmeticCoding::test(QString text)
{
    for(int i=0; i<text.length(); i++) letters_freq[text.at(i)]++;
}

I am getting

arithmeticcoding.cpp:-1: error: undefined reference to `ArithmeticCoding::letters_freq'

Why?

Answer 1

Add this to exactly one of your CPPs

 QMap<QChar,int> ArithmeticCoding::letters_freq;