Unable to initialize a template variable inside a class, why?

I don’t understand why if I initialize this template variable globally, like this:

template <class T> struct null_string { static const std::string value; };
template<class T> const std::string null_string<T>::value = "";
template<class T> std::string const null_str = null_string<const T&>::value;

it works, but if I try to initialize it inside a class, like this:

class foo
 {
  template <class T> struct null_string { static const std::string value; };
  template<class T> const std::string null_string<T>::value = "";
  template<class T> std::string const null_str = null_string<const T&>::value;
 };

it gave me an error:

prove.cpp:8:65: error: invalid use of qualified-name ‘foo::null_string<T>::value’
    8 |     template<class T> const std::string null_string<T>::value = "";
      |                                                                 ^~
prove.cpp:11:78: error: data member ‘null_str’ cannot be a member template
   11 |   template<class T> std::string const null_str = null_string<const T&>::value;
      |  

Do you know why?

>Solution :

Add missing static. Then, either move variable definitions to namespace scope:

class foo
{
    template <class T> struct null_string { static const std::string value; };
    template <class T> static std::string const null_str;
};

template <class T> const std::string foo::null_string<T>::value = "";
template <class T> std::string const foo::null_str = foo::null_string<const T&>::value;

Or make them inline:

class foo
{
    template <class T> struct null_string { inline static const std::string value = ""; };
    template <class T> inline static const std::string null_str = foo::null_string<const T&>::value;
};