Creation of variable with nlohmann data type failing

In nlohmann JSON class The float value type is internally declared as

json::value_t::number_float

But why can't I create an object with that type?

json::value_t::number_float y;

Error:

test.cpp: In function 'int main()':
test.cpp:18:31: error: expected ';' before 'y'
   json::value_t::number_float y ;

What is the issue with it?