c++ - Template function argument as 'const T' -

i given template function member function in class compactwriter:

template<typename t> compactwriter operator%(const field<const t>& field) {     _os << field.getname() << ": " << field.valuereference() << ", ";     return *this; } 

and then, asked second const for.
know according : is there difference between “t” , “const t” in template parameter? there should no difference.
therefore, i'm kind of confused of why question asked.

a little background hoping might trigger might have missed...
template class 'field':

#ifndef serialization_field_h #define serialization_field_h  #include <string> #include <iostream>  template<typename t> class field { private: public:      field(const std::string& name, t& value)             : _name(name)             , _value(value)     {}      const std::string& getname() const { return _name; }     t& valuereference() const { return _value; }  private:     std::string _name;     t& _value; };  template<typename t> const field<t> field(const std::string& name, t& value) {     return field<t>(name, value); }  #endif //serialization_field_h 

and operator calls made in main.cpp:

const std::string name = "dan"; const int age = 35; const double weight = 30 + sqrt(2);  compactwriter writer(std::cout); // initialize writer object.  writer.open();  writer % field("name", name)         % field("age", age)         % field("weight", weight);  writer.close(); return 0; 

the functions writer.open() , writer.close() nothing except adding cout { , }.

if point me reason why second const in operator overload % member function header there, great.

that question being asked in completely different context. const have here significant. otherwise, not legal use fields of const template parameter type.