How to refactor method using template in C++ - c++

Question

I am in need to (or better, I have the chance to) refactor some code in order to make it cleaner.
I would like to use some template, as I think this is a good candidate, in order to reduce the code duplication.
Here is my hpp
class Monetary
{
public:
Monetary();
Monetary(const rapidjson::Value& iMonetary);
virtual ~Monetary();
[...cut...]
private:
static void initMember(const rapidjson::Value& iMonetary, const char* iName, int& oMember);
static void initMember(const rapidjson::Value& iMonetary, const char* iName, std::string& oMember);
private:
int _amount;
int _decimal_place;
std::string _currency;
std::string _type;
};
And here is the implementation for the the initMember methods:
static void Monetary::initMember(const rapidjson::Value& iMonetary, const char* iName, int& oMember)
{
rapidjson::Value::ConstMemberIterator aIterator;
aIterator = iMonetary.FindMember(iName);
if ( aIterator != iMonetary.MemberEnd() &&
aIterator->value.IsNumber() )
{
oMember = iMonetary[iName].GetInt();
}
}
static void Monetary::initMember(const rapidjson::Value& iMonetary, const char* iName, std::string& oMember)
{
rapidjson::Value::ConstMemberIterator aIterator;
aIterator = iMonetary.FindMember(iName);
if ( aIterator != iMonetary.MemberEnd() &&
aIterator->value.IsNumber() )
{
oMember = iMonetary[iName].GetString();
}
}
I was thinking about writing something like
template<typename T>
void Monetary::initMember(const rapidjson::Value& iMonetary, const char* iName, T& oMember)
{
rapidjson::Value::ConstMemberIterator aIterator;
aIterator = iMonetary.FindMember(iName);
if (aIterator == iMonetary.MemberEnd())
{
return;
//throw monetaryException
}
assignFromValue(iMonetary[iName], oMember);
}
template<>
void Monetary::assignFromValue<int>(const rapidjson::Value& iValue, int& oMember)
{
if (!iValue.IsNumber())
{
return;
//throw monetaryException
}
oMember = iValue.GetInt();
}
template<>
void Monetary::assignFromValue<std::string>(const rapidjson::Value& iValue, std::string& oMember)
{
if (!iValue.IsString())
{
return;
//throw monetaryException
}
oMember = iValue.GetString();
}
Is there any clewer way to do that ?

Answer 1

My suggestions:
You don't need to make assignFromValue member functions. If you can implement functionality using non-member functions, you should prefer non-member functions. See How Non-Member Functions Improve Encapsulation and How Non-Member Functions Improve Encapsulation.
You don't need to make assignFromValue function templates. They can be simple overloads.
void assignFromValue(const rapidjson::Value& iValue,
int& oMember)
{
if (!iValue.IsNumber())
{
return;
//throw monetaryException
}
oMember = iValue.GetInt();
}
void assignFromValue(const rapidjson::Value& iValue,
std::string& oMember)
{
if (!iValue.IsString())
{
return;
//throw monetaryException
}
oMember = iValue.GetString();
}

Answer 2

I think I'd do it with a tag-dispatched converter object:
#include <string>
#include <type_traits>
#include <stdexcept>
// simulate your value class
struct Value
{
bool IsNumber() const;
bool IsString() const;
std::string getString() const;
int getInt() const;
};
// a tag type for easy tag dispatching
template<class Type> struct tag {};
// a converter object contains all rules and conversion sequences
struct ValueConverter
{
std::string operator()(tag<std::string>, const Value& v) const
{
if (not v.IsString()) throw std::invalid_argument("not a string");
return v.getString();
}
int operator()(tag<int>, const Value& v) const
{
if (not v.IsNumber()) throw std::invalid_argument("not a number");
return v.getInt();
}
};
// write the member once
template<class Target>
void initMember(const Value& iMonetary, const char* iName, Target& oMember)
{
using target_type = std::decay_t<Target>;
auto converter = ValueConverter();
oMember = converter(tag<target_type>(), iMonetary);
}