Templated value checking and getter

[miloyip]

Provide templated functions:

bool Value::Is<int>() const;
int Value::Get<int>() const;
...

These functions are useful when the user code has templated types.

A difficulty is that current RapidJSON does not provide adapter classes for array and object. As @pah suggested in #162 (comment), we can add some class like that. So we can also have Array Value::GetArray() and Array Get<Array>(), etc.

I create this issue for replacing #153.

[zarlin]

I was missing the templated Is and Get functions as well, so I wrote an implementation. Right now it only supports the 'value' types (bool, numbers and string). I saw another issue that requested float overloads as well, so those are included. The floats are also useful for the special Get function on the bottom of this post.

Is

    //template functions for determining if value is of a type
    template<typename Other> bool Is() const    { static_assert(false, "Type not supported"); }
    template<> bool Is<bool>() const            { return IsBool(); }
    template<> bool Is<int32_t>() const         { return IsInt(); }
    template<> bool Is<int64_t>() const         { return IsInt64(); }
    template<> bool Is<uint32_t>() const        { return IsUint(); }
    template<> bool Is<uint64_t>() const        { return IsUint64(); }
    template<> bool Is<float>() const           { return IsDouble(); }
    template<> bool Is<double>() const          { return IsDouble(); }
#if RAPIDJSON_HAS_STDSTRING
    template<> bool Is<std::string>() const     { return IsString(); }
#else
    template<> bool Is<char*>() const           { return IsString(); }
#endif

Get

    //template functions for obtainign the value as a type
    template<typename Other> Other Get() const  { static_assert(false, "Type not supported"); }
    template<> bool Get<bool>() const           { return GetBool(); }
    template<> int32_t Get<int32_t>() const     { return GetInt(); }
    template<> uint32_t Get<uint32_t>() const   { return GetUint(); }
    template<> int64_t Get<int64_t>() const     { return GetInt64(); }
    template<> uint64_t Get<uint64_t>() const   { return GetUint64(); }
    template<> float Get<float>() const         { return GetDouble(); }
    template<> double Get<double>() const       { return GetDouble(); }
#if RAPIDJSON_HAS_STDSTRING
    template<> std::string Get<std::string>() const { return GetString(); }
#else
    template<> char* Get<char*>() const         { return GetString(); }
#endif

Below is an additional Get function that passes a reference to the target variable the result should be stored in. The type is deduced from the passed target. A bool is returned to indicate success or failure. This is mostly a shortcut, but allows making very clean parsing code.

    //function that tries to obtain a value with the given name
    template <typename Target>
    bool Get(const char* name, Target& target) const
    {
        ConstMemberIterator& v = FindMember(name);
        if (v != MemberEnd() && v->value.Is<Target>())
        {
            target = v->value.Get<Target>();
            return true;
        }
        return false;
    }

example usage

    int foo;
    if (!value.Get("foo", foo))
        foo = 1;//default/fallback value