Examle Builder

#include <iostream>
#include <map>
#include <set>
#include <string>

class Json { };

class Builder {
    class BuilderContext;
    class DictValueContext;

public:
    Builder();
    DictValueContext Key(std::string key);

private:
    Json* json;
    class BaseContext {
    public:
        BaseContext(Builder& builder)
            : builder_(builder)
        {
        }
        DictValueContext Key(std::string key)
        {
            return builder_.Key(std::move(key));
        }

    private:
        Builder& builder_;
    };
    class DictValueContext : public BaseContext {
    public:
        DictValueContext(BaseContext base)
            : BaseContext(base)
        {
        }
        DictValueContext Key(std::string key) = delete;
    };
};

Builder::Builder()
    : json(new Json)
{
}

Builder::DictValueContext Builder::Key(std::string key)
{
    return BaseContext { *this };
}

int main() {

}