rapidjson::Pointer not working in loop

main.cpp

// external libraries
#include "rapidjson/document.h"
#include "rapidjson/pointer.h"

// standard libraries
#include <iostream>
#include <string>
#include <vector>

void parse_json_int(rapidjson::Document &json, const std::string &pointer, int &target)
{
    if (rapidjson::Value *obj = rapidjson::Pointer(pointer.c_str()).Get(json))
    {
        if (obj->IsInt())
        {
            target = obj->GetInt();
        } else
        {
            throw std::runtime_error(
                        std::string("parse_json: ")
                        + "key '" + pointer + "' cannot be parsed as "
                        + "int");
        }
    }
}

int main(int, char**)
{
    // prepare Document
    rapidjson::Document d;
    d.Parse("{\"sp\":[1,2]}");

    // directly give path to integer values
    int val1, val2;
    parse_json_int(d, "/sp/0", val1);
    parse_json_int(d, "/sp/1", val2);
    std::cout << "direct readout val1: " << val1 << std::endl;
    std::cout << "direct readout val2: " << val2 << std::endl;
    std::cout << std::endl;

    {
        std::vector<int> target;
        std::string pointer = "/sp";
        if (rapidjson::Value *obj = rapidjson::Pointer(pointer.c_str()).Get(d))
        {
            if (obj->IsArray())
            {
                const rapidjson::Value &array = obj->GetArray();
                size_t size = array.Size();
                // reserve memory
                target.reserve(array.Size());
                for (size_t i=0; i<array.Size(); i++) {
                    const std::string indexed_ptr = pointer + "/" + std::to_string(i);
                    int elem;
                    parse_json_int(d, indexed_ptr, elem);
                    target.push_back(elem);
                }
            } else
            {
                throw std::runtime_error(
                            std::string("parse_json: ")
                            + "key '" + pointer + "' is not an array");
            }
        }
        // output
        std::cout << "std::vector<int> readouts" << std::endl;
        for (size_t i=0; i<target.size(); i++)
        {
            std::cout << "/sp/"<< i << ": " << target[i] << std::endl;
        }
    }
    return 0;
}