C++ Query builder for SPARQL queries

This is my first post in stack-overflow, so sorry in advance for possible "bad practices".

Context: the goal is to send SPARQL queries through http-requests to a GraphDB data base. Problem: to construct the queries on code in a safe way. Currently done by means of std::string dummyStr = "Hello"+" World" or dummyStr.append("bla") (I was told this was not safe due to XSS, but thats not the issue here)

Question: do you know any query builder library for doing this string concatenation? A search for C++ query builder on the web returned this answer. After implementing the approach with the suggested Qt QSqlQuery class, I'm able to ".prepare" the query, but not to ".bindValue".

QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
db.open();
QSqlQuery startQuery;
/*Prepare query*/
startQuery.prepare("INSERT INTO person (id, forename, surname) "
          "VALUES (:id, :forename, :surname)");
startQuery.bindValue(":id", 1001);
startQuery.bindValue(":forename", "Bart");
startQuery.bindValue(":surname", "Simpson");
/*convert query to std::string*/
QString startQueryString = startQuery.lastQuery();
std::string dummyQuery = startQueryString.toUtf8().constData();

Why can't I bind the values to the placeholders? Is it because I have no "actual" database, but rather a dummy-database just to construct the query?

My actual Query looks something like this: SELECT ?s WHERE { FILTER(STRSTARTS(STR(?s),":referenceIRI")). ?s rdf:type rdfs:Class.} And I would like to treat :referenceIRI as a placeholder.

I've searched overall to try to overcome this problem, as I just need the query-builder functionality.

Also: on my actual SPARQL query I have both ? and :myVal elements, which are the 2 types of placeholders in QSqlQuery for binding values. Any idea on how to by-pass the ? placeholder and just consider the :myVal-type?