QTFStr Tensoflessione::Instabilitaqtf(QTFStr &qtf)
{                                   // ----------
                                    //     ^
                                    // You are passing a reference (which can be modified, as you do below.    
                          
	String risultato;
	risultato << "(σ[, t,0,d]/ f[, t,0,d]) + (σ[, m,y,d]/ k[, crit] * f[, m,y,d]) + K[, m]*(σ[, m,z,d]/ f[, m,z,d]) <= 1 &";
	risultato << "(σ[, t,0,d]/ f[, t,0,d]) + K[, m]*(σ[, m,y,d]/ f[, m,y,d]) + (σ[, m,z,d]/ k[, crit] * f[, m,z,d]) <= 1 ";
	risultato << GetVerificaInst();
	
	qtf.StartTable(3,2,2,10).CharSize(12);
	qtf.TableSubTitle("Instabilità flesso-torsionale");
	qtf.LeftCellMargin(75)("l[, eff]").LeftCellMargin(15).AlignRight()
		(Format("%.2f", GetInstab().leff)).AlignLeft()("mm")("Lunghezza libera di inflessione");
	qtf.LeftCellMargin(75)("σ[, crit]").LeftCellMargin(15).AlignRight()
		(Format("%.2f", GetInstab().sigmam_critico)).AlignLeft()("")("Tensione critica di svergolamento");
	qtf.LeftCellMargin(75)("λ[, rel]").LeftCellMargin(15).AlignRight()
		(Format("%.2f", GetInstab().lambda_relm)).AlignLeft()("")("Snellezza relativa di svergolamento");	
	qtf.LeftCellMargin(75)("k[, crit]").LeftCellMargin(15).AlignRight()
		(Format("%.2f", GetInstab().kcrit)).AlignLeft()("N/mm[` 2]")("Coefficiente di sbandamento laterale");	
	qtf.TableSubTitle(risultato);
	qtf.EndTable();
		
	qtf.TableSubTitle(risultato);
	qtf.EndTable();
		
	return qtf;
       //----------
       //  ^
       // Then, compiler attempts (by default) to copy qtf (But you need to explicitly define a copy constructor, which is missing, and in it, explicity copy the members of your class (using clone() ), They are causing the error.)
       // By the way, returning a QTFStr from this function shoudn't be necessasy at all, since you are already modifying the referenced instance! (it is non-const), unless you really need a copy of it. It is a much cheaper solution.
}

      QTFStr(QTFStr&&) = default;
      QTFStr& operator=(QTFStr&&) = default;

return pick(qtf);