make const value to const ref parameters

python_binding/data/pybind_Tensor.cpp

@@ -257,7 +257,7 @@ static void addScalarCtor(pyTensorClass& mTensor) {
     // though it is not merged: https://github.com/pybind/pybind11/pull/3544/.
     // Hence we use some helper functions defined above to try matching the different numpy scalar types.
     mTensor.def(py::init([](py::object obj,
-                            const std::string backend="cpu") {
+                            const std::string& backend = "cpu") {
         NativeValue native_val;
         DataType native_dtype;
         bool found = getScalarNativeVal(obj, &native_val, &native_dtype);
@@ -283,7 +283,7 @@ static void addScalarCtor(pyTensorClass& mTensor) {
 template<typename T>
 void addArrayCtor(pyTensorClass& mTensor) {
     mTensor.def(py::init([](const py::array_t<T, py::array::c_style|py::array::forcecast> b,
-                            const std::string backend = "cpu") {
+                            const std::string& backend = "cpu") {
         /* Request a buffer descriptor from Python */
         py::buffer_info info = b.request();
         Tensor* newTensor = new Tensor();