From d8458ddebd00b307f4e7ff5bc4d57f7d70b0f733 Mon Sep 17 00:00:00 2001
From: Jonas Rembser <jonas.rembser@cern.ch>
Date: Fri, 20 Sep 2024 20:06:50 +0200
Subject: [PATCH] [Minuit2] Revert recent changes to virtual function signature

This reverts part of 63636f6aeb3 from a few days ago, which also changed
the interface of virtual functions, which was not the intention and
breaks user code.
---
 math/minuit2/inc/Minuit2/FCNAdapter.h              |  9 +--------
 math/minuit2/inc/Minuit2/FCNBase.h                 | 10 +++++-----
 math/minuit2/inc/Minuit2/FCNGradAdapter.h          | 14 +++++++-------
 math/minuit2/inc/Minuit2/FumiliChi2FCN.h           |  4 ++--
 math/minuit2/inc/Minuit2/FumiliFCNAdapter.h        | 14 ++++----------
 math/minuit2/inc/Minuit2/FumiliFCNBase.h           |  6 +++---
 .../inc/Minuit2/FumiliMaximumLikelihoodFCN.h       |  6 ++----
 math/minuit2/inc/Minuit2/FumiliStandardChi2FCN.h   |  6 ++----
 .../Minuit2/FumiliStandardMaximumLikelihoodFCN.h   |  6 ++----
 math/minuit2/inc/Minuit2/GenericFunction.h         |  2 +-
 math/minuit2/inc/Minuit2/ParametricFunction.h      | 10 ++++------
 math/minuit2/src/FumiliStandardChi2FCN.cxx         |  4 ++--
 .../src/FumiliStandardMaximumLikelihoodFCN.cxx     |  4 ++--
 math/minuit2/src/ParametricFunction.cxx            |  4 +---
 math/minuit2/test/MnSim/GaussFcn.cxx               |  2 +-
 math/minuit2/test/MnSim/GaussFcn.h                 |  9 +++------
 math/minuit2/test/MnSim/GaussFcn2.cxx              |  2 +-
 math/minuit2/test/MnSim/GaussFcn2.h                |  3 +--
 math/minuit2/test/MnSim/GaussianModelFunction.h    |  8 +++-----
 math/minuit2/test/MnSim/ParallelTest.cxx           |  2 +-
 math/minuit2/test/MnSim/PaulTest4.cxx              |  8 ++------
 math/minuit2/test/MnSim/ReneTest.cxx               |  4 +---
 math/minuit2/test/MnTutorial/Quad12F.h             |  2 +-
 math/minuit2/test/MnTutorial/Quad1F.h              | 14 +++++---------
 math/minuit2/test/MnTutorial/Quad4F.h              | 14 +++-----------
 math/minuit2/test/MnTutorial/Quad8F.h              |  2 +-
 26 files changed, 61 insertions(+), 108 deletions(-)

diff --git a/math/minuit2/inc/Minuit2/FCNAdapter.h b/math/minuit2/inc/Minuit2/FCNAdapter.h
index 3e5e5a16a3ce7..5aa4ae7d1742a 100644
--- a/math/minuit2/inc/Minuit2/FCNAdapter.h
+++ b/math/minuit2/inc/Minuit2/FCNAdapter.h
@@ -37,19 +37,12 @@ class FCNAdapter : public FCNBase {
 public:
    FCNAdapter(const Function &f, double up = 1.) : fFunc(f), fUp(up) {}
 
-   ~FCNAdapter() override {}
-
-   double operator()(std::span<const double> v) const override { return fFunc.operator()(&v[0]); }
+   double operator()(std::vector<double> const& v) const override { return fFunc.operator()(&v[0]); }
    double operator()(const double *v) const { return fFunc.operator()(v); }
    double Up() const override { return fUp; }
 
    void SetErrorDef(double up) override { fUp = up; }
 
-   // virtual std::vector<double> Gradient(const std::vector<double>&) const;
-
-   // forward interface
-   // virtual double operator()(int npar, double* params,int iflag = 4) const;
-
 private:
    const Function &fFunc;
    double fUp;
diff --git a/math/minuit2/inc/Minuit2/FCNBase.h b/math/minuit2/inc/Minuit2/FCNBase.h
index 412544f4d01d9..7ab93ce5c6099 100644
--- a/math/minuit2/inc/Minuit2/FCNBase.h
+++ b/math/minuit2/inc/Minuit2/FCNBase.h
@@ -74,7 +74,7 @@ class FCNBase : public GenericFunction {
 
    */
 
-   double operator()(std::span<const double> v) const override = 0;
+   double operator()(std::vector<double> const &v) const override = 0;
 
    /**
 
@@ -112,8 +112,8 @@ class FCNBase : public GenericFunction {
 
    virtual bool HasGradient() const { return false; }
 
-   virtual std::vector<double> Gradient(std::span<const double> ) const { return {}; }
-   virtual std::vector<double> GradientWithPrevResult(std::span<const double> parameters, double * /*previous_grad*/,
+   virtual std::vector<double> Gradient(std::vector<double> const&) const { return {}; }
+   virtual std::vector<double> GradientWithPrevResult(std::vector<double> const& parameters, double * /*previous_grad*/,
                                                       double * /*previous_g2*/, double * /*previous_gstep*/) const
    {
       return Gradient(parameters);
@@ -124,10 +124,10 @@ class FCNBase : public GenericFunction {
    };
 
    /// return second derivatives (diagonal of the Hessian matrix)
-   virtual std::vector<double> G2(std::span<const double> ) const { return {};}
+   virtual std::vector<double> G2(std::vector<double> const&) const { return {};}
 
    /// return Hessian
-   virtual std::vector<double> Hessian(std::span<const double> ) const { return {};}
+   virtual std::vector<double> Hessian(std::vector<double> const&) const { return {};}
 
    virtual bool HasHessian() const { return false; }
 
diff --git a/math/minuit2/inc/Minuit2/FCNGradAdapter.h b/math/minuit2/inc/Minuit2/FCNGradAdapter.h
index e7fedf76d46c3..4ddaae08b409a 100644
--- a/math/minuit2/inc/Minuit2/FCNGradAdapter.h
+++ b/math/minuit2/inc/Minuit2/FCNGradAdapter.h
@@ -41,17 +41,17 @@ class FCNGradAdapter : public FCNBase {
 
    bool HasGradient() const override { return true; }
 
-   double operator()(std::span<const double> v) const override { return fFunc.operator()(&v[0]); }
+   double operator()(std::vector<double> const& v) const override { return fFunc.operator()(&v[0]); }
    double operator()(const double *v) const { return fFunc.operator()(v); }
 
    double Up() const override { return fUp; }
 
-   std::vector<double> Gradient(std::span<const double> v) const override
+   std::vector<double> Gradient(std::vector<double> const& v) const override
    {
       fFunc.Gradient(&v[0], &fGrad[0]);
       return fGrad;
    }
-   std::vector<double> GradientWithPrevResult(std::span<const double> v, double *previous_grad, double *previous_g2,
+   std::vector<double> GradientWithPrevResult(std::vector<double> const& v, double *previous_grad, double *previous_g2,
                                               double *previous_gstep) const override
    {
       fFunc.GradientWithPrevResult(&v[0], &fGrad[0], previous_grad, previous_g2, previous_gstep);
@@ -67,7 +67,7 @@ class FCNGradAdapter : public FCNBase {
    }
 
    /// return second derivatives (diagonal of the Hessian matrix)
-   std::vector<double> G2(std::span<const double>  x) const override {
+   std::vector<double> G2(std::vector<double> const&  x) const override {
       if (fG2Func)
          return fG2Func(x);
       if (fHessianFunc) {
@@ -88,7 +88,7 @@ class FCNGradAdapter : public FCNBase {
    }
 
    /// compute Hessian. Return Hessian as a std::vector of size(n*n)
-   std::vector<double> Hessian(std::span<const double>  x ) const override {
+   std::vector<double> Hessian(std::vector<double> const&  x ) const override {
       unsigned int n = fFunc.NDim();
       if (fHessianFunc) {
          if (fHessian.empty() ) fHessian.resize(n * n);
@@ -126,8 +126,8 @@ class FCNGradAdapter : public FCNBase {
    mutable std::vector<double> fHessian;
    mutable std::vector<double> fG2Vec;
 
-   std::function<std::vector<double>(std::span<const double> )> fG2Func;
-   mutable std::function<bool(std::span<const double> , double *)> fHessianFunc;
+   std::function<std::vector<double>(std::vector<double> const& )> fG2Func;
+   mutable std::function<bool(std::vector<double> const& , double *)> fHessianFunc;
 };
 
 } // end namespace Minuit2
diff --git a/math/minuit2/inc/Minuit2/FumiliChi2FCN.h b/math/minuit2/inc/Minuit2/FumiliChi2FCN.h
index 5126312da537a..21827f5ec1aa2 100644
--- a/math/minuit2/inc/Minuit2/FumiliChi2FCN.h
+++ b/math/minuit2/inc/Minuit2/FumiliChi2FCN.h
@@ -87,7 +87,7 @@ class FumiliChi2FCN : public FumiliFCNBase {
 
    */
 
-   virtual std::vector<double> Elements(std::span<const double> par) const = 0;
+   virtual std::vector<double> Elements(std::vector<double> const &par) const = 0;
 
    /**
 
@@ -129,7 +129,7 @@ class FumiliChi2FCN : public FumiliFCNBase {
 
    */
 
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
 
       double chiSquare = 0.0;
diff --git a/math/minuit2/inc/Minuit2/FumiliFCNAdapter.h b/math/minuit2/inc/Minuit2/FumiliFCNAdapter.h
index 56770149b9e44..e5c6995fc06f8 100644
--- a/math/minuit2/inc/Minuit2/FumiliFCNAdapter.h
+++ b/math/minuit2/inc/Minuit2/FumiliFCNAdapter.h
@@ -48,15 +48,13 @@ class FumiliFCNAdapter : public FumiliFCNBase {
 
    FumiliFCNAdapter(const Function &f, unsigned int ndim, double up = 1.) : FumiliFCNBase(ndim), fFunc(f), fUp(up) {}
 
-   ~FumiliFCNAdapter() override {}
-
-   double operator()(std::span<const double> v) const override { return fFunc.operator()(&v[0]); }
+   double operator()(std::vector<double> const &v) const override { return fFunc.operator()(&v[0]); }
    double operator()(const double *v) const { return fFunc.operator()(v); }
    double Up() const override { return fUp; }
 
    void SetErrorDef(double up) override { fUp = up; }
 
-   // virtual std::vector<double> Gradient(std::span<const double> ) const;
+   // virtual std::vector<double> Gradient(std::vector<double> const &) const;
 
    // forward interface
    // virtual double operator()(int npar, double* params,int iflag = 4) const;
@@ -64,22 +62,18 @@ class FumiliFCNAdapter : public FumiliFCNBase {
    /**
        evaluate gradient hessian and function value needed by fumili
      */
-   void EvaluateAll(std::span<const double> v) override;
+   void EvaluateAll(std::vector<double> const &v) override;
 
 private:
-   // data member
-
    const Function &fFunc;
    double fUp;
 };
 
 template <class Function>
-void FumiliFCNAdapter<Function>::EvaluateAll(std::span<const double> v)
+void FumiliFCNAdapter<Function>::EvaluateAll(std::vector<double> const &v)
 {
    MnPrint print("FumiliFCNAdapter");
 
-   // typedef FumiliFCNAdapter::Function Function;
-
    // evaluate all elements
    unsigned int npar = Dimension();
    if (npar != v.size())
diff --git a/math/minuit2/inc/Minuit2/FumiliFCNBase.h b/math/minuit2/inc/Minuit2/FumiliFCNBase.h
index 2f15251c6f883..1e754b9c03b91 100644
--- a/math/minuit2/inc/Minuit2/FumiliFCNBase.h
+++ b/math/minuit2/inc/Minuit2/FumiliFCNBase.h
@@ -84,7 +84,7 @@ class FumiliFCNBase : public FCNBase {
 
    **/
 
-   virtual void EvaluateAll(std::span<const double> par) = 0;
+   virtual void EvaluateAll(std::vector<double> const& par) = 0;
 
    /**
       Return cached Value of objective function estimated previously using the  FumiliFCNBase::EvaluateAll method
@@ -98,7 +98,7 @@ class FumiliFCNBase : public FCNBase {
    **/
 
    virtual const std::vector<double> &Gradient() const { return fGradient; }
-   std::vector<double> Gradient(std::span<const double>) const override { return fGradient;}
+   std::vector<double> Gradient(std::vector<double> const&) const override { return fGradient;}
 
    /**
       Return Value of the i-th j-th element of the Hessian matrix estimated previously using the
@@ -107,7 +107,7 @@ class FumiliFCNBase : public FCNBase {
       @param col col Index of the matrix
    **/
 
-   std::vector<double> Hessian(std::span<const double>) const override { return fHessian;}
+   std::vector<double> Hessian(std::vector<double> const&) const override { return fHessian;}
    virtual double Hessian(unsigned int row, unsigned int col) const
    {
       assert(row < fGradient.size() && col < fGradient.size());
diff --git a/math/minuit2/inc/Minuit2/FumiliMaximumLikelihoodFCN.h b/math/minuit2/inc/Minuit2/FumiliMaximumLikelihoodFCN.h
index 59669e7d5d29b..975f0598ac80d 100644
--- a/math/minuit2/inc/Minuit2/FumiliMaximumLikelihoodFCN.h
+++ b/math/minuit2/inc/Minuit2/FumiliMaximumLikelihoodFCN.h
@@ -48,8 +48,6 @@ class FumiliMaximumLikelihoodFCN : public FumiliFCNBase {
 public:
    FumiliMaximumLikelihoodFCN() {}
 
-   ~FumiliMaximumLikelihoodFCN() override {}
-
    /**
 
    Sets the model function for the data (for example gaussian+linear for a peak)
@@ -84,7 +82,7 @@ class FumiliMaximumLikelihoodFCN : public FumiliFCNBase {
 
    */
 
-   virtual std::vector<double> Elements(std::span<const double> par) const = 0;
+   virtual std::vector<double> Elements(std::vector<double> const &par) const = 0;
 
    /**
 
@@ -126,7 +124,7 @@ class FumiliMaximumLikelihoodFCN : public FumiliFCNBase {
 
    */
 
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
 
       double sumoflogs = 0.0;
diff --git a/math/minuit2/inc/Minuit2/FumiliStandardChi2FCN.h b/math/minuit2/inc/Minuit2/FumiliStandardChi2FCN.h
index 39873f28beb49..a35bc518b9726 100644
--- a/math/minuit2/inc/Minuit2/FumiliStandardChi2FCN.h
+++ b/math/minuit2/inc/Minuit2/FumiliStandardChi2FCN.h
@@ -122,8 +122,6 @@ class FumiliStandardChi2FCN : public FumiliChi2FCN {
       }
    }
 
-   ~FumiliStandardChi2FCN() override {}
-
    /**
 
    Evaluates the model function for the different measurement points and
@@ -141,7 +139,7 @@ class FumiliStandardChi2FCN : public FumiliChi2FCN {
 
    */
 
-   std::vector<double> Elements(std::span<const double> par) const override;
+   std::vector<double> Elements(std::vector<double> const &par) const override;
 
    /**
 
@@ -176,7 +174,7 @@ class FumiliStandardChi2FCN : public FumiliChi2FCN {
 
    **/
 
-   void EvaluateAll(std::span<const double> par) override;
+   void EvaluateAll(std::vector<double> const &par) override;
 
 private:
    std::vector<double> fMeasurements;
diff --git a/math/minuit2/inc/Minuit2/FumiliStandardMaximumLikelihoodFCN.h b/math/minuit2/inc/Minuit2/FumiliStandardMaximumLikelihoodFCN.h
index 48f30aef9d569..2bb52d48dce30 100644
--- a/math/minuit2/inc/Minuit2/FumiliStandardMaximumLikelihoodFCN.h
+++ b/math/minuit2/inc/Minuit2/FumiliStandardMaximumLikelihoodFCN.h
@@ -74,8 +74,6 @@ class FumiliStandardMaximumLikelihoodFCN : public FumiliMaximumLikelihoodFCN {
       fPositions.assign(pos.begin(), pos.end());
    }
 
-   ~FumiliStandardMaximumLikelihoodFCN() override {}
-
    /**
 
    Evaluates the model function for the different measurement points and
@@ -88,7 +86,7 @@ class FumiliStandardMaximumLikelihoodFCN : public FumiliMaximumLikelihoodFCN {
 
    */
 
-   std::vector<double> Elements(std::span<const double> par) const override;
+   std::vector<double> Elements(std::vector<double> const &par) const override;
 
    /**
 
@@ -123,7 +121,7 @@ class FumiliStandardMaximumLikelihoodFCN : public FumiliMaximumLikelihoodFCN {
 
    **/
 
-   void EvaluateAll(std::span<const double> par) override;
+   void EvaluateAll(std::vector<double> const &par) override;
 
 private:
    std::vector<std::vector<double>> fPositions;
diff --git a/math/minuit2/inc/Minuit2/GenericFunction.h b/math/minuit2/inc/Minuit2/GenericFunction.h
index cc7959ae3d717..f7c65118c6262 100644
--- a/math/minuit2/inc/Minuit2/GenericFunction.h
+++ b/math/minuit2/inc/Minuit2/GenericFunction.h
@@ -49,7 +49,7 @@ class GenericFunction {
 
    */
 
-   virtual double operator()(std::span<const double> x) const = 0;
+   virtual double operator()(std::vector<double> const& x) const = 0;
 };
 
 } // namespace Minuit2
diff --git a/math/minuit2/inc/Minuit2/ParametricFunction.h b/math/minuit2/inc/Minuit2/ParametricFunction.h
index 2733b6adca09a..c7b0270772b66 100644
--- a/math/minuit2/inc/Minuit2/ParametricFunction.h
+++ b/math/minuit2/inc/Minuit2/ParametricFunction.h
@@ -65,8 +65,6 @@ class ParametricFunction : public FCNBase {
 
    ParametricFunction(int nparams) : par(nparams) {}
 
-   ~ParametricFunction() override {}
-
    /**
 
    Sets the parameters of the ParametricFunction.
@@ -75,7 +73,7 @@ class ParametricFunction : public FCNBase {
 
    */
 
-   virtual void SetParameters(std::span<const double> params) const
+   virtual void SetParameters(std::vector<double> const& params) const
    {
 
       assert(params.size() == par.size());
@@ -115,7 +113,7 @@ class ParametricFunction : public FCNBase {
 
    */
 
-   double operator()(std::span<const double> x) const override = 0;
+   double operator()(std::vector<double> const& x) const override = 0;
 
    /**
 
@@ -134,7 +132,7 @@ class ParametricFunction : public FCNBase {
 
    */
 
-   virtual double operator()(std::span<const double> x, std::span<const double> params) const
+   virtual double operator()(std::vector<double> const& x, std::vector<double> const& params) const
    {
       SetParameters(params);
       return operator()(x);
@@ -153,7 +151,7 @@ class ParametricFunction : public FCNBase {
 
    */
 
-   virtual std::vector<double> GetGradient(std::span<const double> x) const;
+   virtual std::vector<double> GetGradient(std::vector<double> const& x) const;
 
 protected:
    /**
diff --git a/math/minuit2/src/FumiliStandardChi2FCN.cxx b/math/minuit2/src/FumiliStandardChi2FCN.cxx
index ea0cce62b5d80..8cd7a339e863a 100644
--- a/math/minuit2/src/FumiliStandardChi2FCN.cxx
+++ b/math/minuit2/src/FumiliStandardChi2FCN.cxx
@@ -16,7 +16,7 @@ namespace ROOT {
 
 namespace Minuit2 {
 
-std::vector<double> FumiliStandardChi2FCN::Elements(std::span<const double> par) const
+std::vector<double> FumiliStandardChi2FCN::Elements(std::vector<double> const &par) const
 {
    // Calculate the f(i) contribution to the Chi2. Chi2 = Sum[f(i)**2]
 
@@ -54,7 +54,7 @@ int FumiliStandardChi2FCN::GetNumberOfMeasurements() const
    return fPositions.size();
 }
 
-void FumiliStandardChi2FCN::EvaluateAll(std::span<const double> par)
+void FumiliStandardChi2FCN::EvaluateAll(std::vector<double> const &par)
 {
    // Evaluate chi2 value, gradient and hessian all in a single
    // loop on the measurements
diff --git a/math/minuit2/src/FumiliStandardMaximumLikelihoodFCN.cxx b/math/minuit2/src/FumiliStandardMaximumLikelihoodFCN.cxx
index 3127c89aff33a..ba2ef78ef4245 100644
--- a/math/minuit2/src/FumiliStandardMaximumLikelihoodFCN.cxx
+++ b/math/minuit2/src/FumiliStandardMaximumLikelihoodFCN.cxx
@@ -17,7 +17,7 @@ namespace ROOT {
 
 namespace Minuit2 {
 
-std::vector<double> FumiliStandardMaximumLikelihoodFCN::Elements(std::span<const double> par) const
+std::vector<double> FumiliStandardMaximumLikelihoodFCN::Elements(std::vector<double> const &par) const
 {
 
    //   calculate likelihood element f(i) = pdf(x(i))
@@ -54,7 +54,7 @@ int FumiliStandardMaximumLikelihoodFCN::GetNumberOfMeasurements() const
    return fPositions.size();
 }
 
-void FumiliStandardMaximumLikelihoodFCN::EvaluateAll(std::span<const double> par)
+void FumiliStandardMaximumLikelihoodFCN::EvaluateAll(std::vector<double> const &par)
 {
    // Evaluate in one loop likelihood value, gradient and hessian
 
diff --git a/math/minuit2/src/ParametricFunction.cxx b/math/minuit2/src/ParametricFunction.cxx
index 9bd77a74b978e..c0549dcdb3085 100644
--- a/math/minuit2/src/ParametricFunction.cxx
+++ b/math/minuit2/src/ParametricFunction.cxx
@@ -19,9 +19,7 @@ namespace ROOT {
 
 namespace Minuit2 {
 
-//#include "Minuit2/MnPrint.h"
-
-std::vector<double> ParametricFunction::GetGradient(std::span<const double> x) const
+std::vector<double> ParametricFunction::GetGradient(std::vector<double> const &x) const
 {
    // calculate the numerical gradient (using Numerical2PGradientCalculator)
 
diff --git a/math/minuit2/test/MnSim/GaussFcn.cxx b/math/minuit2/test/MnSim/GaussFcn.cxx
index d31f1c85ca99d..64227039c27fc 100644
--- a/math/minuit2/test/MnSim/GaussFcn.cxx
+++ b/math/minuit2/test/MnSim/GaussFcn.cxx
@@ -16,7 +16,7 @@ namespace ROOT {
 
 namespace Minuit2 {
 
-double GaussFcn::operator()(std::span<const double> par) const
+double GaussFcn::operator()(std::vector<double> const &par) const
 {
 
    assert(par.size() == 3);
diff --git a/math/minuit2/test/MnSim/GaussFcn.h b/math/minuit2/test/MnSim/GaussFcn.h
index 8eb2f7463f3ec..eb43616529ac9 100644
--- a/math/minuit2/test/MnSim/GaussFcn.h
+++ b/math/minuit2/test/MnSim/GaussFcn.h
@@ -24,15 +24,12 @@ class GaussFcn : public FCNBase {
    GaussFcn(std::span<const double> meas, std::span<const double> pos, std::span<const double> mvar)
       : fMeasurements(meas.begin(), meas.end()),
         fPositions(pos.begin(), pos.end()),
-        fMVariances(mvar.begin(), mvar.end()),
-        fErrorDef(1.)
+        fMVariances(mvar.begin(), mvar.end())
    {
    }
 
-   ~GaussFcn() override {}
-
    double Up() const override { return fErrorDef; }
-   double operator()(std::span<const double>) const override;
+   double operator()(std::vector<double> const &) const override;
 
    std::vector<double> Measurements() const { return fMeasurements; }
    std::vector<double> Positions() const { return fPositions; }
@@ -44,7 +41,7 @@ class GaussFcn : public FCNBase {
    std::vector<double> fMeasurements;
    std::vector<double> fPositions;
    std::vector<double> fMVariances;
-   double fErrorDef;
+   double fErrorDef = 1.;
 };
 
 } // namespace Minuit2
diff --git a/math/minuit2/test/MnSim/GaussFcn2.cxx b/math/minuit2/test/MnSim/GaussFcn2.cxx
index 9ed7837eda2d7..56bf16a903af5 100644
--- a/math/minuit2/test/MnSim/GaussFcn2.cxx
+++ b/math/minuit2/test/MnSim/GaussFcn2.cxx
@@ -16,7 +16,7 @@ namespace ROOT {
 
 namespace Minuit2 {
 
-double GaussFcn2::operator()(std::span<const double> par) const
+double GaussFcn2::operator()(std::vector<double> const &par) const
 {
 
    assert(par.size() == 6);
diff --git a/math/minuit2/test/MnSim/GaussFcn2.h b/math/minuit2/test/MnSim/GaussFcn2.h
index f654d68247047..0b9739be23fbc 100644
--- a/math/minuit2/test/MnSim/GaussFcn2.h
+++ b/math/minuit2/test/MnSim/GaussFcn2.h
@@ -29,12 +29,11 @@ class GaussFcn2 : public FCNBase {
    {
       Init();
    }
-   ~GaussFcn2() override {}
 
    virtual void Init();
 
    double Up() const override { return 1.; }
-   double operator()(std::span<const double>) const override;
+   double operator()(std::vector<double> const &) const override;
    double ErrorDef() const override { return Up(); }
 
    std::vector<double> Measurements() const { return fMeasurements; }
diff --git a/math/minuit2/test/MnSim/GaussianModelFunction.h b/math/minuit2/test/MnSim/GaussianModelFunction.h
index d619c25212823..88f924a28cfec 100644
--- a/math/minuit2/test/MnSim/GaussianModelFunction.h
+++ b/math/minuit2/test/MnSim/GaussianModelFunction.h
@@ -82,8 +82,6 @@ class GaussianModelFunction : public ParametricFunction {
 
    GaussianModelFunction(std::span<const double> params) : ParametricFunction(params) { assert(params.size() == 1); }
 
-   ~GaussianModelFunction() override {}
-
    /**
 
    Calculates the Gaussian as a function of the given input.
@@ -97,7 +95,7 @@ class GaussianModelFunction : public ParametricFunction {
 
    */
 
-   double operator()(std::span<const double> x) const override
+   double operator()(std::vector<double> const &x) const override
    {
 
       assert(x.size() == 3);
@@ -128,7 +126,7 @@ class GaussianModelFunction : public ParametricFunction {
 
    */
 
-   double operator()(std::span<const double> x, std::span<const double> param) const override
+   double operator()(std::vector<double> const &x, std::vector<double> const &param) const override
    {
 
       constexpr double two_pi = 2 * 3.14159265358979323846; // M_PI is not standard
@@ -148,7 +146,7 @@ class GaussianModelFunction : public ParametricFunction {
 
    double Up() const override { return 1.0; }
 
-   std::vector<double> GetGradient(std::span<const double> x) const override
+   std::vector<double> GetGradient(std::vector<double> const &x) const override
    {
 
       std::span<const double> param = GetParameters();
diff --git a/math/minuit2/test/MnSim/ParallelTest.cxx b/math/minuit2/test/MnSim/ParallelTest.cxx
index 7646bb87f5f7c..2165284ae343a 100644
--- a/math/minuit2/test/MnSim/ParallelTest.cxx
+++ b/math/minuit2/test/MnSim/ParallelTest.cxx
@@ -59,7 +59,7 @@ struct LogLikeFCN : public FCNBase {
 
    LogLikeFCN(const Data &data) : fData(data) {}
 
-   double operator()(std::span<const double> p) const override
+   double operator()(std::vector<double> const &p) const override
    {
       double logl = 0;
       int ndata = fData.size();
diff --git a/math/minuit2/test/MnSim/PaulTest4.cxx b/math/minuit2/test/MnSim/PaulTest4.cxx
index d177296a5f9be..0f78cd0805148 100644
--- a/math/minuit2/test/MnSim/PaulTest4.cxx
+++ b/math/minuit2/test/MnSim/PaulTest4.cxx
@@ -53,9 +53,7 @@ class PowerLawChi2FCN : public FCNBase {
    {
    }
 
-   ~PowerLawChi2FCN() override {}
-
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
       assert(par.size() == 2);
       PowerLawFunc pl(par[0], par[1]);
@@ -84,9 +82,7 @@ class PowerLawLogLikeFCN : public FCNBase {
    {
    }
 
-   ~PowerLawLogLikeFCN() override {}
-
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
       assert(par.size() == 2);
       PowerLawFunc pl(par[0], par[1]);
diff --git a/math/minuit2/test/MnSim/ReneTest.cxx b/math/minuit2/test/MnSim/ReneTest.cxx
index e06a719075dba..3bc381346f235 100644
--- a/math/minuit2/test/MnSim/ReneTest.cxx
+++ b/math/minuit2/test/MnSim/ReneTest.cxx
@@ -28,9 +28,7 @@ class ReneFcn : public FCNBase {
 public:
    ReneFcn(std::span<const double> meas) : fMeasurements(meas.begin(), meas.end()) {}
 
-   ~ReneFcn() override {}
-
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
       double a = par[2];
       double b = par[1];
diff --git a/math/minuit2/test/MnTutorial/Quad12F.h b/math/minuit2/test/MnTutorial/Quad12F.h
index e43c4143a54fd..217209695e804 100644
--- a/math/minuit2/test/MnTutorial/Quad12F.h
+++ b/math/minuit2/test/MnTutorial/Quad12F.h
@@ -18,7 +18,7 @@ namespace Minuit2 {
 class Quad12F : public FCNBase {
 
 public:
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
 
       double x = par[0];
diff --git a/math/minuit2/test/MnTutorial/Quad1F.h b/math/minuit2/test/MnTutorial/Quad1F.h
index 4ecfb045908a7..1790130c4376b 100644
--- a/math/minuit2/test/MnTutorial/Quad1F.h
+++ b/math/minuit2/test/MnTutorial/Quad1F.h
@@ -18,24 +18,20 @@ namespace Minuit2 {
 class Quad1F : public FCNGradientBase {
 
 public:
-   Quad1F() : fErrorDef(1.) {}
-
-   ~Quad1F() override {}
-
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
 
       double x = par[0];
 
-      return (x * x);
+      return x * x;
    }
 
-   std::vector<double> Gradient(std::span<const double> par) const override
+   std::vector<double> Gradient(std::vector<double> const &par) const override
    {
 
       double x = par[0];
 
-      return (std::vector<double>(1, 2. * x));
+      return std::vector<double>(1, 2. * x);
    }
 
    void SetErrorDef(double up) override { fErrorDef = up; }
@@ -45,7 +41,7 @@ class Quad1F : public FCNGradientBase {
    const FCNBase *Base() const { return this; }
 
 private:
-   double fErrorDef;
+   double fErrorDef = 1.;
 };
 
 } // namespace Minuit2
diff --git a/math/minuit2/test/MnTutorial/Quad4F.h b/math/minuit2/test/MnTutorial/Quad4F.h
index 38d1f68dfe850..d050e2b48d1f9 100644
--- a/math/minuit2/test/MnTutorial/Quad4F.h
+++ b/math/minuit2/test/MnTutorial/Quad4F.h
@@ -18,11 +18,7 @@ namespace Minuit2 {
 class Quad4F : public FCNBase {
 
 public:
-   Quad4F() {}
-
-   ~Quad4F() override {}
-
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
 
       double x = par[0];
@@ -42,11 +38,7 @@ class Quad4F : public FCNBase {
 class Quad4FGrad : public FCNGradientBase {
 
 public:
-   Quad4FGrad() {}
-
-   ~Quad4FGrad() override {}
-
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
 
       double x = par[0];
@@ -57,7 +49,7 @@ class Quad4FGrad : public FCNGradientBase {
       return ((1. / 70.) * (21 * x * x + 20 * y * y + 19 * z * z - 14 * x * z - 20 * y * z) + w * w);
    }
 
-   std::vector<double> Gradient(std::span<const double> par) const override
+   std::vector<double> Gradient(std::vector<double> const &par) const override
    {
 
       double x = par[0];
diff --git a/math/minuit2/test/MnTutorial/Quad8F.h b/math/minuit2/test/MnTutorial/Quad8F.h
index 7ef9e8680b00b..974d443c8e2a9 100644
--- a/math/minuit2/test/MnTutorial/Quad8F.h
+++ b/math/minuit2/test/MnTutorial/Quad8F.h
@@ -18,7 +18,7 @@ namespace Minuit2 {
 class Quad8F : public FCNBase {
 
 public:
-   double operator()(std::span<const double> par) const override
+   double operator()(std::vector<double> const &par) const override
    {
 
       double x = par[0];