全局函数与非静态成员函数指针设计

Question

我有一个连续的优化算法，它以Oracle函数作为模板参数。特定的优化类定义为：

template<class Space, class Solution, class Oracle>
class ConjugateGradient : public ContinuousOptimizerInterface<Space, Solution, Oracle> {
public:

    // Returns the optimal solution found for a given search space
    virtual const Solution& search(const Space& space, Oracle f);
};

作为搜索实现的一部分，我调用oracle函数：

template<class Space, class Solution, class Oracle> 
inline const Solution& ConjugateGradient<Space, Solution, Oracle>::search(const Space& space, Oracle f) {
     // ...
     // get function value and gradient at X
     double fx;
     Solution dfx;
     tie(fx, dfx) = f(X);
     // ..
}

一个使用全局二次函数的简单示例：

typedef tuple<double, VectorXd> (*oracle_f)(const VectorXd&);
static tuple<double, VectorXd> f(const VectorXd& X) {
     double f = pow((4.0-X(0)), 2) + 10.0;
     VectorXd df = 2.0*X - VectorXd::Ones(X.rows())*8.0;
     return make_tuple(f, df);
}

// ...
ConjugateGradient<TestSpace, VectorXd, oracle_f> optimizer;
VectorXd optimal = optimizer.search(TestSpace(), f);

这是可行的，但现在我需要能够将类的非静态成员函数作为Oracle函数传递给ConjugateGradient算法。如何将oracle函数的ConjugateGradient模板声明和实现更改为全局函数或非静态成员函数？另一种方法是创建一个全局包装函数，并使用varargs将参数传递给包装函数，但这是丑陋的，而且类型不安全。

UPDATE：这个示例使用下面答案的绑定思想，但是使用boost::bind而不是std::bind，因为我不在C++11上，而std::bind仅适用于C++11。

#include <Eigen/Dense>
#include <boost/tuple/tuple.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>

using namespace Eigen;
using namespace boost;

// the class is parameterized with the appropriate minimizer strategy
enum Minimizer { kNormalEquations, kConjugateGradient };

template <Minimizer M = kNormalEquations>
class SomeANN : AnnInterface {
private:
    // ...
public:
    // define the oracle function type and member function
    typedef tuple<double, VectorXd> (SomeANN::*oracle_f)(const VectorXd&);
    tuple<double, VectorXd> f(const VectorXd& theta);
};

template <>
inline tuple<double, VectorXd> SomeANN<kConjugateGradient>::f(const VectorXd& theta) {
    double f = 0.0;
    VectorXd df;
    return make_tuple(f, df);
}

// ridge solver using conjugate gradient
template <>
inline void SomeANN<kConjugateGradient>::ridge_solve(const VectorXd& Y) {
    ConjugateGradient<BeginSpace, VectorXd, SomeANN::oracle_f> optimizer;
    // ...
    optimizer.search(BeginSpace(Y.rows()), boost::bind(&SomeANN::f, this, _1));
}

然后我得到了错误：

some_ann.h:163:84: error: no matching function for call to 'ConjugateGradient<BeginSpace, Eigen::Matrix<double, -0x00000000000000001, 1>, boost::tuples::tuple<double, Eigen::Matrix<double, -0x00000000000000001, 1> > (SomeANN<(Minimizer)1u>::*)(const Eigen::Matrix<double, -0x00000000000000001, 1>&)>::search(BeginSpace, boost::_bi::bind_t<boost::tuples::tuple<double,  Eigen::Matrix<double, -0x00000000000000001, 1> >, boost::_mfi::mf1<boost::tuples::tuple<double, Eigen::Matrix<double, -0x00000000000000001, 1> >, SomeANN<(Minimizer)1u>, const Eigen::Matrix<double, -0x00000000000000001, 1>&>,  boost::_bi::list2<boost::_bi::value<SomeANN<(Minimizer)1u>*>, boost::arg<1> > >)'
conjugate_gradient.h:67:2: error: must use '.*' or '->*' to call pointer-to-member function in 'f (...)', e.g. '(... ->* f) (...)'
conjugate_gradient.h:84:3: error: must use '.*' or '->*' to call pointer-to-member function in 'f (...)', e.g. '(... ->* f) (...)'
conjugate_gradient.h:111:5: error: must use '.*' or '->*' to call pointer-to-member function in 'f (...)', e.g. '(... ->* f) (...)'
conjugate_gradient.h:153:4: error: must use '.*' or '->*' to call pointer-to-member function in 'f (...)', e.g. '(... ->* f) (...)'
make[2]: *** [CMakeFiles/some_ann_library.dir/main/cpp/some_ann.cc.o] Error 1
make[1]: *** [CMakeFiles/some_ann_library.dir/all] Error 2
make: *** [all] Error 2

Answer 1

不要更改搜索，传递一个绑定表达式：

auto& solution = cg.search(space, std::bind(
    &MyType::member_function, &myInstance, std::placeholders::_1));