c++ - 全局函数和非静态成员函数指针设计

Question

我有一个持续优化算法，它采用 Oracle 函数作为模板参数。具体优化等级定义为:

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，我调用了 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);
     // ..
}

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);

这工作正常，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。我需要如何将 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可变参数将参数传递给包装函数，但这很丑陋而且不是类型安全的。

更新:此示例使用下面答案的绑定(bind)思想，但使用 boost::bind 而不是 std::bind，因为我不使用 C++11 和 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));
}

然后我收到错误:

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，传递绑定(bind)表达式:

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