将带有 2 个参数的类成员函数传递给 boost bind

Question

Boost 绑定无法推导/替换模板参数：

class A
{
  ...
  bool f(boost::shared_ptr<boost::array<unsigned char, 128>>&, size_t);
  ...
};

A myA;
boost::bind(&A::f, &myA, _1, _2);

我使用的是boost 1.63。编译错误信息指向

boost_1_63_0/include/boost/bind/bind.hpp

:

template<class R, class F, class A> R operator()(type<R>, F & f, A & a, long)
{    
    return unwrapper<F>::unwrap(f, 0)(a[base_type::a1_], a[base_type::a2_], a[base_type::a3_]);
}    

Answer 1

您应该使代码独立：

https://godbolt.org/z/WefoWa9df

#include <boost/array.hpp>
#include <boost/bind/bind.hpp>
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>
#include <iostream>

using b128 = boost::array<unsigned char, 128>;

class A {
  public:
    bool f(boost::shared_ptr<b128>& d, size_t) {
        std::cout << "called with '" << d->at(0) << "'" << std::endl;
        return false;
    }
};

int main() {
    A myA;

    using namespace boost::placeholders;
    auto f = boost::bind(&A::f, &myA, _1, _2);

    auto x = boost::make_shared<b128>();
    x->fill('\x55'); // 'U'

    f(x, 0);
}

打印

called with 'U'

猜猜

我最好的猜测是你正在混合

boost::placeholders

和

std::placeholders

:

// using namespace boost::placeholders;
using namespace std::placeholders;
auto f = boost::bind(&A::f, &myA, _1, _2);

请参阅 https://godbolt.org/z/ecK8aYjv9

/opt/compiler-explorer/libs/boost_1_64_0/boost/bind/bind.hpp:388:42: error: no match for call to '(boost::_mfi::mf2<bool, A, boost::shared_ptr<boost::array<unsigned char, 128> >&, long unsigned int>) (A*&, std::_Placeholder<1>&, std::_Placeholder<2>&)'
  388 |         return unwrapper<F>::unwrap(f, 0)(a[base_type::a1_], a[base_type::a2_], a[base_type::a3_]);
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

奖金

现代化，根本不使用Boost：

#include <functional>
#include <iostream>
#include <memory>

using b128 = std::array<unsigned char, 128>;

class A {
  public:
    bool f(std::shared_ptr<b128> const& d, size_t) const {
        std::cout << "called with '" << d->at(0) << "'" << std::endl;
        return false;
    }
};

int main() {
    A myA;

    using namespace std::placeholders;
    auto f = bind(&A::f, &myA, _1, _2);

    auto x = std::make_shared<b128>();
    x->fill('\x32'); // '2'

    f(x, 0);
}