为什么boost::bind与正向声明不兼容

Question

boost::bind无法绑定通过正向声明声明的参数。

有人能解释一下原因吗？这是一个助推错误吗？

样本代码：

#include "boost/function.hpp" 
#include "boost/bind.hpp" 
#include <vector>
#include <iostream>
class B;
class A
{
public:
    A() {}
    void func1(int i)                      { std::cout << __PRETTY_FUNCTION__ << "(" << i << ")n"; } 
    void func2(const std::string& s)       { std::cout << __PRETTY_FUNCTION__ << "(" << s << ")n"; }
    void func3(const B& b)       { std::cout << __PRETTY_FUNCTION__ << "(" << "unknown" << ")n"; } 
    static void dispatch(      std::vector<A>& vect, boost::function<void(A      &)> const& func)
    {
        for ( std::vector<A>::iterator iter = vect.begin();
              iter != vect.end();
              ++iter )
        {
            func(*iter);
        }
    }
};
int main()
{
    std::vector<A> vect(3);
    A::dispatch(vect, boost::bind(&A::func1, _1, 3));
    A::dispatch(vect, boost::bind(&A::func2, _1, "hello"));
    const B* b = NULL;
    A a;
    a.func3( *b ); // compiles and works!!
    A::dispatch(vect, boost::bind(&A::func3, _1, *b)); // does not compile!!
}

报告的错误为：

main.cpp:37:52: error: invalid use of incomplete type 'class B'
     A::dispatch(vect, boost::bind(&A::func3, _1, *b)); // does not compile

现场演示：http://coliru.stacked-crooked.com/a/5f9437627fdf3c53

Answer 1

这是因为bind按值存储绑定参数。

如果您不希望这样，请将它们包装在引用包装器中：boost::ref(x)或boost::cref(x)。

A::dispatch(vect, boost::bind(&A::func3, _1, boost::cref(*b))); 

编译：

在Coliru上直播

#include "boost/function.hpp" 
#include "boost/bind.hpp" 
#include <vector>
#include <iostream>
class B;
class A
{
public:
    A() {}
    void func1(int i)                      { std::cout << __PRETTY_FUNCTION__ << "(" << i << ")n"; } 
    void func2(const std::string& s)       { std::cout << __PRETTY_FUNCTION__ << "(" << s << ")n"; }
    void func3(const B& b)       { std::cout << __PRETTY_FUNCTION__ << "(" << "unknown" << ")n"; } 
    static void dispatch(      std::vector<A>& vect, boost::function<void(A      &)> const& func)
    {
        for ( std::vector<A>::iterator iter = vect.begin();
              iter != vect.end();
              ++iter )
        {
            func(*iter);
        }
    }
};
int main()
{
    std::vector<A> vect(3);
    A::dispatch(vect, boost::bind(&A::func1, _1, 3));
    A::dispatch(vect, boost::bind(&A::func2, _1, "hello"));
    const B* b = NULL;
    A a;
    a.func3( *b ); // compiles and works!!
    A::dispatch(vect, boost::bind(&A::func3, _1, boost::cref(*b))); 
}