tr1 :: mem_fn和带有默认参数的成员 [英] tr1::mem_fn and members with default arguments

问题描述

我有一个类成员函数，它接受一个默认参数。

I have class with a member function that takes a default argument.

struct Class
{
    void member(int n = 0)
    {}
};

通过std :: tr1 :: mem_fn我可以调用它：

By means of std::tr1::mem_fn I can invoke it:

Class object;

std::tr1::mem_fn(&Class::member)(object,10);

也就是说，如果我想调用对象上的使用默认参数，语法是什么？

That said, if I want to invoke the callable member on the object with the default argument, what's the correct syntax?

std::tr1::mem_fn(&Class::member)(object); // This does not work

g ++抱怨出现以下错误：

g++ complains with the following error:

test.cc:17: error: no match for call to ‘(std::tr1::_Mem_fn<void (Class::*)(int)>) (Class&)’
/usr/include/c++/4.3/tr1_impl/functional:551: note: candidates are: _Res std::tr1::_Mem_fn<_Res (_Class::*)(_ArgTypes ...)>::operator()(_Class&, _ArgTypes ...) const [with _Res = void, _Class = Class, _ArgTypes = int]
/usr/include/c++/4.3/tr1_impl/functional:556: note:                 _Res std::tr1::_Mem_fn<_Res (_Class::*)(_ArgTypes ...)>::operator()(_Class*, _ArgTypes ...) const [with _Res = void, _Class = Class, _ArgTypes = int]

由接受不同参数的成员重载...

Still, the I have the same problem when Class::member is overloaded by members that takes different arguments...

推荐答案

默认函数在调用时绑定，进入任何种类的包装器，因为它们的实现方式。当你通过& Class :: member ， mem_fn ：*）（int），并且看不到默认参数。使用 tr1 :: bind ，可以明确绑定默认参数： std :: tr1 :: bind（& Class :: member，0） 或者你可以使用它，因为你会 mem_fn ，但是你不能在一个对象。

Default functions are bound at call time, but can't be bound into any sort of wrapper implicitly, because of the way they are implemented. When you pass &Class::member, mem_fn only sees a void (Class::*)(int), and can't see the default argument. Using tr1::bind, you can bind the default argument explictly: std::tr1::bind(&Class::member, 0) or you can use it as you would mem_fn, but you can't do both in one object. You would have to write your own wrapper class for that.

对于重载，你必须明确指定 mem_fn

As for overloads, you will have to explicitly specify the template arguments for mem_fn so the right function pointer is picked as in mem_fn<void(int)>(&Class::member).

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