是否可能创建函数本地闭包前C ++ 11？ [英] Is it possible to create function-local closures pre-C++11?

问题描述

使用C ++ 11，我们可以获得lambdas，并且可以在实际需要的地方创建函数/函子/闭包，而不是它们真正属于的地方。

在C ++ 98/03中，一个很好的函数局部函数/闭包的方法如下：

With C++11, we get lambdas, and the possibility to create functions/functors/closures on-the-fly where we actually need them, not somewhere where they don't really belong.
In C++98/03, a nice way to make function-local functors/closures would've been the following:

struct{
  void operator()(int& item){ ++item; }
}foo_functor;
some_templated_func(some_args, foo_functor);

< 。我的火车虽然然后采取以下方式：

Sadly, you can't use local types for templates (Visual Studio allows this with language extensions enabled). My train of though then went the following way:

struct X{
  static void functor(int& item){ ++item; }
};
some_templated_func(some_args, &X::functor);

显然的问题是，你不能保存任何状态，因为本地结构/

我接下来想解决这个问题是使用 std :: bind1st 和 std的混合： ：mem_fun 和非静态方法&变量，但不幸的是 std :: mem_fun 不知何故阻塞了 std :: mem_fn（& X :: functor）这也可能是因为本地struct /类不能在模板中使用：

The obvious problem being, that you can't save any state, since local structs/classes can't have static members.
My next thought to solving that problem was using a mix of std::bind1st and std::mem_fun and non-static methods & variables, but unfortunately std::mem_fun somehow chokes with std::mem_fn(&X::functor), which again might be because local struct/classes can't be used in templates:

// wanted, not working solution
struct X{
  int n_;
  X(int n) : n_(n) {}
  void functor(int& item) const { item += n_; }
};
X x(5);
some_templated_func(some_args,std::bind1st(std::mem_fun(&X::functor),&x));

在VC9& VC10（具有 / Za ，禁用的语言扩展），具有以下错误

Fails under VC9 & VC10 (with /Za, disabled language extensions) with the following error

error C2893: Failed to specialize function template 'std::const_mem_fun1_t<_Result,_Ty,_Arg> std::mem_fun(_Result (_Ty::* )(_Arg) const)'
With the following template arguments:
'void'
'main::X'
'int &'

或在gcc 4.3.4下出现此错误

Or under gcc 4.3.4 with this error

error: no matching function for call to ‘mem_fun(void (main()::X::*)(int&))’

有趣的是，VC9 / VC10还是扼杀了上面的例子，即使语言扩展启用：

Funnily enough, VC9 / VC10 still chokes on the above example, even with language extensions enables:

error C2535: 'void std::binder1st<_Fn2>::operator ()(int &) const' : member function already defined or declared

那么，标题中标注的功能是什么？或者在最后一个例子中，我如何使用 std :: bind1st 或 std :: mem_fun 时犯错误？

So, is the functionality stated in the title somehow, anyhow achievable? Or am I making a mistake in the last example in how I use std::bind1st or std::mem_fun?

推荐答案

bind1st 仅适用于二进制函数，限制。 mem_fn 仅适用于非静态成员函数;对于你的应用程序，你会想要 ptr_fun 。

bind1st only works for binary functions, and in general it's very restricted. mem_fn works with non-static member functions only; for your application you would want ptr_fun.

真正的C ++ 03最好的工作是Boost Bind，或者我将在这里演示 tr1 :: bind 这在我看来更便携。

Really the best tool for the job in C++03 is Boost Bind, or I'll demonstrate here with tr1::bind which is (in my opinion) more portable.

#include <tr1/functional>
#include <iostream>
#include <algorithm>

using namespace std::tr1::placeholders;

int nums[] = { 1, 2, 4, 5, 6, 8 };

int main() {
    struct is_multiple {
        static bool fn( int mod, int num ) { return num % mod == 0; }
    };

    int *n = std::find_if( nums, nums + sizeof nums/sizeof*nums,
                        std::tr1::bind( is_multiple::fn, 3, _1 ) );

    std::cout << n - nums << '\n';
}

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