在gcc 4.7中使用std :: bind编译错误 [英] Compile errors using std::bind in gcc 4.7

问题描述

在代码的不同位置使用 std :: bind 时遇到很多麻烦。有时它会起作用，有时却不会，所以我认为我做的事情根本上是错误的。

I am having a lot of trouble using std::bind in various places of my code. Sometimes it works, sometimes it doesn't, so I assume I am doing something fundamentally wrong.

据我了解，以下 std :: bind 的基本用法应该可以正常工作：

As far as I understand, the following basic use of std::bind should work fine:

#include <functional>

int foo(int a, int b){ return a+b; }

int main(){

    using namespace std::placeholders;

    // works
    auto bar_auto=std::bind(foo,1,_2);

    // compile error
    std::function<int(int)> bar_fun=std::bind(foo,1,_2);

    int quux=1;
    // compile error
    std::function<int(int)> bar_fun_lvalue=std::bind(foo,quux,_2);

}

肯定是 bar_auto 是 std :: function< int（int）> （ foo 的类型，带有1 int 自变量绑定），为什么 bar_fun 无法编译？我加入了 bar_fun_lvalue ，因为一些谷歌搜索显示我右值曾经是有问题的。但这并不能解决任何问题。

Surely the type of bar_auto is std::function<int(int)> (type of foo with 1 int argument bound), so why does bar_fun fail to compile? I included bar_fun_lvalue because some googling showed me that rvalues used to be problematic. but this didn't fix anything.

它类似于此错误，但是它太旧了，我不希望它具有相关性。

It resembles this bug, but that's so old I don't expect it to be relevant.

gcc的输出不是特别有启发性：

gcc's output is not particularly enlightening:

bindnew包含的文件中。 cpp：1：0：
/usr/include/c++/4.7/functional：在'static _Res
std :: _ Function_handler< _Res（_ArgTypes ...），_Functor> :: __ M_invoke（ const
std :: __ Any_data& ;, _ArgTypes ...）[with _Res = int; _Functor =
std :: _ Bind））（int，int）>; _ArgTypes =
{int}]'：/usr/include/c++/4.7/functional:2298:6：需要
'std :: function< _Res（_ArgTypes ...）> :: function （_Functor，类型名
std :: enable_if<（！std :: is_integral< _Functor> :: value），
std :: function< _Res（_ArgTypes ...）> :: __ Useless> ::类型）[with _Functor =
std :: _ Bind））（int，int）>; _Res = int;
_ArgTypes = {int};类型名称std :: enable_if<（！std :: is_integral< _Functor> :: value），std :: function< _Res（_ArgTypes
...）> :: __ Useless> :: type = std :: function： ：_Useless]'
bindnew.cpp：15：52：从这里需要
/usr/include/c++/4.7/functional:1912:40：错误：调用
时没有匹配项' （std :: _ Bind））（int，int）>）（int）'
/usr/include/c++/4.7/functional:1140:11：注意：候选对象是：
/ usr / include /c++/4.7/functional:1211:2：注意：模板_Result std :: _ Bind< _Functor（_Bound_args ...）> :: operator（）（_ Args&&...）[with _Args = {_Args .. 。}; _Result =
_Result; _Functor = int（*）（int，int）; _Bound_args = {int，std :: _ Placeholder< 2>}] /usr/include/c++/4.7/functional:1211:2：注意：

模板参数推导/替换失败：
/ usr / include / c ++ / 4.7 / functional：1206：35：错误：无法在传递
的参数中将
'std :: _ No_tuple_element'转换为'int'/usr/include/c++/4.7/functional： 1225：2：注意：模板_Result std :: _ Bind< _Functor（_Bound_args ...）> :: operator（）（_ Args& ...）const [with _Args = {_Args ...};
_Result = _Result; _Functor = int（*）（int，int）; _Bound_args = {int，std :: _ Placeholder< 2>}] /usr/include/c++/4.7/functional:1225:2：注意：

模板参数推导/替换失败：
/ usr / include / c ++ / 4.7 / functional：1219：35：错误：无法在传递
的参数中将
'std :: _ No_tuple_element'转换为'int'/usr/include/c++/4.7/functional： 1239：2：注意：模板_Result std :: _ Bind< _Functor（_Bound_args ...）> :: operator（）（_ Args& ...）volatile [with _Args = {_Args ...};
_Result = _Result; _Functor = int（*）（int，int）; _Bound_args = {int，std :: _ Placeholder< 2>}] /usr/include/c++/4.7/functional:1239:2：注意：

模板参数推导/替换失败：
/ usr / include / c ++ / 4.7 / functional：1233：35：错误：无法在传递
的参数中将
'std :: _ No_tuple_element'转换为'int'/usr/include/c++/4.7/functional： 1253：2：注意：模板_Result std :: _ Bind< _Functor（_Bound_args ...）> :: operator（）（_ Args& ...）const volatile [with _Args = {_Args
... }; _Result = _Result; _Functor = int（*）（int，int）; _Bound_args =
{int，std :: _ Placeholder< 2>}] /usr/include/c++/4.7/functional:1253:2：
注意：模板参数推导/替换失败：
/usr/include/c++/4.7/functional:1247:35：错误：无法在参数传递中将
'std :: __ No_tuple_element'转换为'int'

In file included from bindnew.cpp:1:0: /usr/include/c++/4.7/functional: In instantiation of ‘static _Res std::_Function_handler<_Res(_ArgTypes ...), _Functor>::_M_invoke(const std::_Any_data&, _ArgTypes ...) [with _Res = int; _Functor = std::_Bind))(int, int)>; _ArgTypes = {int}]’: /usr/include/c++/4.7/functional:2298:6: required from ‘std::function<_Res(_ArgTypes ...)>::function(_Functor, typename std::enable_if<(! std::is_integral<_Functor>::value), std::function<_Res(_ArgTypes ...)>::_Useless>::type) [with _Functor = std::_Bind))(int, int)>; _Res = int; _ArgTypes = {int}; typename std::enable_if<(! std::is_integral<_Functor>::value), std::function<_Res(_ArgTypes ...)>::_Useless>::type = std::function::_Useless]’ bindnew.cpp:15:52: required from here /usr/include/c++/4.7/functional:1912:40: error: no match for call to ‘(std::_Bind))(int, int)>) (int)’ /usr/include/c++/4.7/functional:1140:11: note: candidates are: /usr/include/c++/4.7/functional:1211:2: note: template _Result std::_Bind<_Functor(_Bound_args ...)>::operator()(_Args&& ...) [with _Args = {_Args ...}; _Result = _Result; _Functor = int (*)(int, int); _Bound_args = {int, std::_Placeholder<2>}] /usr/include/c++/4.7/functional:1211:2: note:
template argument deduction/substitution failed: /usr/include/c++/4.7/functional:1206:35: error: cannot convert ‘std::_No_tuple_element’ to ‘int’ in argument passing /usr/include/c++/4.7/functional:1225:2: note: template _Result std::_Bind<_Functor(_Bound_args ...)>::operator()(_Args&& ...) const [with _Args = {_Args ...}; _Result = _Result; _Functor = int (*)(int, int); _Bound_args = {int, std::_Placeholder<2>}] /usr/include/c++/4.7/functional:1225:2: note:
template argument deduction/substitution failed: /usr/include/c++/4.7/functional:1219:35: error: cannot convert ‘std::_No_tuple_element’ to ‘int’ in argument passing /usr/include/c++/4.7/functional:1239:2: note: template _Result std::_Bind<_Functor(_Bound_args ...)>::operator()(_Args&& ...) volatile [with _Args = {_Args ...}; _Result = _Result; _Functor = int (*)(int, int); _Bound_args = {int, std::_Placeholder<2>}] /usr/include/c++/4.7/functional:1239:2: note:
template argument deduction/substitution failed: /usr/include/c++/4.7/functional:1233:35: error: cannot convert ‘std::_No_tuple_element’ to ‘int’ in argument passing /usr/include/c++/4.7/functional:1253:2: note: template _Result std::_Bind<_Functor(_Bound_args ...)>::operator()(_Args&& ...) const volatile [with _Args = {_Args ...}; _Result = _Result; _Functor = int (*)(int, int); _Bound_args = {int, std::_Placeholder<2>}] /usr/include/c++/4.7/functional:1253:2: note: template argument deduction/substitution failed: /usr/include/c++/4.7/functional:1247:35: error: cannot convert ‘std::_No_tuple_element’ to ‘int’ in argument passing

推荐答案

占位符位置对象（例如，当您使用 _2 时）不是参数的位置在您调用的函数中，但在创建的可调用对象中为参数的占位符。相反，总是从 _1 开始并增加。

The placeholder position objects (e.g. when you use _2) is not the position of the argument in the function you call, but a placeholder for the argument in the created callable object. Instead always start with _1 and increase.

所以：

auto bar_auto=std::bind(foo,1,_1);

等。

这意味着您可以通过简单地操作

This means you can switch the arguments in the object created by std::bind by simply doing like

auto bar_auto=std::bind(foo,_2,_1);

调用 bar_auto 对象，第一个参数将是 foo 的第二个参数，调用中的第二个参数将是 foo 。

When you "call" the bar_auto object, the first argument will be the second argument to foo, and the second argument in the call will be the first argument to foo.

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