主页 / coding / 问题 / 79117948
Accepted
Oodini
Asked: 2024-10-23 20:41:17 +0800 CST

使用 std::for_each 和 std::invoke 在容器中调用 std::functions

  • 772

我想知道是否有可能只用一条指令而不是 lambda 来调用容器中的回调（这是一种练习）。

我尝试了std::invoke。这是我的代码：

#include <algorithm>
#include <functional>
#include <iostream>

int main()
{
    std::function<void(void)> foo = []() { std::cout << "Foo !"; };
    std::function<void(void)> bar = []() { std::cout << "Bar !"; };

    const std::vector<std::function<void(void)>> callbacks = { foo, bar };
    
    std::for_each(callbacks.cbegin(), callbacks.cend(), std::invoke);

    return 0;
}

但我在编译时遇到很多错误：

main.cpp: In function ‘int main()’:
main.cpp:12:22: error: no matching function for call to ‘for_each(std::vector >::const_iterator, std::vector >::const_iterator, )’
   12 |         std::for_each(callbacks.cbegin(), callbacks.cend(), std::invoke);
      |         ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/11/algorithm:62,
                 from main.cpp:1:
/usr/include/c++/11/bits/stl_algo.h:3814:5: note: candidate: ‘template _Funct std::for_each(_IIter, _IIter, _Funct)’
 3814 |     for_each(_InputIterator __first, _InputIterator __last, _Function __f)
      |     ^~~~~~~~
/usr/include/c++/11/bits/stl_algo.h:3814:5: note:   template argument deduction/substitution failed:
main.cpp:12:22: note:   couldn’t deduce template parameter ‘_Funct’
   12 |         std::for_each(callbacks.cbegin(), callbacks.cend(), std::invoke);
      |         ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/11/algorithm:74,
                 from main.cpp:1:
/usr/include/c++/11/pstl/glue_algorithm_defs.h:42:1: note: candidate: ‘template __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void> std::for_each(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Function)’
   42 | for_each(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Function __f);
      | ^~~~~~~~
/usr/include/c++/11/pstl/glue_algorithm_defs.h:42:1: note:   template argument deduction/substitution failed:
main.cpp:12:22: note:   candidate expects 4 arguments, 3 provided
   12 |         std::for_each(callbacks.cbegin(), callbacks.cend(), std::invoke);
      |         ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Call Stack
#   Function    File:Line
Local Variables
Variable    Value
Registers
Register    Value
Display Expressions
Expression  Value   
Breakpoints and Watchpoints
    #   Description

我也尝试过这样的方式：

std::for_each(callbacks.cbegin(), callbacks.cend(), std::invoke<void(void)>);

但我仍然有问题：

In file included from /usr/include/c++/11/algorithm:62,
                 from main.cpp:1:
/usr/include/c++/11/bits/stl_algo.h: In instantiation of ‘_Funct std::for_each(_IIter, _IIter, _Funct) [with _IIter = __gnu_cxx::__normal_iterator<const std::function<void()>*, std::vector<std::function<void()> > >; _Funct = void (*)(void (&&)())]’:
main.cpp:12:15:   required from here
/usr/include/c++/11/bits/stl_algo.h:3820:12: error: invalid initialization of reference of type ‘void (&&)()’ from expression of type ‘const std::function’
 3820 |         __f(*__first);
      |         ~~~^~~~~~~~~~
c++

1 个回答

  1. Best Answer

    如果没有 lambda 就无法做到这一点，因为std::invoke（像 中的许多其他函数一样std）不允许获取其地址。
    您可以在此处查看有关它的更多信息：我可以获取标准库中定义的函数的地址吗？

    如果有帮助的话，下面是使用 lambda 执行此操作的方法：

    对于范围中的每个元素（在您的情况下是回调函数），将使用它作为参数调用 lambda。
    然后您可以使用它std::invoke来调用回调函数元素：

    #include <algorithm>
#include <functional>
#include <iostream>

int main()
{
    std::function<void(void)> foo = []() { std::cout << "Foo !"; };
    std::function<void(void)> bar = []() { std::cout << "Bar !"; };

    const std::vector<std::function<void(void)>> callbacks = { foo, bar };

    std::for_each(callbacks.cbegin(), 
                  callbacks.cend(), 
                  [](std::function<void(void)> cb) { std::invoke(cb); });
}

    输出：

    Foo !Bar !

    现场演示

    • 2

相关问题