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Oodini's questions

Martin Hope
Oodini
Asked: 2024-11-21 06:01:26 +0800 CST
  • 7

我正在阅读 Nicolai M. Josuttis 的《C++17 - 完整指南》一书。

第 14 章的标题是“使用可变参数声明”。我不明白其中的示例代码：

// Part 1: "inherit" all function call operators of passed based types:
template<typename... Ts>
struct overload : Ts...
{
    using Ts::operator()...;
}

// Part 2: base types are deduced from passed arguments
// >>>> What is it ?? -> see answer from Ted Lyngmo
// EDIT: That's also based on the fact that overload can use
//       aggregate initialization, and so that overload can be
//       instancied with a constructor with any number and types
//       (but we have to help it to deduce the types).
template<typename... Ts>
overload(Ts...) -> overload<Ts...>;

// Part 3
auto twice = overload {
                [](std::string& s) { s += s; }
                [](auto& v) { v *= 2; }
             };

代码分为3个部分：

  • 第 1 部分：我知道我们声明一个最终将具有 3 个函数调用运算符的类。

  • 第 2 部分：我不明白这一点…… - 我们在这里声明什么？你能解释一下语法吗，尤其是overload(Ts...)？

  • 第 3 部分：我们使用聚合初始化来初始化基类函数调用运算符。

如果我必须再次阅读前面的章节，请告诉我哪一章！

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Martin Hope
Oodini
Asked: 2024-10-23 20:41:17 +0800 CST
  • 7

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);
      |         ~~~^~~~~~~~~~
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