我使用“overload”模式为std::variant创建一个访问者,将方法定义为带有泛型lambda的类间das,以提供一个空的默认方法。
// imagine declaration of the overload template + the deduction helper here ...
using Foodstuff = std::variant<Fruit, Vegetable, Meat, Dairy, Fish>;
int things_eaten{0};
auto picky_eater = overload{
// Eats fruit
[&things_eaten](const Fruit&) { things_eaten++; },
// Eats dairy
[&things_eaten](const Dairy&) {things_eaten++; },
// Doesn't eat anything else - I need an empty handler for the other foodstuffs
[](auto) {}
};
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然后我用它来std::visit
一个食物的向量。在MSVC 2022(v17.7.6)中编译这个,目标是C17语言标准,我没有得到编译器警告(at /W3),程序按照我在“release”目标中的预期运行,但是当它退出函数时,我将其更改为debug,我得到了异常
运行时检查失败#2 -变量'picky_eater'周围的堆栈已损坏。
注意到其他的lambda有一个捕获,而最后的泛型lambda没有,如果我把最后一个重载改为[&things_eaten](auto) {}
,这个异常就消失了。
此外,如果我将语言标准更改为C20,原始版本不会生成该异常。
这个异常是否在原始代码中识别出了一个真实的问题,C20是否引入了一些东西(也许是模板演绎的变化?)来解决它?这是MSVC C17在调试buuilds中运行时检查的一个奇怪的bug吗(我没有其他编译器或分析工具可以比较)。
#include <variant>
#include <vector>
#include <string>
#include <iostream>
// Overload pattern
template <typename... Ts>
struct overload : Ts...
{
using Ts::operator()...;
};
// Template deduction helper; needed in C++17
template <typename... Ts>
overload(Ts...)->overload<Ts...>;
///////////////////////////////////////////////////////////
struct Fruit {
std::string name;
};
struct Vegetable {
std::string name;
};
struct Meat {
std::string name;
};
struct Dairy {
std::string name;
};
struct Fish {
std::string name;
};
using Foodstuff = std::variant<Fruit, Vegetable, Meat, Dairy, Fish>;
using Meal = std::vector<Foodstuff>;
int main() {
auto meal = Meal{ Fruit{"Apple"}, Vegetable{"Tomato"}, Meat{"Beef"}, Vegetable{"Potato"}, Dairy{"Yoghurt"} };
int things_eaten{ 0 };
auto picky_eater = overload{
// Eats fruit
[&things_eaten](const Fruit&) { things_eaten++; },
// Eats dairy
[&things_eaten](const Dairy&) { things_eaten++; },
// Doesn't eat anything else - I need an empty handler for the other foodstuffs
// If I write the generic lambda this way, MSVC in C++17 langauge standard will
// give an exception in Debug builds:
// "Run-Time Check Failure #2 - Stack around the variable 'picky_eater' was corrupted."
[](auto) {}
// But if I just add the same capture, even though it's not used, that error goes away?
// [&things_eaten](auto) {}
};
for (const auto& ingredient : meal) {
std::visit(picky_eater, ingredient);
}
std::cout << "I ate " << things_eaten << " things" << std::endl;
return 0;
// As the program exits, we get the exception
}
型
1条答案
按热度按时间kqlmhetl1#
您的代码看起来是正确的,可以进一步简化为
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在Visual Studio中保留堆栈损坏错误。
我向MS团队报告:https://developercommunity.visualstudio.com/t/Run-Time-Check-Failure-2---Stack-around/10510860
根据他们的repospone,这是一个真实的编译器错误,最早在2019年报告:https://developercommunity.visualstudio.com/t/Capturing-lambdas-in-function-object-res/475396