错误：变量“不能被隐式捕获，因为没有指定默认捕获模式” [英] Error: variable "cannot be implicitly captured because no default capture mode has been specified"

问题描述

我想要关注此示例使用 remove_if 的lambda。这是我的尝试：

I am trying to follow this example to use a lambda with remove_if. Here is my attempt:

int flagId = _ChildToRemove->getId();
auto new_end = std::remove_if(m_FinalFlagsVec.begin(), m_FinalFlagsVec.end(), 
        [](Flag& device) { 
            return device.getId() == flagId; 
        });

m_FinalFlagsVec.erase(new_end, m_FinalFlagsVec.end());

但无法编译：

error C3493: 'flagId' cannot be implicitly captured because no default capture mode has been specified

b $ b

如何在lambda表达式中包含外部参数 flagId ？

推荐答案

您必须指定要捕获的 flagId 。这是 [] 部分的用途。现在它没有捕获任何东西。您可以按值或参考来捕获（更多信息）。像：

You must specify flagId to be captured. That is what the [] part is for. Right now it doesn't capture anything. You can capture (more info) by value or by reference. Something like:

auto new_end = std::remove_if(m_FinalFlagsVec.begin(), m_FinalFlagsVec.end(),
        [&flagId](Flag& device)
    { return device.getId() == flagId; });

这通过引用捕获。如果你想捕获const值，你可以这样做：

Which captures by reference. If you want to capture by const value, you can do this:

auto new_end = std::remove_if(m_FinalFlagsVec.begin(), m_FinalFlagsVec.end(),
        [flagId](Flag& device)
    { return device.getId() == flagId; });

或可变值：

auto new_end = std::remove_if(m_FinalFlagsVec.begin(), m_FinalFlagsVec.end(),
        [flagId](Flag& device) mutable
    { return device.getId() == flagId; });

很遗憾，没有直接的方法来捕获const引用。我个人只是声明一个临时const引用和捕获通过ref：

Sadly there is no straightforward way to capture by const reference. I personally would just declare a temporary const ref and capture that by ref:

const auto& tmp = flagId;
auto new_end = std::remove_if(m_FinalFlagsVec.begin(), m_FinalFlagsVec.end(),
            [&tmp](Flag& device)
        { return device.getId() == tmp; }); //tmp is immutable

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