使用std :: for_each lambda函数的错误

Question

我有一点问题,我无法弄清楚为什么这段代码不起作用:

std::for_each(users.begin(), users.end(), [](Wt::WString u)
{
    std::cout << "ilosc: " << users.size() << std::endl;
    userBox_->addItem(u);
});

编译时得到的错误:

GameWidget.cpp: In lambda function:
GameWidget.cpp:352:30: error: 'users' is not captured
GameWidget.cpp:353:4: error: 'this' was not captured for this lambda function
GameWidget.cpp: In member function 'virtual void GameWidget::updateUsers()':
GameWidget.cpp:354:3: warning: lambda expressions only available with -std=c++11 or -std=gnu++11 [enabled by default]
GameWidget.cpp:354:4: error: no matching function for call to 'for_each(std::set<Wt::WString>::iterator, std::set<Wt::WString>::iterator, GameWidget::updateUsers()::<lambda(Wt::WString)>)'
GameWidget.cpp:354:4: note: candidate is:
In file included from /usr/include/c++/4.7/algorithm:63:0,
                 from GameWidget.h:11,
                 from GameWidget.cpp:9:
/usr/include/c++/4.7/bits/stl_algo.h:4436:5: note: template<class _IIter, class _Funct> _Funct std::for_each(_IIter, _IIter, _Funct)
GameWidget.cpp:354:4: error: template argument for 'template<class _IIter, class _Funct> _Funct std::for_each(_IIter, _IIter, _Funct)' uses local type 'GameWidget::updateUsers()::<lambda(Wt::WString)>'
GameWidget.cpp:354:4: error:   trying to instantiate 'template<class _IIter, class _Funct> _Funct std::for_each(_IIter, _IIter, _Funct)'

我正在使用gcc 4.7.3,所以可能C++ 11支持可用于我的编译器.

userBox_是一个集合,并BOOST_FOREACH适用于此代码:

BOOST_FOREACH(Wt::WString i, users)
{
    std::cout << "ilosc: " << users.size() << std::endl;
    userBox_->addItem(i);
}

谢谢你的回答,我很好奇为什么会这样.

Answer 1

您编写的lambda不捕获任何上下文变量.要做到这一点,最简单的方法是添加&到lambda的捕获列表中.这将通过引用捕获所有上下文变量,您将能够在lambda中访问它们.

std::for_each(users.begin(), users.end(), [&](Wt::WString u)
{
    std::cout << "ilosc: " << users.size() << std::endl;
    userBox_->addItem(u);
});

我不明白你为什么要users.size()在循环中打印,因为看起来每次迭代输出都是一样的.如果将其移动到循环外部,并希望对lambda捕获的内容进行更精细的控制,则可以修改捕获列表以仅捕获this指针.这将允许您访问成员变量userBox_.

std::cout << "ilosc: " << users.size() << std::endl;
std::for_each(users.begin(), users.end(), [this](Wt::WString u)
{
    userBox_->addItem(u);
});

MSDN有一篇很好的文章非常详细地解释了lambda表达式语法.

最后,在你的情况下,没有必要std::for_each和一个lambda.使用基于范围的for循环将项添加到集合中会更简洁.

for( auto const& u: users ) {
  userBox_->addItem(u);
}