boost::lambda::if_then 用于 copy_if
我一直在尝试通过以下代码模拟 copy_if 但我的编译器（g++-4.0）只是不断抱怨。 技术上有什么问题？ 感谢您的帮助！
template <class STL> // a std container of class A, but I don't know if it's a list or vector or deque
void export_(STL& Alist) { //a member function
for_each(Alist0.begin(), Alist0.end(), //member data
boost::lambda::if_then(
boost::lambda::bind(&A::get_StatusTag, boost::lambda::_1) == OK, //some global enum returned by A::get_StatusTag
boost::lambda::bind(&STL::push_back, Alist, boost::lambda::_1)
)
);
}
I have been trying to emulate a copy_if by the following codes but my compiler (g++-4.0) just keeps complaining. What's technically wrong? Thanks for your help!
template <class STL> // a std container of class A, but I don't know if it's a list or vector or deque
void export_(STL& Alist) { //a member function
for_each(Alist0.begin(), Alist0.end(), //member data
boost::lambda::if_then(
boost::lambda::bind(&A::get_StatusTag, boost::lambda::_1) == OK, //some global enum returned by A::get_StatusTag
boost::lambda::bind(&STL::push_back, Alist, boost::lambda::_1)
)
);
}
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lambda::bind 通过复制进行绑定。 表达式
bind(&STL::push_back, AList, _1)因此创建了一个包含AList副本的函子。 由于该函子的operator()是const，所以应用它会失败，因为它最终会调用const对象（其内部副本）上的非常量成员（push_back）。解决方案：绑定到 AList 的引用，使用：
bind(&STL::push_back, boost::ref(AList), _1)lambda::bind binds by copy. The expression
bind(&STL::push_back, AList, _1)thus creates a functor holding a copy of AList. Since this functor's operator() is const, applying it fails because it ends up calling a non-const member (push_back) on a const object (its internal copy).Solution: bind to a reference to AList, using:
bind(&STL::push_back, boost::ref(AList), _1)作为参考，这里有一个更正常的 copy_if 实现，取自 http://www.richelbilderbeek.nl/CppCopy_if .htm
然后你可以像这样使用它：
当然假设我复制的行不是代码中失败的行。
For reference, here's a more normal implementation of copy_if, taken from http://www.richelbilderbeek.nl/CppCopy_if.htm
Then you'd use it like:
Assuming of course that the line I've copied isn't the line which fails in your code.
我的猜测是编译器不喜欢这行代码，
我想知道它是否未能从绑定对象而不是 _1 中找到正确的运算符方法。 您应该尝试将其包装在另一个 lambda 表达式中。
可以这样想：
My guess is the compiler does not like the line
I wonder if it's failing to find the right operator method from the bind object and not the _1. You should try wrapping it in another lambda expression.
Think of it this way: