让我们考虑以下代码:
class Widget{
};
int main(){
Widget w;
auto lambda = bind([](Widget&& ref){ return; }, std::move(w));
return 0;
}
它会触发错误
no match for call to ‘(std::_Bind<main()::<lambda(Widget&&)>(Widget)>) ()’
lambda();
我的问题是:为什么会出现错误?毕竟,我对右值引用进行了显式转换——我的意思是 std::move(w)
我通过右值引用来论证——我的意思是Widget&& ref
.
怎么了?
而且下面的代码有效,让我更担心的是:
class Widget{
};
int main(){
Widget w;
auto lambda = bind([](Widget& ref){ return; }, std::move(w));
return 0;
}
最佳答案
如果您写下 std::bind
的示意性作用,它可能会变得更清楚。
// C++14, you'll have to write a lot of boilerplate code for C++11
template <typename FuncT, typename ArgT>
auto
bind(FuncT&& func, ArgT&& arg)
{
return
[
f = std::forward<FuncT>(func),
a = std::forward<ArgT>(arg)
]() mutable { return f(a); }; // NB: a is an lvalue here
}
由于您可以多次调用 std::bind
给您的函数对象,它不能“用完”捕获的参数,因此它将作为左值引用传递。您向 bind
本身传递一个右值这一事实仅意味着在初始化 a
的行上没有复制。
如果您尝试使用上面显示的原理图 bind
编译您的示例,您还会从编译器中获得更有帮助的错误消息。
main.cxx: In instantiation of ‘bind(FuncT&&, ArgT&&)::<lambda()> mutable [with FuncT = main()::<lambda(Widget&&)>; ArgT = Widget]’:
main.cxx:10:33: required from ‘struct bind(FuncT&&, ArgT&&) [with FuncT = main()::<lambda(Widget&&)>; ArgT = Widget]::<lambda()>’
main.cxx:11:31: required from ‘auto bind(FuncT&&, ArgT&&) [with FuncT = main()::<lambda(Widget&&)>; ArgT = Widget]’
main.cxx:18:59: required from here
main.cxx:11:26: error: no match for call to ‘(main()::<lambda(Widget&&)>) (Widget&)’
]() mutable { return f(a); }; // NB: a is an lvalue here
^
main.cxx:11:26: note: candidate: void (*)(Widget&&) <conversion>
main.cxx:11:26: note: conversion of argument 2 would be ill-formed:
main.cxx:11:26: error: cannot bind ‘Widget’ lvalue to ‘Widget&&’
main.cxx:18:33: note: candidate: main()::<lambda(Widget&&)> <near match>
auto lambda = bind([](Widget&&){ return; }, std::move(w));
^
main.cxx:18:33: note: conversion of argument 1 would be ill-formed:
main.cxx:11:26: error: cannot bind ‘Widget’ lvalue to ‘Widget&&’
]() mutable { return f(a); }; // NB: a is an lvalue here
关于c++ - std::bind 和右值引用,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/34877699/