[英]No matching function compile error when passing lambda expression to a templated caller function?
Code:代码:
#include <iostream>
template <class FunctorType>
void caller(const FunctorType& func) {
func();
}
int main() {
double data[5] = {5., 0., 0., 0., 0.};
auto peek_data = [data]() { std::cout << data[0] << std::endl; };
auto change_data = [data]() mutable { data[0] = 4.2; };
caller(peek_data); // This works
caller(change_data); // This doesn't
return 0;
}
If I compile this with clang++ -std=c++11 mutable_lambda.cpp
, I got error: no matching function for call to object of type 'const (lambda at mutable_lambda.cpp:8:22)'
.如果我用
clang++ -std=c++11 mutable_lambda.cpp
编译它,我得到error: no matching function for call to object of type 'const (lambda at mutable_lambda.cpp:8:22)'
。
Question: Why passing the second lambda expression with mutable copy capture failed to compile?问题:为什么传递带有可变副本捕获的第二个 lambda 表达式编译失败? Thanks in advance!
提前致谢!
A mutable
lambda has a non- const
operator()
. mutable
lambda 具有非const
operator()
。 You are trying to call this non- const
operator()
through a const
reference.您正在尝试通过
const
引用调用此非const
operator()
。 That doesn't work for the same reason that calling any non- const
non-static member function doesn't work through a const
reference.这不起作用的原因与调用任何非
const
非静态成员函数不能通过const
引用工作的原因相同。
If you want to allow caller
to modify the passed function object (through a non- const
operator()
call) if it is passed as non- const
argument, then take it by forwarding reference instead of const
reference:如果要允许
caller
修改传递的函数对象(通过非const
operator()
调用),如果它作为非const
参数传递,则通过转发引用而不是const
引用来获取它:
template <class FunctorType>
void caller(FunctorType&& func) {
func(); // maybe std::forward<FunctorType>(func)();
}
Also note that change_data
does not change the data
in main
.另请注意,
change_data
不会更改main
中的data
。 It changes a copy of it stored inside the lambda.它更改了存储在 lambda 中的副本。 If you want it to change the
data
in main
, you need to capture data
by-reference and then the lambda itself also doesn't need to be mutable
anymore:如果您希望它更改
main
中的data
,则需要通过引用捕获data
,然后 lambda 本身也不再需要是mutable
的:
auto change_data = [&data]() { data[0] = 4.2; };
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