lambda表达式被拒绝（C ++ 11）

Question

virtual Answer<const Taaal& const> askWho(bool(const Taaal& const)) = 0;
virtual Answer<const Taaal& const> askWho(const Taaal& const t) {
    return askWho([&](auto q) {
        return q == t;
    });
}

is being rejected with "no instance of overloaded function matches the argument list" (additionally, I'm being told auto is not allowed, but replacing that with an explicit type leaves the first problem)

Taaal is the (abstract) class that these methods are declared in. Answer is #included from another header, and its definition shouldn't matter.

Answer 1

Only a capture-less lambda defines an implicit conversion to a function pointer. Your lambda expression captures its context because of the capture default & . Since your lambda cannot be stateless, you could change the parameter type of the other overload

virtual Answer<const Taaal&> askWho(std::function<bool(const Taaal&)>) = 0;

---

If your lambda didn't need to capture context variables, you could've dropped the & from the capture list and the conversion would've worked as expected

virtual Answer<const Taaal&> askWho(const Taaal& t) {
    return askWho([](auto q) {
//                ^^ - nothing here
        return ...;
    });
}

Note that the auto q parameter type for the lambda only works if you compile your code in C++14 mode ( -std=c++1y on gcc and clang). In C++11 mode you'll have specify the parameter type.

Answer 2

auto in lambdas is only allowed in C++14.

You must use the real parameter type, polymorphic lambdas are not included in C++11.