递归 lambda C++17

Question

I have the following situation, I have a class with functions which looks like this:

void GetTest(int id, std::function<void(Test)> cb);
void GetTest2(long id, std::function(void(Test2)> cb);

The cb method will be called after the method is done which will run in the background, the order of execution of the CB method isn't guaranteed.

Now I have the case where I need to get a vector of IDs. So I tried to write a template function to call these methods and call a callback after all IDs were queried.

I came up with the following method:

template <typename T, typename T2>
void QueryList(std::vector<T2> list, std::function<void(std::shared_ptr<std::vector<T>>)> finalCallback) {
    std::shared_ptr<std::vector<T>> ret = std::make_shared<std::vector<T>>();

    auto callback = [this, list, ret, finalCallback](size_t i, auto callback, T res) {
        ret->push_back(res);

        if (i + 1 < list.size()) {
            GetTest(list[i + 1], [callback, i](T test) { callback(i + 1, callback, test); });
        } else {
            finalCallback(ret);
        }
    };

    GetTest(list[0], [callback](T test) { callback(0, callback, test); });
}

But this results in the following error error C3779: 'QueryList::<lambda_6d76c4236d9122e99fd2ab44986c7643>::operator ()': a function that returns 'auto' cannot be used before it is defined

Answer 1

You should specify the return type of the lambda:

template <typename T, typename T2>
void QueryList(std::vector<T2> list, std::function<void(std::shared_ptr<std::vector<T>>)> finalCallback) {
    std::shared_ptr<std::vector<T>> ret = std::make_shared<std::vector<T>>();

    auto callback = [this, list, ret, finalCallback](size_t i, auto callback, T res) -> void {
//                                                                                   ^^^^^^^
        ret->push_back(res);

        if (i + 1 < list.size()) {
            GetTest(list[i + 1], [callback, i](T test) { callback(i + 1, callback, test); });
        } else {
            finalCallback(ret);
        }
    };

    GetTest(list[0], [callback](T test) { callback(0, callback, test); });
}