构造函数中的lambda实例化(lambda instantiation in constructor)

构造函数中的lambda实例化(lambda instantiation in constructor)

我有这样一个构造函数：

ConcurrentHashMap(int expected_size, int expected_threads_count, const Hash& hasher = Hash()) { this->my_hash_ = hasher; if (expected_size != kUndefinedSize) table.reserve(expected_size); }

当我为hasher参数传递lambda函数时：

auto lambda = [](const std::pair<int, int>& x) { return pair_hash(x); };

我得到错误：

: In instantiation of ‘ConcurrentHashMap<K, V, Hash>::ConcurrentHashMap(int, int, const Hash&) [with K = std::pair<int, int>; V = std::__cxx11::basic_string<char>; Hash = Correctness_Constructors_Test::TestBody()::<lambda(const std::pair<int, int>&)>]’: required from here

和：

error: use of deleted function ‘Correctness_Constructors_Test::TestBody()::<lambda(const std::pair<int, int>&)>::<lambda>()’

我怎样才能克服这个问题？

I have such a constructor:

ConcurrentHashMap(int expected_size, int expected_threads_count, const Hash& hasher = Hash()) { this->my_hash_ = hasher; if (expected_size != kUndefinedSize) table.reserve(expected_size); }

When I pass a lambda function for the hasher argument:

auto lambda = [](const std::pair<int, int>& x) { return pair_hash(x); };

I get the errors:

: In instantiation of ‘ConcurrentHashMap<K, V, Hash>::ConcurrentHashMap(int, int, const Hash&) [with K = std::pair<int, int>; V = std::__cxx11::basic_string<char>; Hash = Correctness_Constructors_Test::TestBody()::<lambda(const std::pair<int, int>&)>]’: required from here

And:

error: use of deleted function ‘Correctness_Constructors_Test::TestBody()::<lambda(const std::pair<int, int>&)>::<lambda>()’

How can I overcome this problem?

最满意答案

这里的问题是你在构造函数成员初始化列表中默认构造my_hash_ （因为你没有提供），然后你在构造函数体中分配它。 由于my_hash_是一个lambda，因此它不是默认构造的，因为lambdas不是默认构造的。 您需要在成员初始化列表中初始化my_hash_ ，如

ConcurrentHashMap(int expected_size, int expected_threads_count, const Hash& hasher = Hash()) : my_hash_(hasher) { //... }

The issue here is you default construct my_hash_ in your constructor member initialization list (because you do not provide one) and then you assign to it in the constructor body. Since my_hash_ is a lambda it is not default constructable as lambdas are not default constructable. You need to initialize my_hash_ in the member initializer list like

ConcurrentHashMap(int expected_size, int expected_threads_count, const Hash& hasher = Hash()) : my_hash_(hasher) { //... }