Multimap插入键typeinfo与std :: make

Multimap插入键typeinfo与std :: make_pair vs std :: pair构造函数(Multimap insert key typeinfo with std::make_pair vs std::pair constructor)

我想了解为什么以下代码使用std :: pair构造函数是有效的：

码：

std::multimap< std::type_index, void * > mm; mm.insert(std::pair<std::type_index, void *>(typeid(int), 0));

并在使用std :: make_pair时出现编译错误：

错误：

C2248 'type_info::type_info' cannot access private member declared in class 'type_info'

代码：

std::multimap< std::type_index, void * > mm; mm.insert(std::make_pair(typeid(int), 0));

有什么线索吗？ 谢谢 ！

I would like to understand why the following code is valid using std::pair constructor :

Code:

std::multimap< std::type_index, void * > mm; mm.insert(std::pair<std::type_index, void *>(typeid(int), 0));

And gives compiling error when using std::make_pair:

Error:

C2248 'type_info::type_info' cannot access private member declared in class 'type_info'

Code :

std::multimap< std::type_index, void * > mm; mm.insert(std::make_pair(typeid(int), 0));

Any clues ? Thank you !

最满意答案

typeid的返回类型是std::type_info对象，它没有定义构造函数。 当make_pair从传入的参数中推导出其输出对的模板参数时，它会推导出std::pair<std::type_info, int> 。 然后，由于上述原因，它无法创建所需的对。

您的另一行使用显式模板参数创建对： std::pair<std::type_index, void *> 。 这一次，你正在创建一个std::type_index对象，它有一个构造函数，它接受一个std::type_info - 正是你给它的东西。 所以没问题。

如果还给出了显式模板参数，那么make_pair行将编译：

mm.insert(std::make_pair<std::type_index, void *>(typeid(int), 0));

The return type of the typeid is a std::type_info object, which has no constructors defined. As make_pair deduces the template arguments for its output pair from the parameters passed in, it deduces std::pair<std::type_info, int>. It then fails to create the required pair for the above reason.

Your other line creates the pair with explicit template parameters: std::pair<std::type_index, void *>. This time, you are creating an std::type_index object, which does have a constructor, which takes an std::type_info - exactly what you're giving it. So no problems.

Your make_pair line would compile if it was also given explicit template parameters:

mm.insert(std::make_pair<std::type_index, void *>(typeid(int), 0));