引用前向声明 class 作为列表的模板

Question

我不能使用对 incomplate（前向声明）class 的引用作为列表的模板。但是指针工作正常。对于我读过的不完整类型，向量、列表和转发列表是允许的（地图、集合和其他不允许）

这是例子

#include <list>
#include <vector>

class MyClass;

void foo(const MyClass&);            //works fine
std::vector<const MyClass&> ref_vec; //error
std::list<const MyClass&> ref_list;  //error
std::vector<const MyClass*> p_vec;   //works fine
std::list<const MyClass*> p_list;    //works fine

class MyClass
{};

void main(){}

Answer 1

这里的问题与前向声明无关。您可以通过将程序更改为以下内容来查看：

#include <list>
#include <vector>

class MyClass
{};

void foo(const MyClass&);            //works fine
std::vector<const MyClass&> ref_vec; //error
std::list<const MyClass&> ref_list;  //error
std::vector<const MyClass*> p_vec;   //works fine
std::list<const MyClass*> p_list;    //works fine

int main(){}

标记为 //error 和 /works fine 的行将以完全相同的方式失败。问题是 std::vector and std::list are not allowed to have a type which is a reference. The reason for this in C++11 and beyond is that elements must be Erasable, which references are not (more details are in the question Why can't I make a vector of references?).

引用前向声明 class 作为列表的模板

Reference to forward declared class as template for list

c++

forward-declaration