Why can't I make a vector of references?
When I do this:
std::vector<int> hello;
Everything works great. However, when I make it a vector of references instead:
std::vector<int &> hello;
I get horrible errors like
error C2528: 'pointer' : pointer to reference is illegal
I want to put a bunch of references to structs into a vector, so that I don't have to meddle with pointers. Why is vector throwing a tantrum about this? Is my only option to use a vector of pointers instead?
The component type of containers like vectors must be assignable. References are not assignable (you can only initialize them once when they are declared, and you cannot make them reference something else later). Other non-assignable types are also not allowed as components of containers, e.g. vector<const int>
is not allowed.
yes you can, look for std::reference_wrapper
, that mimics a reference but is assignable and also can be "reseated"
By their very nature, references can only be set at the time they are created; i.e., the following two lines have very different effects:
int & A = B; // makes A an alias for B
A = C; // assigns value of C to B.
Futher, this is illegal:
int & D; // must be set to a int variable.
However, when you create a vector, there is no way to assign values to it's items at creation. You are essentially just making a whole bunch of the last example.
Ion Todirel already mentioned an answer YES using std::reference_wrapper
. Since C++11 we have a mechanism to retrieve object from std::vector
and remove the reference by using std::remove_reference
. Below is given an example compiled using g++
and clang
with option-std=c++11
and executed successfully.
#include <iostream>
#include <vector>
#include<functional>
class MyClass {
public:
void func() {
std::cout << "I am func \n";
}
MyClass(int y) : x(y) {}
int getval()
{
return x;
}
private:
int x;
};
int main() {
std::vector<std::reference_wrapper<MyClass>> vec;
MyClass obj1(2);
MyClass obj2(3);
MyClass& obj_ref1 = std::ref(obj1);
MyClass& obj_ref2 = obj2;
vec.push_back(obj_ref1);
vec.push_back(obj_ref2);
for (auto obj3 : vec)
{
std::remove_reference<MyClass&>::type(obj3).func();
std::cout << std::remove_reference<MyClass&>::type(obj3).getval() << "\n";
}
}
TL; DR
Use std::reference_wrapper
like this:
#include <functional>
#include <string>
#include <vector>
#include <iostream>
int main()
{
std::string hello = "Hello, ";
std::string world = "everyone!";
typedef std::vector<std::reference_wrapper<std::string>> vec_t;
vec_t vec = {hello, world};
vec[1].get() = "world!";
std::cout << hello << world << std::endl;
return 0;
}
Demo
Long answer
As standard suggests, for a standard container X
containing objects of type T
, T
must be Erasable
from X
.
Erasable
means that the following expression is well formed:
allocator_traits<A>::destroy(m, p)
A
is container's allocator type, m
is allocator instance and p
is a pointer of type *T
. See here for Erasable
definition.
By default, std::allocator<T>
is used as vector's allocator. With the default allocator, the requirement is equivalent to the validity of p->~T()
(Note the T
is a reference type and p
is pointer to a reference). However, pointer to a reference is illegal, hence the expression is not well formed.