Cannot pass vector of subclass when base class required as a param

Question

I have a class A and its subclass B and a method that takes std::vector<A>. And I cannot make it work by passing std::vector<B>.

I though that if subclass B can be casted to A, I should be able to pass vector of Bs to method that takes vector of As. How can I do that properly?

#include <iostream>
#include <vector>

class A {

    public:

        A() {}
};

class B: public A {

    public: 

        B(): A() {}
};

void method(std::vector<A> a) {}

int main() {
    std::vector<B> b;
    method(b);

    return 0;
}

When compiled:

g++ ex-cast-tag.cpp -std=c++11

ex-cast-tag.cpp:22:5: error: no matching function for call to 'method'
    method(b);
    ^~~~~~
ex-cast-tag.cpp:18:6: note: candidate function not viable: no known conversion from 'vector<B>' to 'vector<A>'
  for 1st argument
void method(std::vector<A> a) {}
     ^
1 error generated.

Thanks in advance!

Template specializations with different arguments are not related, even if the arguments are related. Try making `method()` a template function. — , Jul 19 '18 at 19:33
Related: https://stackoverflow.com/questions/51428853/cast-stdlistx-to-stdlisty — πάντα ῥεῖ, Jul 19 '18 at 19:33
@Ripi2 it does not change anything. Result is the same. Compiler says `no know conversion from vector to vector`. — Tomasz Szulc, Jul 19 '18 at 19:34
@Ripi2 Both the parameter type and the value passed in must be `vector`. — Code-Apprentice, Jul 19 '18 at 19:37
"Use pointers" in the sense that you can have a vector of base class pointers, and insert derived class pointers in there. You will not need `vector`, only `vector`. — , Jul 19 '18 at 19:38
closing this a dupe is rather unfortunate, as the alleged dupe is pre 2011, and answers using `enable_if`, for example, are not allowed, but are useful here. — Walter, Jul 19 '18 at 20:31

score 3 · Answer 1 · answered Jul 19 '18 at 20:40

One solution is to use a template. For example

template<typename C>
std::enable_if_t<std::is_base_of<A,C>::value>
method(std::vector<C> const&a) 
{
    // assume that a[i] is an A (derived or actually the same)

}

Here, I have used SFINAE to ensure that C is actually A or derived from A. But you can also use static_assert instead. In this case you get nicer error messages, but the overloading behaviour of method() is different (i.e. when this template is considered).

Code-Apprentice · Answer 2 · 2018-07-19T19:52:34.377

1

If B is a subclass of A, it does not follow that std::vector<B> is a subclass of std::vector<A>. In order to use inheritance correctly here, you need to change the function to accept a parameter of type std::vector<A*> then pass it a std::vector<A*> whose pointers all point to instances of type B. Alternatively, you can make the function into a template function.

edited Jul 19 '18 at 19:52

answered Jul 19 '18 at 19:35

Code-Apprentice

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Passing `std::vector` to method that takes `std::vector` does not work either. Template works. – Tomasz Szulc Jul 19 '18 at 19:39
@TomaszSzulc I edited to clarify how to do that correctly. – Code-Apprentice Jul 19 '18 at 19:52

Cannot pass vector of subclass when base class required as a param

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