为什么我不能添加shared_ptr <Derived> 到地图 <key_type,shared_ptr<Base> &gt;在此代码中？

Question

I'd like to add a shared_ptr<derived> to a map with shared_ptr<base> values (as below http://ideone.com/hd68yc ) but it fails (we reach EXIT_FAILURE ):

#include <iostream>
#include <map>
#include <memory>
using namespace std;

class base{
    public:
        base(const int& s = 1):setting{s}{};
        int setting;
};
class derived : public base{
    public:
        derived(const int& s):setting{s}{};
        int setting;
};

int main() {
    map<string,shared_ptr<base>> m_s;
    m_s.insert(make_pair("Name",make_shared<derived>(4)));
    // None of these worked either...
    //m_s.emplace("Name",make_shared<derived>(4));
    //m_s["Name"] = make_shared<derived>(4);

    if(4 == m_s.at("Name")->setting){
        return EXIT_SUCCESS;
    }else{
        cout << "setting is " << m_s.at("Name")->setting << " when 4 expected";
        return EXIT_FAILURE;
    }
}

Answer 1

You cannot initialize base class member in constructor initialization list, but you can call proper base class constructor:

class derived : public base{
    public:
        derived(const int& s):base{s}{};
};

Your naive "fix" to put member setting into derive class does not fix the issue, but hides it, allow code to compile, but breaking the logic.

Answer 2

For a start, the derived class contains an extra setting member. Getting rid of that revealed that I can't use the derived(const int& s):setting{s}{}; ctor syntax since it refers to a setting member that is inherited.

In the end, using the following fixed my problem:

class correct_derived : public base{
    public:
        correct_derived(const int& s){
            setting = s;
        }
};

Answer 3

m_s.insert(make_pair("Name",make_shared<derived>(4)));

if(4 == m_s.at("Name")->setting) {

As far as I can tell you store a Pointer in a map and then compare it to an int (4). You would have to de-reference the stored shared pointer to get its value which you then can compare to '4'. That might be the reason you end up at EXIT_FAILURE.