由于未初始化的成员，在构造函数案例崩溃之前调用的C ++函数

Question

I have this program

#include <iostream>
#include <memory>

using namespace std;

class B {
public:
    void printB() {
        cout << __FUNCTION__ << endl;
        cout << mModuleName << endl;
    }
    void setModuleName() {
        mModuleName = "BBBBBBB";
    }

private:
    string mModuleName;
};

class A {
public:
    void printA() {
        cout << __FUNCTION__ << endl;
        b.printB();
    }
    B b;
};

int main(int argc, char ** argv) {
    cout << " START " << endl;

    std::shared_ptr<A> a;

    a->printA();

    a = std::make_shared<A>();

    cout << " END " << endl;
    return 0;
}

Here is the output:

vagrant@vagrant:~/development$ ./a.out
 START
printA
printB
Segmentation fault (core dumped)

This obviously crashes when reaching to line where he tries to access mModuleName. I can fix it by moving printA() after make_shared line but I would like more robust solution to prevent SIGSEGV. Any suggestions?

Answer 1

This isn't an actual problem, you're simply doing something that is obviously going to crash because you are dereferencing a null pointer.

You can do this as an alternative and call make_shared in the same line you declare the object.

int main(int argc, char ** argv) {
    std::shared_ptr<A> a = std::make_shared<A>();
    a->printA();
    return 0;
}

There is no real reason to split up the object declaration and initialization in this case.

Answer 2

I can fix it by moving printA() after make_shared line but I would like more robust solution to prevent SIGSEGV. Any suggestions?

One possible method is to use a non-member function.

namespace MyApp
{
   void print(A* aptr)
   {
      if ( aptr != nullptr )
      {
         aptr->printA();
      }
   }
}

and in main use

MyApp::print(a);