Question

This is the simplest I could work my problem down to, sorry about the length:

#include <vector>
#include <iostream>

class Bar
{
private:
    std::vector<int> intVector_;
public:
    Bar() {};
    void addInt(int newInt) 
    { 
        intVector_.push_back(newInt); 
        std::cout << intVector_.size() << " "; 
    };
    int getIntVectorSize() { return intVector_.size(); };
};

class Foo
{
private:
    Bar bar_;
public:
    Foo() { bar_ = Bar(); };
    Bar getBar() { return bar_; };
};

int main(char argc, char* argv[])
{
    Foo foo = Foo();
    foo.getBar().addInt(1);
    std::cout << foo.getBar().getIntVectorSize() << " ";
    foo.getBar().addInt(2);
    std::cout << foo.getBar().getIntVectorSize() << " ";
    foo.getBar().addInt(3);
    std::cout << foo.getBar().getIntVectorSize() << " ";
}

My problem is that adding an int to the vector only seems to last for the duration of addInt(). My output for the size of the vector looks like this:

1 0 1 0 1 0

I’m rather new to C++ and all this reference/pointer business, so I am stumped as to how I can fix this, or if this is even possible. Thanks for any help!

Answer 1

It’s because getBar() returns a Bar by value, which makes a copy of bar_ for every call of the function, and you are modifying the vector of the temporary.

You can avoid this by returning a reference:

class Foo
{
private:
    Bar bar_;
public:
    Foo() { bar_ = Bar(); }; // you prob. want to use an initialiser list btw.
    Bar& getBar() { return bar_; };
//     ^ notice the ampersand
};

This way, any modifications done on the return value of getBar are done on bar_ and not a temporary copy that is destroyed at the end of the statement.