C++ pointer to string through class

Question

I have a class and in there I have a struct which contains a string. In main I declare the class, then create a struct to reach that string. How it looks like:

Class *myClass = new Class;
Class::Struct *myStruct;

myStruct = &myClass->getStruct(); // the struct is in a vector so I get it through a function

cout << myStruct->string << "\n";

So, with this method I can't reach the string in the struct, but I can the int's, float's, etc. If I change the code to this:

Class::Struct myStruct;

myStruct = myClass->getStruct();

cout << myStruct.string << "\n";

then it works.

My question is, why is the second one working, and the first not, and why only the string? It's not about life and death, because I don't really need that string, I just tested the program, but I'm really curious what's going on, because I'm still learning the language, and especially pointers.

Thanks in advance!

EDIT: Ok, so the definitions

class Class {
public:
    struct Struct {
        string name;
        float data;
    };

    Class();
    Struct getStruct( int i );

private:
    vector<Struct>  tempStruct;
};

Class::Class() {
    Struct str;
    str.name = "test";
    str.data = 1.0f;

    tempStruct.push_back( str );
}

struct Class::getStruct( int i ) {
    return tempStruct[i];
}

EDIT2: That was a little mistake, but I forgot the &. But you helped me to find it.

Show the class and struct definition. It's hard to guess with so little info. — Olaf Dietsche
– Olaf Dietsche, Commented Oct 26, 2012 at 14:04
It almost certaintly the case that the getStruct() returns a copy of the struct from the vector, meaning in the first case myStruct has the address of a temporary object resulting in myStruct being a dangling pointer. — hmjd
– hmjd, Commented Oct 26, 2012 at 14:06

Nikos C. · Accepted Answer · 2012-10-26 14:06:12Z

1

This is because getStruct() returns a copy of the struct. This is called a "temporary". The temporary dies immediately. So now you have a pointer to a dead temporary. It doesn't exist anymore so the pointer to it is invalid. Either return a pointer or reference to the struct in getStruct(), or hold a reference to its return value like this:

const Class::Struct& myStruct = myClass->getStruct();

answered Oct 26, 2012 at 14:06

Nikos C.

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6 Comments

Zharf Over a year ago

the function should return a reference, not a copy (or a (usually) pointer)

Nikos C. Over a year ago

@Zharf Well, we don't know for sure what it should return since we can't see the code of the class. I added it to the answer though, to be more complete.

matthew3r Over a year ago

But as I wrote before, the problem is only with the string. I can write out for example the value of the float, but not the value of the string. And that's what I can not understand.

Nikos C. Over a year ago

@matthew3r This is due to the memory still containing remnants of the data. In other words, you're accessing corpses. In programming, corpses decay very quickly though.

Zharf Over a year ago

@matthew3r it's technically undefined behaviour, don't count on it

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