C++ calling constructor to initialize an object yields errors

Question

What's wrong with this:

struct FileListItem {
    string  sOriginalFn;
    time_t  ttTimeTaken;

    FileListItem(){}
    FileListItem(string _sOriginalFn, time_t _ttTimeTaken) :
        sOriginalFn (_sOriginalFn), ttTimeTaken (_ttTimeTaken)  { }
};

struct FileList : vector<FileListItem> {
    int iCurItm;
    FileList() : vector(), iCurItm(-1) {};

    void Add(string _sOriginalFn, time_t _ttTimeTaken) { 
        push_back(FileListItem(_sOriginalFn, _ttTimeTaken)); 
    }
}

I get a run-time "read access violation" the first time Add is called.

I then try:

struct FileList : vector<FileListItem> {
    int iCurItm;
    FileList() : vector(), iCurItm(-1) {};

    FileListItem Itm;                    // <--- new member
    void Add(string _sOriginalFn, time_t _ttTimeTaken) { 
        Itm(_sOriginalFn, _ttTimeTaken); // <--- E0980 pointing to "Itm"
        push_back(Itm); 
    }
}

and get a compile time error:

E0980 - call of an object of a class type without appropriate operator() or conversion functions to pointer-to-function type.

I must have forgotten or missed something since I stopped programming 30 years ago, when Borland C++ was IT...

It is usually considered bad practice to inherit from the standard library. It's better to have a struct containing a vector than to inherit from std::vector — Clonk
– Clonk, Commented Aug 6, 2018 at 7:28
Sorry, I was wrong, you can access push_back even with private inheritance from within you derived class. Note that there is a typo in FileList() : vector(), iCurItm(-1) {};, the semicolon is superfluous at the end. — Daniel Langr
– Daniel Langr, Commented Aug 6, 2018 at 7:30
VS2017 did not give any runtime error when Add was called. what is your compiler? — miradham
– miradham, Commented Aug 6, 2018 at 7:31
Using VS2017. I eliminated , iCurItm(-1). and eliminated the superfluous ; typo. And still - getting the same runtime error. miradham - can you show the full program you used to test it - maybe it's something about the way I first create or call my objects? — Itay1
– Itay1, Commented Aug 6, 2018 at 9:53
We need a full reproducible example of the code being run in order to understand where that error is coming from — Rerito
– Rerito, Commented Aug 6, 2018 at 10:59

Rerito · Accepted Answer · 2018-08-06 07:42:50Z

3

In the line

Itm(_sOriginalFn, _ttTimeTaken);

You are not calling the constructor for your object. Instead, the compiler interprets this as a function call on the functor Itm. Therefore, it tries to find a valid operator() for the given arguments, to no-avail.

Do not use constructor member initializer list syntax elsewhere than... Constructor member initializer list. Instead, what you have to do here is to assign a new object to your member.

Then, you don't want, ever to inherit from std containers. Their destructor is not virtual so you are exposing yourself to a realm of suffering...

Use composition or type aliasing instead:

struct FileList {
    std::vector<FileListItem> flist;
    int curItem;

    FileList() : flist(), curItem(-1) {}

    void Add(string const& _sOriginalFn, time_t _ttTimeTaken) {
    // No need to construct the object before, let vector handle it
    flist.emplace_back(_sOriginalFn, _ttTimeTaken);
};

As for the read access violation, from the code I see, nothing explains it. Could it be some access related to curItem somewhere in your code that isn't displayed here?

edited Aug 6, 2018 at 7:42

answered Aug 6, 2018 at 7:23

Rerito

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5 Comments

miradham Over a year ago

this does not explain read access violation OP has faced first

Daniel Langr Over a year ago

Just note that this will copy from rvalue arguments.

Rerito Over a year ago

@DanielLangr indeed. I didn't want to overcomplicate things with a perfect forwarding design

Itay1 Over a year ago

I eliminated iCurItm from the constructor, and also eliminated the entire constructor - runtime error is exactly the same. Any ideas? @miradham - can you show the full program you used to test it - maybe it's something about the way I first create or call my objects?

Itay1 Over a year ago

OK, Got it! @miradham's code ran OK, but the stubborn memory over-run continued as long as FileList.Add was (elsewhere in the code, as @Retiro suggested) invoked through a local variable defined: FileList &FL;. Once it changed to FileList *FL; - There was no problem. I must have done something absolutely TERRIBLE! though I wonder how, with thousands of compiler errors/warnings, this was not caught. Thanks everybody for help and advice!

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