CODEWITHSUNDEEP
c++qtstructconstructorqvector
mrg95
mrg95

Reputation: 2418

QVector of struct - no appropriate default constructor available

I'm very confused as to why this isn't working. I must be misunderstanding something key about QVectors...

I've created an MCVE to show the issue:

#include <QCoreApplication>
#include <QVector>

struct ChunkRequest
{
    ChunkRequest(int x, int z)
    {
        this->x = x;
        this->z = z;
    }

    int x;
    int z;
};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QVector<ChunkRequest> requestedChunks;

    requestedChunks.append(ChunkRequest(1, 2));

    return a.exec();
}

Compiling throws an error C2512: 'ChunkRequest' : no appropriate default constructor available

I am able to create a ChunkRequest variable with ChunkRequest req(1, 2); but as soon as I try to append it to my QVector the error throws.

I am legitimately confused as to why.

EDIT: After reading your comments, it is clear to me that QVector requires a default constructor in order to determine the size of each element in the array. But this doesn't answer why.

If a struct has a certain number of members and each member has a known size in memory (even pointers to dynamic memory are known size) then I don't see why QVector requires a default constructor? The size should be known at compile time... right?

Upvotes: 5

Views: 1528

Answers (2)

dtech
dtech

Reputation: 49289

it is clear to me that QVector requires a default constructor in order to determine the size of each element in the array. But this doesn't answer why.

it has absolutely nothing to do with it, and constructors in no way determined the size of objects, their member layout does.

What mandates the default constructor is the QVector constructors and methods that allow setting a size. That constructor will be used to initialize all those elements.

Without a default constructor, the values would be uninitialized and pretty much useless garbage data.

Having a default constructor tells you "I can construct meaningful objects of that type without passing any parameters".

It doesn't matter if you personally don't use any of those constructors or methods, it is required by the class implementation nonetheless.

As of why it also uses construction for what is basically reserved memory, that would probably merit a question on its own, because at least for me there is no good reason to do that at all, it seems like a potential overhead.

Upvotes: 2

MrEricSir
MrEricSir

Reputation: 8242

In C++ specifying any constructor for a struct (or class) instructs the compiler not to provide a default constructor automatically. It's a quirk of the language.

This becomes a problem for container classes such as QVector which can resize on the fly (either internally or explicitly.) When allocating new objects to fill the space it will call the default constructor -- but if there isn't a default constructor available a compile-time error will occur.

The problem can be solved by specifying a default constructor, even if it doesn't do anything, for example:

ChunkRequest() {}

Upvotes: 4

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