I need help adding an array to a vector in C++

Question

I'm following along with the OpenGL Super Bible 5th edition, and they define a vector(vector as in math) as

typedef float   M3DVector3f[3];

I'm trying to add an instance of this to an std::vector(the 're sizable array' in c++), however I keep getting an error saying:

array initialization needs curly braces

Full Error

The way I defined the std::vector and the way I'm adding to it is:

std::vector<M3DVector3f> vertices;

float vertex[3];
sscanf_s(line.c_str(), "%*s %f %f %f", &vertex[0], &vertex[1], &vertex[2]);

M3DVector3f v = {vertex[0], vertex[1], vertex[3]};

vertices.push_back(v);

I've gathered that the problem is with the vertices.push_back(v) call, because I don't get an error when I comment that out. Could someone explain to me and help me figure out why it won't let me add this vector to my vector?

I just want to say thanks to everyone to who helped me. I took GMan's advice and created a new class for all of my Vectors. This worked out a lot better because it let me overload operators so now my code looks all clean and tidy. :) — krej
– krej, Commented Nov 23, 2010 at 5:17

GManNickG · Accepted Answer · 2010-11-23 02:33:46Z

4

Arrays cannot be (directly) copied or assigned, and standard containers requires types to be copyable and assignable.

However, you can (and probably should) do this instead:

struct M3DVector3f // a basic vector class
{
    float x;
    float y;
    float z;
};

Which is copyable and just as usable.

answered Nov 23, 2010 at 2:33

GManNickG

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7 Comments

In silico Over a year ago

It's also more readable as it shows what each component represents.

Chubsdad Over a year ago

why doesn't the implementation throw error while declaring 'vertices' instead, if it does not meet the requirements of the container?

GManNickG Over a year ago

@Chubsdad: Because it doesn't have to. The use of these requirements doesn't occur until the instantiation of the functions that use it. (This is even more important in C++0x, where element requirements are applied on a per-function basis.)

Chubsdad Over a year ago

@GMan: From a design point of view, I would love to see the declaration itself fail rather than pushing the surprise element to the actual point of usage, especially when almost nothing can be done with such a declaration e.g. for some trivial functions like size(). Looks like it is a limitation of the language mechanism

GManNickG Over a year ago

@Chubsdad: It could be done (by inheriting from a mixin class that does a variety of static_assert's and concept checks), but again after C++03 such behavior would be wrong.

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Chubsdad · Accepted Answer · 2010-11-23 02:40:02Z

1

For starters, vertex[3] above should be vertex[2] as vertex[3] lies outside the upper bound of the array. However, that is not the crux of the issue here. The template instantiation of vector is being done with an array type. There is no default copy constructor, per se, for array types that will perform any copy deeper than the simple pointer copy. You may find that this works better if instead you try:

std::vector<M3DVector3f*> vertices;
M3DVector3f* v = new M3DVector3f;

sscanf_s(line.c_str(), "%*s %f %f %f", &((*v)[0]), &((*v)[1]), &((*v)[2]));

vertices.push_back(v);

There may be another, more elegant solution utilizing smart pointers to clean up each entry in the vector when you go to destroy it later. :-)

edited Nov 23, 2010 at 2:40

Chubsdad

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answered Nov 23, 2010 at 2:31

Dave

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1 Comment

krej Over a year ago

I tried this, however the use of the 'new' gives me this error: cannot convert from 'float ' to 'M3DVector3f ()'

Chubsdad · Accepted Answer · 2010-11-23 02:53:14Z

0

Clarifying a bit on GMan's response above

The following code also throws the same error as you have got while doing a push_back.

typedef float MYBUF[3];

int main(){
   MYBUF m1;

   MYBUF m2(m1);     // Same Error as in OP
}

This is exactly what the vector::push_back does with any T. It tries to make a copy (and hence the term copyable) of the input argument and such a copy is not allowed by the C++ language for array types.

answered Nov 23, 2010 at 2:53

Chubsdad

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Comments

Tony Delroy · Accepted Answer · 2010-11-23 04:03:40Z

0

Not sure how well it will suit you, but you could experiment with some less direct solution, like the following (which compiles without error):

#include <iostream>
#include <vector>

typedef float M3DVector3f[3];

struct X
{
    X(M3DVector3f& p) { x_[0] = p[0]; x_[1] = p[1]; x_[2] = p[2]; }
    operator M3DVector3f&() { return x_; }
    M3DVector3f x_;
};

int main()
{
    M3DVector3f v = { 1, 2, 3 };
    std::vector<X> xs;
    xs.push_back(v);
}

answered Nov 23, 2010 at 4:03

Tony Delroy

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