I'm looking to build a 1D std::array of structs, each of whose size is 16 bytes. The 1D array is a flattening of a class representing a 3D array (basically an std::array wrapper that has some 3D specific operators and other fluff). My first attempt is an array of size 256 x 256 x 32, so roughly 35MB, which throws a SIGSEGV error.
A simplified example of everything looks like this:
Structs.cpp
struct Coord {
int x;
int y;
int z;
Coord() { }
Coord(int x_, int y_, int z_) { x = x_; y = y_; z = z_; }
}
int TR (int arg) {
// ... Some transformation
}
struct MyStruct {
Coord position;
int terrain;
MyStruct() { }
MyStruct(int x_, int y_, int z_, int terrain_) {
terrain = terrain_;
position = Coord(TR(x_), TR(y_), TR(z_));
}
}
ArrayWrapper.hpp
#include <array>
template <typename T, int HSIZE, int VSIZE> struct ArrayWrapper {
private:
std::array<T, HSIZE*HSIZE*VSIZE> narray;
public:
void set(T obj, int x, int y, int z) {
narray[x + z*HSIZE + y*HSIZE*HSIZE] = obj;
}
T& operator() (int x, int y, int z) {
return narray.at(x + z*HSIZE + y*HSIZE*HSIZE);
}
CoordinateMap.cpp
#include "ArrayWrapper.hpp"
#include "Structs.cpp"
const int HSIZE = 256;
const int VSIZE = 32;
class CMap {
private:
ArrayWrapper<MyStruct, HSIZE, VSIZE>* coords_ = new ArrayWrapper<MyStruct, HSIZE, VSIZE>;
ArrayWrapper<MyStruct, HSIZE, VSIZE> coords = *coords_;
public:
// ... Getter, setter, and a bunch of other methods,
~CMap() { delete coords; }
}
If I anywhere try to say CMap something; I get a SIGSEGV. I know that the stack is relatively small, so I'm attempting to allocate this structure on the heap by using new. Many people (on this site and others) say "Finding a large range of contiguous memory is difficult, even if it's on the heap," but don't give an indication of what a reasonable expectation of the size of contiguous memory is. I would think 32MB in a modern-day computer is doable.
What might be throwing a Seg. fault here?
ArrayWrapper<MyStruct, HSIZE, VSIZE> coords = *coords_;
Should be...
ArrayWrapper<MyStruct, HSIZE, VSIZE>& coords = *coords_;
... which makes sense. The 1st line is making a copy of coords_' reference, which, in this case, defeats the purpose of using new, since that copy is put on the stack.
Related
I am currently porting some C code I wrote to C++ for fun. I am struggling with a malloc() call I make in C, with h and w being constants for simplicity reasons, but later exchanged with runtime constants:
double (*g2)[h][w] = malloc(h * w * sizeof(double));
In C, this is an implicit conversion of a void*, and this of course doesn't fly with C++.
I already tried casting this with reinterpret_cast<double[h][w]>, but this is still an invalid cast.
I was wondering, how can I make this work in C++ since this would save me a lot of work?
As an alternative I'll probably use a matrix class with indirection:
struct Matrix : std::vector<double> {
unsigned matSize;
std::vector<double*> indirection;
Matrix() : matSize(0) {}
Matrix(unsigned n) : matSize(n) {
resize(n*n);
indirection.resize(n);
for(unsigned i = 0; i < n; ++i) {
indirection[i] = &(*this)[i*n];
}
}
double& operator()(unsigned i, unsigned j) {
return indirection[i][j];
}
const double& operator()(unsigned i, unsigned j) const {
return indirection[i][j];
}
};
Porting involves more than just making it work, line by line, so:
C:
double (*g2)[h][w] = malloc(h * w * sizeof(double));
...
g2[y][x] = ...;
C++:
std::vector<double> g2(h*w);
...
g2[y+x*h] = ...; // or
g2[y*w+x] = ...;
Using that syntax is inconvenient for accessing elements so you might want to wrap it inside a simple class. Example:
#include <iostream>
#include <iterator>
#include <vector>
class arr2d {
public:
arr2d(size_t h, size_t w) : data_(h * w), w_(w) {}
inline double& operator()(size_t y, size_t x) {
return data_[y * w_ + x];
}
inline double operator()(size_t y, size_t x) const {
return data_[y * w_ + x];
}
// getting pointer to a row
inline double* operator[](size_t y) {
return &data_[y * w_];
}
inline double const* operator[](size_t y) const {
return &data_[y * w_];
}
inline size_t width() const { return w_; }
private:
std::vector<double> data_;
size_t w_;
};
int main() {
arr2d g2(3, 4);
g2(2, 3) = 3.14159;
// alternative access:
g2[1][2] = 1.23456;
std::cout << g2[2][3] << "\n";
double* row = g2[2];
std::copy(row, row + g2.width(), std::ostream_iterator<double>(std::cout, ", "));
std::cout << "\n";
}
Output:
3.14159
0, 0, 0, 3.14159,
A non-initializing version could look like:
class arr2d {
public:
arr2d(size_t h, size_t w) : data_(new double[w * h]), w_(w) {}
inline double& operator()(size_t y, size_t x) { return data_[y * w_ + x]; }
inline double operator()(size_t y, size_t x) const { return data_[y * w_ + x]; }
inline double* operator[](size_t y) { return &data_[y * w_]; }
inline double const* operator[](size_t y) const { return &data_[y * w_]; }
inline size_t width() const { return w_; }
private:
std::unique_ptr<double[]> data_;
size_t w_;
};
But note that the
std::copy(row, row + g2.width(), std::ostream_iterator<double>(std::cout, ", "));
from the first example would lead to undefined behaviour.
Also note that this version will delete the copy constructor and copy assignment operator. You'll have to implement them yourself if you need them.
The creation time for the non-initializing version is of course hard to beat with any initializing version, but for access times, one might think that a lookup table, or indirection as you call it, for the rows would speed up things compared to doing the multiplication and addition in one go.
My results:
8x8 http://quick-bench.com/f8zcnU9P8oKwMUwLRXYKZnLtcLM
1024x1024 http://quick-bench.com/0B2rQeUkl-WoqGeG-iS1hdP4ah8
4096x4096 http://quick-bench.com/c_pGFmB2C9_B3r3aRl7cDK6BlxU
It seems to vary. The lookup version is faster for the 4096x4096 matrix but the naive version is faster for the two smaller ones. You need to compare using sizes close to what you'll be using and also check with different compilers. I sometimes get completely opposite "winners" when changing compiler.
Since you don't mind inheriting from std::vector or keeping extra data for a lookup-table, this could be an option. It seems to outperform the other versions slightly.
class arr2d : protected std::vector<double*> {
public:
using std::vector<double*>::operator[]; // "row" accessor from base class
arr2d(size_t h, size_t w) :
std::vector<double*>(h),
data_(new double[w * h]),
w_(w),
h_(h)
{
for(size_t y = 0; y < h; ++y)
(*this)[y] = &data_[y * w];
}
inline size_t width() const { return w_; }
inline size_t height() const { return h_; }
private:
std::unique_ptr<double[]> data_;
size_t w_, h_;
};
Here are Philipp-P's (OP:s) own measurements for the different 2D-array implementations:
8x8 http://quick-bench.com/vMS6a9F_KrUf97acWltjV5CFhLY
1024x1024 http://quick-bench.com/A8a2UKyHaiGMCrf3uranwOCwmkA
4096x4096 http://quick-bench.com/XmYQc0kAUWU23V3Go0Lucioi_Rg
Results for 5-point stencil code for the same versions:
8x8 http://quick-bench.com/in_ZQTbbhur0I4mu-NIquT4c0ew
1024x1024 http://quick-bench.com/tULLumHZeCmC0HUSfED2K4nEGG8
4096x4096 http://quick-bench.com/_MRNRZ03Favx91-5IXnxGNpRNwQ
In C++ it's not advised to manually allocate memory unless necessary. Let the standard library and templates do the work for you.
They can be very useful and are great to learn if you want to get into C++! You can save a lot of time this way and write some better code.
For example, what is this data type used for? If it fits for your usage, you may instead consider creating a 2D array using std::array:
std::array<std::array<double, w>, h>
If you need to re-size the arrays regularly, std::vector could be used instead. It has effectively the same performance as an array, given that is is all it is under the hood. You can reserve() or resize() as necessary and push_back uses a 1.5x increasing scheme and is good at its job.
EDIT: Since size is known, arrays may be better here. Taken suggestion from comments.
I recommend you using std::vector. Just wrap it if you want 2D array syntax.
#include <iostream>
#include <vector>
class Array2D {
std::vector<double> _v;
size_t _width;
size_t _height;
public:
Array2D(size_t height, size_t width, double initVal = 0.0)
: _v(width * height, initVal),
_width(width),
_height(height)
{}
double* operator[](size_t y) {
return _v.data() + y * _width;
}
};
int main(int, char**) {
size_t rows = 5;
size_t cols = 3;
Array2D a(rows, cols, 0.2);
for (size_t i = 0; i < cols; ++i)
a[4][i] = -0.1 * i;
std::cout << a[4][2] << std::endl; //-0.2
return 0;
}
You can do this, which should work in both C and C++:
double *g2 = (double*) malloc(h * w * sizeof(double));
Though, as others have stated, this is not the way to approach this issue in C++ in general. For instance, you should use a std::vector instead:
#include <vector>
std::vector<double> g2(h * w);
In both cases, you end up with a dynamically allocated 2D array of doubles in a single contiguous block of memory. And as such, you need to then use g2[(row*w)+col] syntax to access the individual elements, where 0 <= row < h and 0 <= col < w.
I have this code:
class Vector3
{
public:
Vector3() : x(values[0]), y(values[1]), z(values[2])
{ x = y = z = 0; }
float& x;
float& y;
float& z;
private:
float[3] values;
};
class Model
{
public:
Vector3 vertices[64];
};
I'm doing this vector class because I want to deal with the values as X, Y, Z in the code, but for some operations I need a contiguous array of values to be passed to a function.
So the whole array of vertices[64] need to be [x0][y0][z0][x1][y1][z1][x2][y2][z2] etc.
But if I do this:
//Get first address:
void* firstAddress = &vertices[0];
//Or
void* firstAddress = vertices;
I don't have the contiguous array as I need it (the data is all messed up), and I'm guessing it's because of the pointers I have in the Vector3 class.
Is there any way I can do get this functionality that I want? (Having a single array of float but dealing with values as x,y,z)
Firstly, the Standard doesn't define how references should be implemented, but they'll almost certainly occupy actual memory in your class much a pointer members would, ruining the contiguous data packing you're hoping for....
If your focus is more on the vertices container, and you just want x/y/z member access to elements in it, then you could try something like:
template <size_t N>
class Vertices
{
public:
class Proxy
{
public:
Proxy(float* p) : x(p[0]), y(p[1]), z(p[2]) { }
float& x;
float& y;
float& z;
};
Proxy operator[](size_t n) { return Proxy(&d_[n * 3]); }
const Proxy operator[](size_t n) const { return Proxy(&d_[n * 3]); }
private:
float d_[N * 3];
};
You could have member functions instead:
class V3
{
float data[3];
public:
V3() : data{0,0,0} {}
float & x() { return data[0]; }
float & y() { return data[1]; }
float & z() { return data[2]; }
};
You could also omit the constructor and have an aggregate, if that's more suitable.
What you have and array of values pointers (pointing to arrays of 3 floats) with 3 float references (x, y, z). You probably ant something more like:
float & x() { return values[0]; }
float & y() { return values[1]; }
float & z() { return values[2]; }
If I understand your requirement for continuous data in the Model, then your Vector is really an alias to a specific triple of data in the model. No float storage required.
class Model
class Vector
Vector(Model *model_, size_t idx_) : model(model_),idx(idx_) { };
Model *model;
size_t idx;
float & x() { return model->data[3*idx]; }
float & y() { return model->data[3*idx+1]; }
float & z() { return model->data[3*idx+2]; }
float data[64 * 3];
Vector vectors[64];
Model() {
...
for( size_t ii = 0; ii < 64; ii++ ) {
vectors[ii] = new Vector(this,ii);
}
Vector & vector(size_t idx) { return vectors[ii]; }
For a guaranteed contiguous array you need to either copy the data into the array, or use compiler-specific guarantees about memory layout, in particular that
there will be no padding,
and if you want to also access triplets of the array as Vector3 instances, that
accessing a Vector3 at arbitrary address will not cause a trap or inefficiency (we're into alignment here).
It so happens that some common libraries such as OpenCV do make such assumptions for their internal image buffers.
But I'm not entirely sure that the code I've seen has not been platform-adapted. So, in practice you have these choices:
copy the data contiguously to an array (or from it), and/or
use an existing library that provides this kind of functionality, such as OpenCV.
Note that using member functions instead of references buys you nothing wrt. to the contiguous array problem, but it does make the Vector3 potentially assignable.
On further reflection, i was maybe too trigger-happy writing the above. For if you can guarantee total size 3*sizeof(float), and that's almost a given (just get rid of those references), then you are guaranteed that you can access a Vector3 at any address that can hold a float, since C++ guarantees arrays with no padding, and since in such an array a Vector3 can end up at any address that can hold a float. So the in-practice problem reduces to making a decision about supporting compilers or compiler configurations that are unable to make Vector3 of size 3*sizeof(float).
I.e.
struct Vector3
{
float x, y, z;
auto operator[]( int i ) -> float& { return (&x)[i]; }
auto operator[]( int i ) const -> float const& { return (&x)[i]; }
};
static_assert( sizeof( Vector3 ) == 3*sizeof( float ), "Ungood Vector3 size" );
using the fact that members with no intervening access specifier are guaranteed to be in increasing address order.
Disclaimer: off the cuff code, not touched by compiler's hands.
If you keep your Vector3 class a POD you should be able to simply cast your vertices to a float array:
struct Vector3 {
float x;
float y;
float z;
};
class Model
{
public:
Vector3 vertices[64];
float* data() {
return reinterpret_cast<float*>(vertices);
}
};
int main() {
Model m;
for(int i = 0; i < 64; ++i) {
m.vertices[i] = {10+i,100+i,1000+i};
}
float *data = m.data();
for(int i= 0; i < 64*3; ++i) {
std::cout << data[i] << ", ";
}
}
Only problem could be the Allignment of the structure, but should you use c++11, there is a standard way of alligning the structure, using allignasalignas(alignof(float[3])). I don't know if this is really required.
Also, c++11 gives quite a few options on what to do with Vector3, while still considering it as a POD type.
I need to create a function that has a parameter which is a multi-dimensional array with two dimensions being user-specified, e.g.
int function(int a, int b, int array[a][b])
{
...
}
How would I do that in C++ ?
Are the dimensions known at compile-time? In that case, turn them into template parameters and pass the array by reference:
template<int a, int b>
int function(int(&array)[a][b])
{
...
}
Example client code:
int x[3][7];
function(x);
int y[6][2];
function(y);
Assuming the dimensions are not known at compile time, you emulate a two dimensional array with a one dimensional array:
int& getat(int x, int y, int r, int c, int *array) {return array[y*c+x];}
int function(int a, int b, int *array) {
getat(4, 2, a, b, array) = 32; //array[4,2] = 32
}
or, for safety, wrap it all in a class:
template <class T>
class array2d {
std::vector<T> data;
unsigned cols, rows;
public:
array2d() : data(), cols(0), rows(0) {}
array2d(unsigned c, unsigned r) : data(c*r), cols(c), rows(r) {}
T& operator()(unsigned c, unsigned r) {
assert(c<cols&&r<rows);
return data[r*cols+c];
}
};
or, best yet, use Boost's Multidimensional Array, which will be better than anything mere mortals could write.
I'm not sure if this work, because your question and code are not the same, according to your code the function can have 3 parameters, so this would work:
int function(int a, int b, int** &array)
{
array = new int*[a];
for (int i =0;i<a;i++)
array[i] = new int[b];
// I don't know why you are returning int, probably doing something here....
}
However your question says that your function can take only one parameter, so:
if the dimensions are known at compile time, then Fred's Answer is the best (it charmed me in fact! :) ).
if not, I can't see any possible solution that allows passing more than one user-specified value other than encapsulating all these values in one object.
Like this:
class Foo {
public:
Foo(int d1, int d2)
{ a = d1; b = d2; }
int a,b;
int** array;
};
int function(Foo &f)
{
f.array = new int*[f.a];
for (int i = 0;i<f.a;i++)
f.array[i] = new int[f.b];
// I don't know why you are returning int, probably doing something here....
}
Though I find it a bad idea, in fact the function could be a parameterless method instead:
class Foo {
public:
Foo(int d1, int d2)
{ a = d1; b = d2; }
void Create() // Or could do this right in the Constructor
{
array = new int*[a];
for (int i = 0;i<a;i++)
array[i] = new int[b];
}
private:
int a,b;
int** array;
};
Still this is a bad idea, because you are reinventing the wheel, as there are a perfect class in the STL to do all the work for you:
vector< vector<int> > v; // Now v is a 2D array
It's been a long time since I worked with C++, but I have a class that uses 3-dimensional data and I can't figure out how I can make this work. I need the sizes of the dimensions to be defined in the constructor. I tried this in the header:
class CImage
{
public:
float values[][][];
...
}
and this in the constructor:
CImage::CImage(int cols, int rows, int depth)
{
values[cols][rows][depth];
}
but this returns the error: "declaration of `values' as multidimensional array must have bounds for all dimensions except the first".
Also using this in the constructor does not work:
values = new float[cols][rows][depth];
I also tried using vector, but without much success. Header:
vector<vector<vector<float> > > values;
Nothing in constructor. No compiler errors, but when I try to set a value:
values[c][r][d] = value;
the program crashes.
It seems so basic, but I just can't figure it out...
The program crashes when accessing that vector because it is empty, i.e. there are no elements at those indexes.
The best way to go about this is to make a linear, one-dimensional, vector (or even an array), and access it with a pair of operator()'s, see C++FAQ Lite for details. Or use boost::multi_array.
For example:
#include <vector>
#include <iostream>
class CImage
{
int X, Y, Z;
std::vector<float> values;
public:
CImage(int cols, int rows, int depth)
: X(cols), Y(rows), Z(depth),
values(cols*rows*depth) {}
float operator()(int x, int y, int z) const
{
return values[Z*Y*x + Z*y + z];
// or you lay it out differently in memory
// if you please, transparent to the user:
// return values[x + X*y + X*Y*z];
}
float& operator()(int x, int y, int z)
{
return values[Z*Y*x + Z*y + z];
// likewise, in a different layout
// return values[x + X*y + X*Y*z];
}
};
int main()
{
CImage ci(3,3,3);
ci(2,2,2) = 7.0;
std::cout << ci(2,2,2) << '\n';
}
Just to poiny out why Cubbi is right, this would be the constructor of the 3d-vector:
vector<vector<vector<float>>> values;
// create vector [dim0][dim1][dim2]
// init value: init
size_t dim0 = 3;
size_t dim1 = 3;
size_t dim2 = 3;
float init = 0.42f;
values = vector<vector<vector<float>>>
(
dim0,
vector<vector<float>>
(
dim1,
vector<float>
(
dim0,
init
)
)
);
Nice, isn't it?
Besides, you cannot declare a float values[][][]; because arrays reside on the stack, so the compiler has to know at compile time what size that array has (exception to this: C99 variable length arrays, but this would'nt be C++). You could declare a float*** values; (using new float[c][r][d]; in the ctor), but this is awful, too.
There's no way to do something like this, in C++ is there?
union {
{
Scalar x, y;
}
Scalar v[2];
};
Where x == v[0] and y == v[1]?
Since you are using C++ and not C, and since they are of the same types, why not just make x a reference to v[0] and y a reference to v[1]
How about
union {
struct {
int x;
int y;
};
int v[2];
};
edit:
union a {
struct b { int first, second; } bee;
int v[2];
};
Ugly, but that's more accurate
Try this:
template<class T>
struct U1
{
U1();
T v[2];
T& x;
T& y;
};
template<class T>
U1<T>::U1()
:x(v[0])
,y(v[1])
{}
int main()
{
U1<int> data;
data.x = 1;
data.y = 2;
}
I've used something like this before. I'm not sure its 100% OK by the standard, but it seems to be OK with any compilers I've needed to use it on.
struct Vec2
{
float x;
float y;
float& operator[](int i) { return *(&x+i); }
};
You can add bounds checking etc to operator[] if you want ( you probably should want) and you can provide a const version of operator[] too.
If you're concerned about padding (and don't want to add the appropriate platform specific bits to force the struct to be unpadded) then you can use:
struct Vec2
{
float x;
float y;
float& operator[](int i) {
assert(i>=0);
assert(i<2);
return (i==0)?x:y;
}
const float& operator[](int i) const {
assert(i>=0);
assert(i<2);
return (i==0)?x:y;
}
};
I was looking for a similair thing and eventually came up with a solution.
I was looking to have a data storage object that I could use as both an array of values and as individual values (for end-user flexibility in writing Arduino libraries).
Here is what I came up with:
class data{
float _array[3];
public:
float& X = _array[0];
float& Y = _array[1];
float& Z = _array[2];
float& operator[](int index){
if (index >= 3) return _array[0]; //Make this action whatever you want...
return _array[index];
}
float* operator&(){return _array;}
};
int main(){
data Test_Vector;
Test_Vector[0] = 1.23; Test_Vector[1] = 2.34; Test_Vector[2] = 3.45;
cout<<"Member X = "<<Test_Vector.X;
cout<<"Member Y = "<<Test_Vector.Y;
cout<<"Member Z = "<<Test_Vector.Z;
float* vector_array = &Test_Vector;
cout<<"Array = {"<<vector_array[0]<<", "<<vector_array[1]<<", "<<vector_array[2]<<"}";
}
Thanks to Operator overloading, we can use the data object as if was an array and we can use it for pass-by-reference in function calls (just like an array)!
If someone with More C++ experience has a better way of applying this end product, I would love to see it!
EDIT: Changed up the code to be more cross-platform friendly
Given your example:
union
{
struct
{
Scalar x, y;
};
Scalar v[2];
};
As others have noted, in general, the standard does not guarantee that there will be no padding between x and y, and actually compilers inserting padding in structures is pretty common behavior.
On the other hand, with solutions like:
struct U
{
int v[2];
int& x;
int& y;
};
U::U()
: x(v[0])
, y(v[1])
{}
what I don't like mainly is the fact that I have to mention x, y twice. For cases where I have more than just a few elements (say 10), this becomes much less readable and harder to maintain - e.g. if you want to change the order of x,y then you have to change the indexes below too (well not mandatory but otherwise order in memory wouldn't match order of fields, which would not be recommended). Also, U can no longer be a POD since it needs a user-defined constructor. And finally, the x & y references consume additional memory.
Hence, the (acceptable for me) compromise I've come up with is:
struct Point
{
enum CoordType
{
X,
Y,
COUNT
};
int coords[CoordType::COUNT];
};
typedef Point::CoordType PtCoord;
With this you can then do:
Point p;
for ( int i = 0; i < PtCoord::COUNT; i++ )
p.coords[i] = 100;
std::cout << p.coords[PtCoord::X] << " " << p.coords[PtCoord::Y] << std::endl;
// 100 100
A bit sophisticated but I prefer this over the references suggestion.
Depending on what "Scalar" is, yes, you can do that in C++. The syntax is almost exactly (maybe even exactly exactly, but I'm rusty on unions) what you wrote in your example. It's the same as C, except there are restrictions on the types that can be in the unions (IIRC they must have a default constructor). Here's the relevant Wikipedia article.
With C++11 you have anonymous unions and structs which just export their definitions to the enclosing scope, so you can do this:
typedef int Scalar;
struct Vector
{
union
{
struct
{
Scalar x, y;
};
Scalar v[2];
};
};