I'm pretty sure this is possible, because I'm pretty sure I've seen it done. I think it is awesome, but I will gladly accept answers along the lines of "this is a terrible idea because ____".
Say we have a basic struct.
struct vertex
{
float x, y, z;
};
Now, I want to implement aliases on these variables.
vertex pos;
vertex col;
vertex arr;
pos.x = 0.0f; pos.y = 0.5f; pos.z = 1.0f;
col.r = 0.0f; col.g = 0.5f; col.b = 1.0f;
arr[0] = 0.0f; arr[1] = 0.5f; arr[2] = 1.0f;
Ideally the third syntax would be indistinguishable from an array. That is, if I sent arr as a reference parameter to a function expecting an array of floats into which it will store data (eg many of the OpenGL glGet functions), it would work fine.
What do you think? Possible? Possible but stupid?
What I would do is make accessors:
struct Vertex {
float& r() { return values[0]; }
float& g() { return values[1]; }
float& b() { return values[2]; }
float& x() { return values[0]; }
float& y() { return values[1]; }
float& z() { return values[2]; }
float operator [] (unsigned i) const { return this->values_[i]; }
float& operator [] (unsigned i) { return this->values_[i]; }
operator float*() const { return this->values_; }
private:
float[3] values_;
}
Nameless nested structs in a union are not standard C++. This, however, should work:
struct Vertex
{
private:
typedef float Vertex::* const vert[3];
static const vert v;
public:
typedef size_t size_type;
float x, y, z;
const float& operator[](size_type i) const {
return this->*v[i];
}
float& operator[](size_type i) {
return this->*v[i];
}
};
const Vertex::vert Vertex::v = {&Vertex::x, &Vertex::y, &Vertex::z};
EDIT: A little more information. The struct uses an array of 3 pointer-to-data-members to access the data in the overloaded [] operators.
The line "typedef float Vertex::* const vert" means that vert is a pointer to a float member of the Vertex struct. The [3] means that it's an array of 3 of these. In the overloaded operator[], this array is indexed and the pointer-to-data-member is dereferenced and the value returned.
Additionally, this method should work regardless of packing issues - the compiler is free to pad the Vertex structure however it likes and it'll still work just fine. An anonymous union will run into problems if the floats are packed differently.
Use a union?
union vertex
{
struct { float x, y, z; };
struct { float r, g, b; };
float arr[3];
};
I wouldn't recommend it - it will lead to confusion.
Added:
As noted by Adrian in his answer, this union with anonymous struct members is not supported by ISO C++. It works in GNU G++ (with complaints about not being supported when you turn on '-Wall -ansi -pedantic'). It is reminiscent of the pre-pre-standard C days (pre-K&R 1st Edn), when structure element names had to be unique across all structures, and you could use contracted notations to get to an offset within the structure, and you could use member names from other structure types - a form of anarchy. By the time I started using C (a long time ago, but post-K&R1), that was already historical usage.
The notation shown with anonymous union members (for the two structures) is supported by C11 (ISO/IEC 9899:2011), but not by earlier versions of the C standard. Section 9.5 of ISO/IEC 14882:2011 (C++11) provides for anonymous unions, but GNU g++ (4.9.1) does not accept the code shown with -pedantic, identifying "warning: ISO C++ prohibits anonymous structs [-Wpedantic]".
Since the idea will lead to confusion, I'm not particularly concerned that it isn't standard; I would not use the mechanism for this task (and I'd be leery of using anonymous structures in a union even if it was beneficial).
A concern was raised:
The three (x-y-z, r-g-b and the array) do not necessarily align.
It is a union with three elements; the three elements start at the same address. The first two are structures containing 3 float values. There's no inheritance and there are no virtual functions to give different layouts, etc. The structures will be laid out with the three elements contiguous (in practice, even if the standard permits padding). The array also starts at the same address, and subject to 'no padding' in the structures, the elements overlap the two structures. I really don't see that there would be a problem.
References?
template<typename T>
struct vertex {
vertex() :
r(data[0]), g(data[1]), b(data[2]),
x(data[0]), y(data[1]), z(data[2])
{
}
T *operator *() {
return data;
}
const T *operator *() const {
return data;
}
T data[3];
T &r, &g, &b;
T &x, &y, &z;
};
You can get this with a union as others have mentioned. Overloading color and position onto the same structure like this may not be a good idea ( for example, adding two colors usually means you want to saturate to 1.0, whereas adding vectors happens linearly ), but overlaying a float[] on top of them like that is perfectly fine and a well accepted means of interchanging data with GL/DirectX/etc.
I recommend you avoid referring to the same member by different aliases in the same function scope, though, because this will drive you into a nasty hardware stall called a load-hit-store. In particular, avoid this if you can:
vector foo;
foo.x = 1.0f;
return foo[0] + foo[1];
Following structure will have the requested behavior:
struct vertex
{
private:
float data[3];
public:
float &x, &y, &z;
float &r, &g, &b;
vertex() : x(data[0]), y(data[1]), z(data[2]), r(data[0]), g(data[1]), b(data[2]) {
}
float& operator [](int i) {
return data[i];
}
};
I guess you can do some macro magic to get what you want.
But that will look ugly. Why do you want to use same struct, vertex for 3 different types? Why can't you define class for color?
Also keep in mind that vertex and color are not same. If you change something to vertex, that will affect the color also, if you have the same class for both.
I am not sure whether I understood the question correctly. But it looks like you need to overload the operator[] to provide array like access to your struct/class. See the example mentioned here: Operator overloading
I have a template and two Vector classes below, one crazy, one sane. The template implements a simple fixed at compile time array of values. It is designed for subclassing and uses a protected array variable to avoid you having to jump through hoops to access the array. (Some folks might not like such a design. I say, if your subclasses are calling your overloaded operators, coupling might be a good idea.)
The crazy class allows you to have member variables called x, y, z and it acts like an array for calls to glGetFloatV. The sane one just has accessor functions x(), y(), z() and still works with glGetFloatV. You can use either class as a basis for other vector objects you might pass to the OpenGL library. Although the classes below are specific to points, you can obviously just do a search/replace to turn them into a rgb color classes.
The crazy class is crazy because the cost of the syntactic sugar vec.x instead of vec.x() is 3 reference variables. That could take up a lot of space in a large application. Use the simpler sane version.
template <typename T, int N>
class FixedVector {
protected:
T arr[N];
public:
FixedVector();
FixedVector(const T* a) {
for (int i = 0; i < N; ++i) {
arr[i] = a[i];
}
}
FixedVector(const T& other) {
for (int i = 0; i < N; ++i) {
arr[i] = other.arr[i];
}
}
FixedVector& operator=(const T& other) {
for (int i = 0; i < N; ++i) {
arr[i] = other.arr[i];
}
return *this;
}
T* operator&() { return arr; }
const T* operator&() const { return arr; }
T& operator[](int ofs) {
assert(ofs >= 0 && ofs < N);
return arr[ofs];
}
const T& operator[](int ofs) const {
assert(ofs >= 0 && ofs < N);
return arr[ofs];
}
};
class CrazyPoint : public FixedVector<float, 3> {
public:
float &x, &y, &z;
CrazyPoint()
: x(arr[0]), y(arr[1]), z(arr[2])
{ arr[0] = arr[1] = arr[2] = 0.0; }
CrazyPoint(const float* a)
: x(arr[0]), y(arr[1]), z(arr[2])
{
arr[0] = a[0];
arr[1] = a[1];
arr[2] = a[2];
}
CrazyPoint(float a, float b, float c)
: x(a), y(b), z(c)
{
arr[0] = a;
arr[1] = b;
arr[2] = c;
}
};
class SanePoint : public FixedVector<float, 3> {
public:
float& x() { return arr[0]; }
float& y() { return arr[1]; }
float& z() { return arr[2]; }
SanePoint() { arr[0] = arr[1] = arr[2] = 0.0; }
SanePoint(float a, float b, float c)
{
arr[0] = a;
arr[1] = b;
arr[2] = c;
}
};
// usage
SanePoint normal;
glGetFloatV(GL_CURRENT_NORMAL, &normal);
Bad idea in my opinion, at least for the example given: the downside is that, for just about any solution to this, you're probably going to be able to freely assign "rgb" instances to/from "xyz" instances, which is probably rarely sensible or correct. ie you risk giving up some useful type safety.
Personally, for the example you give, I'd subclass rgb and xyz types from a base boost::array<float,3> or similar. So both of them inherit operator[], can be passed to functions expecting arrays, and passed with more type safety to things expecting colours/coordinates. It's often you want to treat an xyz or an rgb as an array, but rare you want to treat an xyz as an rgb or vice-versa. (rgb IS-A array: OK. xyz IS-A array: OK. rgb IS-A xyz ???? I don't think so!)
Of course that means access to x,y,z & r,g,b needs to be by accessor (forwarding to the appropriate operator[](...) ) rather than direct to the member. (You'd need C#'s properties for that).
You can try adding references to variables, like this:
struct test {
float x, y, z;
float &r, &g, &b;
test() : r(x), g(y), b(z) {}
};
But your structure gets bigger (from 12 bytes to 40 bytes).
To use [] on it, use overloading of operator[], as mentioned before.
Just a warning about using reference members pointing to value members. You need to define a copy constructor (and possibly also assignment operator), if you ever copy such an object (like transfer it by value). The default copy constructor will leave you with a copy whose reference members point to the value members of the original object, not the ones of the new object. This is certainly not something you want.
Considering you also end up with larger objects, as already pointed out, I think using accessor methods is to be preferred over reference members.
I think the poster was looking for something very simple, with no performance overhead - as you would want with something like a 3D vector class. So adding virtual functions (vtable indirection cost), additional members (memory cost), unions (each new name may require a recompile of all users), or even preprocessor magic (increased program size, cross-type equivalency) is undesirable.
Real world use case would be to take a templated Vector3 class (which could be double-based or float-based) and apply it to other scenarios in a user-friendly manner. It is likely defined with [x, y, z] members, but if you want to use it for rotations, you might want [psi, theta, phi], for speeds [dx, dy, dz], etc.
For the entire type, you can use the following to alias it at compile time:
using Rotation3 = Vector3;
But there appears to be nothing as simple or performant that you can do for aliasing the underlying variables, right?
Related
Sometimes there is a need to have a Vector3f class, which has x, y and z members, and can be indexed as a float[3] array at the same time (there are several questions here at SO already about this).
Something like:
struct Vector3f {
float data[3];
float &x = data[0];
float &y = data[1];
float &z = data[2];
};
With this, we can write this:
Vector3f v;
v.x = 2.0f;
v.y = 3.0f;
v.z = 4.0f;
glVertex3fv(v.data);
But this implementation is bad, because references take space in the struct (which is quite unfortunate. I don't see any reason why references cannot be removed in this particular case, maybe it is missed optimization from the compiler's part).
But, with [[no_unique_address]] I had this idea:
#include <new>
template <int INDEX>
class Vector3fProperty {
public:
operator float() const {
return propertyValue();
}
float &operator=(float value) {
float &v = propertyValue();
v = value;
return v;
}
private:
float &propertyValue() {
return std::launder(reinterpret_cast<float*>(this))[INDEX];
}
float propertyValue() const {
return std::launder(reinterpret_cast<const float*>(this))[INDEX];
}
};
struct Vector3f {
[[no_unique_address]]
Vector3fProperty<0> x;
[[no_unique_address]]
Vector3fProperty<1> y;
[[no_unique_address]]
Vector3fProperty<2> z;
float data[3];
};
static_assert(sizeof(Vector3f)==12);
So, basically, I have properties in the struct, which handles the access to x, y and z. These properties should not take space, as they are empty, and have the attribute of [[no_unique_address]]
What do you think about this approach? Does it have UB?
Note, this question is about a class, for which all these are possible:
Vector3f v;
v.x = 1;
float tmp = v.x;
float *c = v.<something>; // there, c points to a float[3] array
If this is going to live in a header, and you have some confidence in your compiler's optimizing capabilities, you can probably stick to a plain-old operator[]() overload and expect the compiler to be smart enough to elide the call and return the element that you want. E.g.:
class Vec3f {
public:
float x;
float y;
float z;
float &operator[](int i) {
if(i == 0) {
return x;
}
if(i == 1) {
return y;
}
if(i == 2) {
return z;
}
}
};
I tossed this into Compiler Explorer (https://godbolt.org/z/0X4FPL), which showed clang optimizing the operator[] call away at -O2, and GCC at -O3. Less exciting than your approach, but simple and should work under most circumstances.
But this implementation is bad, because references take space in the struct (which is quite unfortunate. I don't see any reason why references cannot be removed in this particular case, maybe it is missed optimization from the compiler's part).
This looks like a complicated issue. Standard-layout classes have to be compatible between each other. And so compilers are not allowed to eliminate any member, regardless of how they are defined. For non standard-layout? Who knows. For more info read this: Do the C++ standards guarantee that unused private fields will influence sizeof?
From my experience compilers never remove class members, even if they are "unused" (e.g. formally sizeof does use them).
Does it have UB?
I think this is UB. First of all [[no_unique_address]] only means that the member need not have a unique address, not that it must not have a unique address. Secondly it is not clear where your data member starts. Again, compilers are free to use or not paddings of previous [[no_unique_address]] class members. Meaning your accessors may access incorrect piece of memory.
Another problem is that you want to access "outer" memory from the "inner" class. AFAIK such thing is also UB in C++.
What do you think about this approach?
Assuming it is correct (which is not) I still don't like it. You want getters/setters but C++ does not support this feature. So instead of doing those weird, complicated constructs (imagine other people maintaining this code) how about simply do
struct Vector3f {
float data[3];
float x() {
return data[0];
}
void x(float value) {
data[0] = value;
}
...
};
You say this code is ugly. Maybe it is. But it is simple, easy to read and maintain. There's no UB, it does not depend on potential hacks with unions, and does exactly what you want, except for beauty requirement. :)
GLM implements this kind of functionality using anonymous structs inside an anonymous union
I can't personally guarantee that this is standard-compliant, but most major compilers (MSVC, GCC, Clang) will support this idiom:
struct Vector3f {
union {
struct {
float x, y, z;
};
struct {
float data[3];
};
};
Vector3f() : Vector3f(0,0,0) {}
Vector3f(float x, float y, float z) : x(x), y(y), z(z) {}
};
int main() {
Vector3f vec;
vec.x = 14.5;
std::cout << vec.data[0] << std::endl; //Should print 14.5
vec.y = -22.345;
std::cout << vec.data[1] << std::endl; //Should print -22.345
std::cout << sizeof(vec) << std::endl; //On most platforms will print 12
}
The non-standard behavior is in the anonymous struct used to group the letters together, which GCC will issue a warning about. As far as I know, the union itself should be valid, because the datatypes are all identical, but you should still check with your compiler documentation if you're unsure whether this is valid or not.
As an added convenience, we can also overload the brackets operator to shorten our syntax a little:
struct Vector3f {
/*...*/
float& operator[](size_t index) {return data[index];}
float operator[](size_t index) const {return data[index];}
};
int main() {
Vector3f vec;
vec.x = 14.5;
std::cout << vec[0] << std::endl; //Should print 14.5
vec.y = -22.345;
std::cout << vec[1] << std::endl; //Should print -22.345
std::cout << sizeof(vec) << std::endl; //On most platforms will print 12
}
Just for clarity, accessing inactive members in the way I am is valid according to the C++ standard, because those members share a "common subsequence":
If two union members are standard-layout types, it's well-defined to examine their common subsequence on any compiler.
CPP Reference: Union Declaration
Because x and data[0] are
Both floats,
Both occupy the same memory,
Are both standard Layout types as the standard defines them,
It's perfectly valid to access one or the other regardless of which is currently active.
As stated, this is impossible: pointer arithmetic is defined only within an array, and there’s no way (without putting a reference in the class, which takes up space in current implementations) to have v.x refer to an array element.
I'm starting to get grips with c++ and I thought I would learn about creating vectors and doing manipulations on them and in doing so I would get to grips with pointers and references.
Maybe I've been looking at it too long but I'm not quite sure what is going on the with the section of code related to the return statement with the book I'm following along has given me, more specifically the return ((&x)[i]); part.
Listed below:
float& operator [](int i)
{
return ((&x)[i]);
}
My understanding is that & - address operator would give you the address of a particular variable. Now if I remember correctly pointers and arrays go hand in hand, so when you get a reference to the x variable like so (&x)[i] you are able to access the next element using an offset operator but I didn't know you could do this with structs.
Am I on the right track or have I lost it?
Using the following links to help me:
http://www.cplusplus.com/doc/tutorial/pointers/
https://en.cppreference.com/w/cpp/language/operator_member_access
struct Vector3D
{
float x, y, z;
Vector3D() = default;
Vector3D(float a, float b, float c)
{
x = a;
y = b;
z = c;
}
float& operator [](int i)
{
return ((&x)[i]);
}
const float& operator [](int i) const
{
return ((&x)[i]);
}
};
int main(int argc, char const *argv[])
{
Vector3D vec = Vector3D(1,2,3);
auto test = vec[1];
std::cout << test << std::endl;
return 0;
}
I understand that the value printed here would be 2 but I don't quite get the &x reference being able to access the struct member values?
What
return ((&x)[i]);
does is take the address of x, and then using [i], it gets the object at the offset of i from x. Basically, you are pretending you have an array of int's, instead of 3 seperate int's, with x being the first element of the array.
This is undefined behavior for a couple of reason though. First, you cannot access an element that is out of bounds for the array. An object can be considered an array of one so (&variable)[0] is legal, as it is just variable (ignoring that & could be overloaded). So any index besides 0 is UB. Secondly, the compiler is allowed to put padding between member variables of a class. This means that you can't know, without asserting the class is the size you expect, that y and z are adjacent to x in memory.
The correct way to write the function would be use use a switch statement like
float& operator [](int i)
{
switch(i)
{
case 0: return x;
case 1: return y;
case 2: return z;
default: // throw an exception or return one of the members or do something else to indicate an error
}
}
This is a C++ followup for another question of mine
In the old days of pre-ISO C, the following code would have surprised nobody:
struct Point {
double x;
double y;
double z;
};
double dist(struct Point *p1, struct Point *p2) {
double d2 = 0;
double *coord1 = &p1->x;
double *coord2 = &p2->x;
int i;
for (i=0; i<3; i++) {
double d = coord2[i] - coord1[i]; // THE problem
d2 += d * d;
}
return sqrt(d2);
}
Unfortunately, this problematic line uses pointer arithmetic (p[i] being by definition *(p + i)) outside of any array which is explicitely not allowed by the standard. Draft 4659 for C++17 says in 8.7 [expr.add]:
If the expression P points to element x[i] of an array object x with n elements,
the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) element
x[i + j] if 0 <= i + j <= n; otherwise, the behavior is undefined.
And the (non-normative) note 86 makes it even more explicit:
An object that is not an array element is considered to belong to a single-element array for this purpose. A
pointer past the last element of an array x of n elements is considered to be equivalent to a pointer to a hypothetical element
x[n] for this purpose.
The accepted answer of the referenced question uses the fact that the C language accepts type punning through unions, but I could never find the equivalent in the C++ standard. So I assume that a union containing an anonymous struct member and an array would lead to Undefined Behaviour in C++ — they are different languages...
Question:
What could be a conformant way to iterate through members of a struct as if they were members of an array in C++? I am searching for a way in current (C++17) versions, but solutions for older versions are also welcome.
Disclaimer:
It obviously only applies to elements of same type, and padding can be detected with a simple assert as shown in that other question, so padding, alignment, and mixed types are not my problem here.
Use an constexpr array of pointer-to-member:
#include <math.h>
struct Point {
double x;
double y;
double z;
};
double dist(struct Point *p1, struct Point *p2) {
constexpr double Point::* coords[3] = {&Point::x, &Point::y, &Point::z};
double d2 = 0;
for (int i=0; i<3; i++) {
double d = p1->*coords[i] - p2->*coords[i];
d2 += d * d;
}
return sqrt(d2);
}
IMHO the easiest way is to just implement operator[]. You can make a helper array like this or just create a switch...
struct Point
{
double const& operator[] (std::size_t i) const
{
const std::array coords {&x, &y, &z};
return *coords[i];
}
double& operator[] (std::size_t i)
{
const std::array coords {&x, &y, &z};
return *coords[i];
}
double x;
double y;
double z;
};
int main()
{
Point p {1, 2, 3};
std::cout << p[2] - p[1];
return 0;
}
struct Point {
double x;
double y;
double z;
double& operator[]( std::size_t i ) {
auto self = reinterpret_cast<uintptr_t>( this );
auto v = self+i*sizeof(double);
return *reinterpret_cast<double*>(v);
}
double const& operator[]( std::size_t i ) const {
auto self = reinterpret_cast<uintptr_t>( this );
auto v = self+i*sizeof(double);
return *reinterpret_cast<double const*>(v);
}
};
this relies on there being no packing between the doubles in your `struct. Asserting that is difficult.
A POD struct is a sequence of bytes guaranteed.
A compiler should be able to compile [] down to the same instructions (or lack thereof) as a raw array access or pointer arithmetic. There may be some problems where this optimization happens "too late" for other optimzations to occur, so double-check in performance sensitive code.
It is possible that converting to char* or std::byte* insted of uintptr_t would be valid, but there is a core issue about if pointer arithmetic is permitted in this case.
You could use the fact that casting a pointer to intptr_t doing arithmetic and then casting the value back to the pointer type is implemetation defined behavior. I believe it will work on most of the compilers:
template<class T>
T* increment_pointer(T* a){
return reinterpret_cast<T*>(reinterpret_cast<intptr_t>(a)+sizeof(T));
}
This technic is the most efficient, optimizers seems not to be able to produce optimal if one use table look up: assemblies-comparison
I read somewhere a piece of code for defining 3d coordinate. So, they used x, y & z coordinate like this below:
union
{
struct
{
float x, y, z;
};
float _v[3];
};
My question is that why a union is required here and also what is the advantage of using struct with an array?
Important note: this construct leads to undefined behavior. What follows is a description of its authors' intentions, which, unfortunately, many compilers translate to precisely the behavior the authors expect, which in turn leads to proliferation of code like that.
union construct is not really required here: the authors used it for convenience. The reason they put a union there is to give themselves a syntax for accessing x, y, and z in two distinct ways:
By specifying the field name - that is, coord.x, coord.y, or coord.z, or
By specifying the field index - that is, coord._v[0], coord._v[1], or coord._v[2].
An approach that provides comparable functionality without running into undefined behavior is to use inline member functions for your access:
struct Vector3D {
int v[3];
int& x() { return v[0]; }
int& y() { return v[1]; }
int& z() { return v[2]; }
};
int main() {
Vector3D coord;
coord.v[0] = 5;
cout << coord.x() << endl;
coord.y() = 10;
cout << coord.v[1] << endl;
return 0;
}
Demo.
As Gill Bates says, in this way you can (maybe) access the 3 coordinates both as x, y, z and as v[0], v[1], v[2]
But as per #Richard Critten comment, this is actually an UB.
You can get the same result in a "safe" way with something like this:
struct Coordinates
{
Coordinates():x(v[0]), y(v[1]), z(v[2])
{
}
int v[3];
int& x;
int& y;
int& z;
};
i.e., using references to the array values and initializing them in constructor
Size of this struct will be obviously different (larger) than the union you show in OP
This way you can address the 3d coordinate as a struct..
foo.x;
foo.y; // etc
But it also allows you to get the 3 variables as an array while occupying the same space (that's the plan at least), accessing them like..
foo._v[0] // x, etc
Anyway, that's the idea. But the moment the struct has any sort of padding between members your array will be misaligned and you will end up reading garbage values. Long story short, this code has undefined behaviour, a bad implementation that shouldn't be used.
I would like to iterate through a pre-allocated float array with a custom container that does not owns the data, but acts on a segment of it. Example, naming the container class LinhaSobre:
std::unique_ptr<float[]> data(new float[720]);
...
//creates container to iterate 26 floats starting from from data[12]
LinhaSobre cont(data.get()+12, 26);
//sets those elements to 1.5
for(size_t i = 0; i < cont.size(); i++)
cont[i] = 1.5f;
Here's a possible implementation of the operator[] :
//...
//LinhaSobre has a member mem0 which is initialized
//as a pointer to where the interval starts
float & LinhaSobre::operator[] (size_t i)
{
return *(mem0+i);
}
Notice that I'm returning a reference from LinhaSobre::operator[] to data that it does not owns. It should not interfere with the data's lifetime (constructors, destructors).
Now I want to expose the stored data by another pattern, std::array<float,4>, and not pure float. Example, naming the new class LinhaSobre4f:
std::unique_ptr<float[]> data(new float[720]);
...
//creates container to iterate 4 array<float, 4> starting from from data[12]
LinhaSobre4f l(data.get()+(3*4), 4);
//sets those elements to {1.5f, 2.5f, 3.5f, 4.5f};
for(size_t i = 0; i < l.size(); i++)
l[i] = { {1.5f, 2.5f, 3.5f, 4.5f} };
Notice that I treat the items as an array.
This would lead to some changes in the container class, my main concern is with the operator[], here's the full class code:
struct LinhaSobre4f
{
LinhaSobre4f(float * pos_begin, size_t size_):
pos0(pos_begin),
size_(size_){}
std::array<float, 4> & operator[](size_t i)const
{
std::array<float,4> * r =
reinterpret_cast<std::array<float,4>*> (pos0+(4*i));
return *r;
}
size_t size()const
{
return size_;
}
private:
float * pos0;
size_t size_;
};
The operator[] returns a reference to a block of memory treated as an std::array<float,4> that never really existed as such, but given the std::array memory layout guaranties, it works. I'm dubious about this, is it OK? (aside from memory alignment, which I'll guarantee). Am I allowed to expose an object like this, semantically? What is the correct term for this? (I've used fake object in the title).
Here's a live demo of the example. Here's another (the other link sometimes fails)
The C++ standard (I'm reading C++11) defines a std::array as follows:
The conditions for an aggregate (8.5.1) shall be met.
You are not guaranteed that a std::array is a POD. The C++ standard guarantees only that it's a class aggregate.
Based on that, I believe that your usage of reinterpret_cast to convert a POD array of floats to a std::array is undefined behavior.
Chances are that it'll work, with your compiler, but you are not guaranteed that this will be portable, or legal.
You might create a plain old reference_type:
struct LinhaSobre4f {
struct Ref {
Ref(float *m): m(m){};
Ref &operator=(std::initializer_list<float> const &l) {
std::copy(l.begin(), l.end(), m);
return *this;
}
private:
float *m;
};
Ref operator[](size_t i) { return m + 4 * i; }
private:
float *m;
};
Adding on Sam Varshavchik's answer, you may be interested in the span type (formerly known as array_view).
The span type is an abstraction that provides a view over a contiguous sequence of objects, the storage of which is owned by some other object (more details in P0122R1, CppCoreGuidelines and Guidelines Support Library Review: span<T>).
Conceptually, a span is simply a pointer to some storage and a count of the elements accessible via that pointer. It's so small that it can be passed by value.
An open source (header only), reference implementation is available at https://github.com/Microsoft/GSL (the implementation generally assumes a platform that implements C++14 support. There are specific workarounds to support MSVC 2013 and 2015).