Depending on a variable, I need to select the SeedPositions32 or SeedPositions16 array for further use. I thought a pointer would allow this but I can't seed to make it work. How do you declare a pointer to a C++11 std::array? I tried the below.
array<int>* ArrayPointer;
//array<typedef T, size_t Size>* ArrayPointer;
array<int,32> SeedPositions32 = {0,127,95,32,64,96,31,63,16,112,79,48,15,111,80,
47,41,72,8,119,23,104,55,87,71,39,24,7,56,88,103,120};
array<int,16> SeedPositions16 = {...}
std::array has a template parameter for size. Two std::array template instantiations with different sizes are different types. So you cannot have a pointer that can point to arrays of different sizes (barring void* trickery, which opens its own can of worms.)
You could use templates for the client code, or use std::vector<int> instead.
For example:
template <std::size_t N>
void do_stuff_with_array(std::array<int, N> the_array)
{
// do stuff with the_array.
}
do_stuff_with_array(SeedPositions32);
do_stuff_with_array(SeedPositions16);
Note that you can also get a pointer to the data:
int* ArrayPtr = SeedPositions32.data();
but here, you have lose the size information. You will have to keep track of it independently.
You can simply access the content of the std::array as a raw C-like array pointer using the std::array::data() member function:
int* arrayPointer = useSeedPositions32 ? SeedPositions32.data() : SeedPositions16.data();
In his answer juanchopanza explained very well why what you want cannot work.
The question is why would you want to do that? There is no way where you could use a (pointer to) std::array<int,32> in place of std::array<int,16>.
The point of std::array<> is to keep track of the number of elements at compile time (and also to avoid memory allocation for small fixed-sized arrays). If you instead want the number of elements to be managed at run time, you should presumably not use std::array<>, but std::vector.
The alternative of obtaining a pointer to the underlying data (using std::array::data() as proposed in other answers) and keeping track of the number of elements by yourself is somewhat dangerous and not really recommendable. The problem is that you must ensure that the pointer is never dangling.
Finally, I cannot find any possible use case. In order to use your pointer, you must declare both an array<int,32> and an array<int,16> object, yet use only one of them.
Why don't you simply only declare a array<int,32> and use only its first 16 elements if not all 32 are needed?
You could do something like this:
int * myArray = use32 ? &SeedPositions32[0] : &SeedPositions16[0];
Related
For my project I need to store pointers to objects of type ComplicatedClass in an array. This array is stored in a class Storage along with other information I have omitted here.
Here's what I would like to do (which obviously doesn't work, but hopefully explains what I'm trying to achieve):
class ComplicatedClass
{
...
}
class Storage
{
public:
Storage(const size_t& numberOfObjects, const std::array<ComplicatedClass *, numberOfObjects>& objectArray)
: size(numberOfObjects),
objectArray(objectArray)
{}
...
public:
size_t size;
std::array<ComplicatedClass *, size> objectArray;
...
}
int main()
{
ComplicatedClass * object1 = new ComplicatedClass(...);
ComplicatedClass * object2 = new ComplicatedClass(...);
Storage myStorage(2, {object1, object2});
...
return 0;
}
What I am considering is:
Using std::vector instead of std::array. I would like to avoid this because there are parts of my program that are not allowed to allocate memory on the free-store. As far as I know, std::vector would have to do that. As a plus I would be able to ditch size.
Changing Storage to a class template. I would like to avoid this because then I have templates all over my code. This is not terrible but it would make classes that use Storage much less readable, because they would also have to have templated functions.
Are there any other options that I am missing?
How can I pass and store an array of variable size containing pointers to objects?
By creating the objects dynamically. Most convenient solution is to use std::vector.
size_t size;
std::array<ComplicatedClass *, size> objectArray;
This cannot work. Template arguments must be compile time constant. Non-static member variables are not compile time constant.
I would like to avoid this because there are parts of my program that are not allowed to allocate memory on the free-store. As far as I know, std::vector would have to do that.
std::vector would not necessarily require the use of free-store. Like all standard containers (besides std::array), std::vector accepts an allocator. If you implement a custom allocator that doesn't use free-store, then your requirement can be satisfied.
Alternatively, even if you do use the default allocator, you could write your program in such way that elements are inserted into the vector only in parts of your program that are allowed to allocate from the free-store.
I thought C++ had "free-store" instead of heap, does it not?
Those are just different words for the same thing. "Free store" is the term used in C++. It's often informally called "heap memory" since "heap" is a data structure that is sometimes used to implement it.
Beginning with C++11 std::vector has the data() method to access the underlying array the vector is using for storage.
And in most cases a std::vector can be used similar to an array allowing you to take advantage of the size adjusting container qualities of std::vector when you need them or using it as an array when you need that. See https://stackoverflow.com/a/261607/1466970
Finally, you are aware that you can use vectors in place of arrays,
right? Even when a function expects c-style arrays you can use
vectors:
vector<char> v(50); // Ensure there's enough space
strcpy(&v[0], "prefer vectors to c arrays");
I have this method:
void createSomething(Items &items)
{
int arr[items.count]; // number of items
}
But it's throwing an error:
expression must have a constant value
I found just this solution:
int** arr= new int*[items.count];
so I'm asking is there a better way how do handle this?
You can use a std::vector
void createSomething(Items &items)
{
std::vector<int> arr(items.count); // number of items
}
The reason your first method won't work is that the size of an array must be know at compile time (without using compiler extensions), so you have to use dynamically sized arrays. You can use new to allocate the array yourself
void createSomething(Items &items)
{
int* arr = new int[items.count]; // number of items
// also remember to clean up your memory
delete[] arr;
}
But it is safer and IMHO more helpful to use a std::vector.
Built in arrays & std::array always require a constant integer to determine their size. Of course in case of dynamic arrays (the one created with new keyword) can use a non-constant integer as you have shown.
However std::vector (which of course internally a dynamic array only) uses a is the best solution when it comes to array-type applications. It's not only because it can be given a non-constant integer as size but also it can grown as well as dynamically quite effectively. Plus std::vector has many fancy functions to help you in your job.
In your question you have to simply replace int arr[items.count]; with :-
std::vector<int> arr(items.count); // You need to mention the type
// because std::vector is a class template, hence here 'int' is mentioned
Once you start with std::vector, you would find yourself preferring it in 99% cases over normal arrays because of it's flexibility with arrays. First of all you needn't bother about deleting it. The vector will take care of it. Moreover functions like push_back, insert, emplace_back, emplace, erase, etc help you make effective insertions & deletions to it which means you don't have to write these functions manually.
For more reference refer to this
In my code I use buffers currently allocated this way:
char* buf1 = (char*)malloc(size);
However at some points in the code I want to reassign the pointer to some place else in memory. The problem is that there are other places in the code that still need to be able to access the pointer buf1.
What's the best way to do this in C++? Right now I am considering writing a struct with a single char* in it, then allocating an object of this struct type and passing it to the places where I need to, and referring to wrapped pointer to get the current value of buf1.
However it seems that this is similar to what unique_ptr does. If I use unique_ptr how can I wrap a char* with it? I had some trouble with testing this and I'm not sure it's supported.
To clarify: these buffers are bytes of varying sizes.
In general, this question cannot be answered. There are simply way too many things you could be wanting to be doing with an array of char. Without knowing what it actually is that you want to do, its impossible to say what may be good abstractions to use…
If you want to do stuff with strings, just use std::string. If you want a dynamically-sized buffer that can grow and shrink, use std::vector.
If you just need a byte buffer the size of which is determined at runtime or which you'd just generally want to live in dynamic storage, I'd go with std::unique_ptr. While std::unique_ptr<T> is just for single objects, the partial specialization std::unique_ptr<T[]> can be used for dealing with dynamically allocated arrays. For example:
auto buffer = std::unique_ptr<char[]> { new char[size] };
Typically, the recommended way to create an object via new and get an std::unique_ptr to it would be to use std::make_unique. And if you want your buffer initialized to some particular value, you should indeed use std::make_unique<char[]>(value). However, std::make_unique<T[]>() will value-initialize the elements of the array it creates. In the case of a char array, that effectively means that your array will be zero-initialized. In my experience, compilers are, unfortunately, unable to optimize away the zero-initialization, even if the entire buffer would be overwritten first thing right after being created. So if you want an uninitialized buffer for the sake of avoiding the overhead of initialization, you can't use std::make_unique. Ideally, you'd just define your own function to create a default-initialized array via new and get an std::unique_ptr to it, for example:
template <typename T>
inline std::enable_if_t<std::is_array_v<T> && (std::extent_v<T> == 0), std::unique_ptr<T>> make_unique_default(std::size_t size)
{
return std::unique_ptr<T> { new std::remove_extent_t<T>[size] };
}
and then
auto buffer = make_unique_default<char[]>(new char[size]);
It seems that C++20 will include this functionality in the form of std::make_unique_default_init. So that would be the preferred method then.
Note that, if you're dealing with plain std::unique_ptr, you will still have to pass around the size of the buffer separately. You may want to bundle up an std::unique_ptr and an std::size_t if you're planning to pass around the buffer
template <typename T>
struct buffer_t
{
std::unique_ptr<T[]> data;
std::size_t size;
};
Note that something like above struct represents ownership of the buffer. So you'd want to use this, e.g., when returning a new buffer from a factory function, e.g.,
buffer_t makeMeABuffer();
or handing off ownership of the buffer to someone else, e.g.,
DataSink(buffer_t&& buffer)
You would not want to use it just to point some function to the buffer data and size do some processing without transferring ownership. For that, you'd just pass a pointer and size, or, e.g., use a span (starting, again, with C++20; also available as part of GSL)…
I have this method:
void createSomething(Items &items)
{
int arr[items.count]; // number of items
}
But it's throwing an error:
expression must have a constant value
I found just this solution:
int** arr= new int*[items.count];
so I'm asking is there a better way how do handle this?
You can use a std::vector
void createSomething(Items &items)
{
std::vector<int> arr(items.count); // number of items
}
The reason your first method won't work is that the size of an array must be know at compile time (without using compiler extensions), so you have to use dynamically sized arrays. You can use new to allocate the array yourself
void createSomething(Items &items)
{
int* arr = new int[items.count]; // number of items
// also remember to clean up your memory
delete[] arr;
}
But it is safer and IMHO more helpful to use a std::vector.
Built in arrays & std::array always require a constant integer to determine their size. Of course in case of dynamic arrays (the one created with new keyword) can use a non-constant integer as you have shown.
However std::vector (which of course internally a dynamic array only) uses a is the best solution when it comes to array-type applications. It's not only because it can be given a non-constant integer as size but also it can grown as well as dynamically quite effectively. Plus std::vector has many fancy functions to help you in your job.
In your question you have to simply replace int arr[items.count]; with :-
std::vector<int> arr(items.count); // You need to mention the type
// because std::vector is a class template, hence here 'int' is mentioned
Once you start with std::vector, you would find yourself preferring it in 99% cases over normal arrays because of it's flexibility with arrays. First of all you needn't bother about deleting it. The vector will take care of it. Moreover functions like push_back, insert, emplace_back, emplace, erase, etc help you make effective insertions & deletions to it which means you don't have to write these functions manually.
For more reference refer to this
Coming from a PHP background, I'm trying to learn C++, since I find it an interesting language. As a practice I want to create a simple Vector class using templates, which is not to hard. However, I run into one problem.
I have created the following template class:
template <typename T>
class Vector
{
public:
Vector(int length);
~Vector(void);
int getLength();
T& operator[] (const int index);
private:
T *_items;
int _count;
};
template <typename T>
Vector<T>::Vector(int length)
{
_items = new T[length];
_count = length;
}
template <typename T>
T& Vector<T>::operator[]( const int index )
{
if (index >= getLength() || index < 0)
throw exception("Array out of bounds.");
return _items[index];
}
All functions are implemented, but they're not relevant to my question, so I haven't copied them here.
This class works as expected, with one exception:
If I want to create a vector of array's, it doesn't work.
e.g.:
Vector<int[2]> someVector(5);
What I obviously want is that the _items property of the vector class will be an int[5][2]. However, since the compiler replaces the 'T' with 'int[2]', the _items property will be int[2][5] (or at least, that's what I understood from debugging, please correct me if I'm wrong). As a result, the [] operator doesn't work correctly anymore, and therefore this whole class is useless for arrays.
Is there a way to solve this problem, so that this class also works for arrays? And if that's not possible, is there a way to prevent this class being initialized with arrays?
Edit: Thanks for all your responses so far. However, I might have been not entirely clear with my question. First of all, I created this class to get used to c++, I know there is a std::vector class, and I also now that it's better to use a vector of vectors. That's not really the problem. I just want to understand templates better, and c++ in general, so I want to know how to deal with this type of problems. I want to be able to create classes which don't make the program crash. If I, or someone else, would use this class right now and tried to use primitive arrays instead of vectors for this class, at some point the program will crash since the array is wrong (Vector(y) becomes int[x][y] instead of int[y][x] internally). So I want a solution which either creates the correct array, or prevents the vector being initialized with arrays at all.
Quite simply, don't ever use the in-built primitive arrays. For anything. They suck, tremendously. Always use a class wrapper such as boost::array, if you don't have TR1 or C++0x both of which also provide an array class. A Vector<boost::array<int, 2>> will trivially compile.
Why not use primitive arrays?
They have hideous implicit conversions to pointers, forget their size at the drop of a hat, aren't first-class citizens (can't assign to them, for example), don't check their bounds, for example. boost::array<int, 2> is exactly a primitive array without crappy conversions, is fully generic- for example, your Vector template works fine with a boost::array<int, 2> off the bat, and it can do bounds checking in the at() function.
I'm not suggesting using a dynamic array instead. boost::array is not a dynamically sized array. It is a constant size, value type array, that will not convert to a pointer, it has functions for use in the Standard library like begin, end, size, and it can be treated like any other type- unlike arrays, which have a dozen special rules- as you've found out.
I see a few things wrong with this.
If you are making a generic vector class, why deal with a vector of arrays rather than a vector of vectors?
Rather than defining a vector of type int[2], try making a vector of vectors of size 2 with something like this:
Vector<Vector<int>> vector(5);
for (int i = 0; i < 5; i++)
{
vector[i] = Vector<int>(2);
}
Rather than using a constant sized array, try using a pointer.
Rather than using a pointer to arrays of size 2, use a pointer to a pointer, and allocate two spaces for that pointer. Pointers are a lot easier to deal with in C++ because you can simply pass the address of the pointer rather than having to copy the entire array. It's generally very bad form to pass arrays.
Add a default parameter to your constructor
Rather than declaring the constructor with Vector(int length);, try using Vector(int length = 0); so that if the user doesn't specify a length it will default to a size of zero.
Finally, are you aware that there is in fact an std::vector, or are you aware of this and simply trying to replicate it? Templates are known to be one of the hardest subjects of C++, good luck!
The condition must say index >= getLength(), because getLength() is not an allowed index value too (the first element's index is 0, the last's is getLength()-1)