I'm new to this website and a bit new in programming. I'm using vector for the first time and I want to print content of it but I'm getting addresses instead of normal words. I don't know how to do that in other way than this.
My vector:
vector<Component*> vect;
and my code for printing:
void Frame::print() {
for (int i = 0; i < vect.size(); i++)
cout << vect[i] << endl;
}
You are storing pointers in your vector. The value of a pointer is a memory address, therefore you are seeing addresses in your output. You need to dereference the pointers to access the actual Component objects:
void Frame::print()
{
for (int i = 0; i < vect.size(); i++)
{
Component *component = vect[i];
cout << *component << endl; // <-- not the extra '*'
}
}
In order for this to work, an operator<< overload for Component is also required:
ostream& operator<<(ostream &out, const Component &comp)
{
// print comp values to out as needed...
return out;
}
Recommended reading:
Operator overloading
You also need to study up on pointers and references in general.
vect is a vector (aka a lump of contiguous storage) of Component * (aka Component pointers) that is it's a chunk of memory with addresses of other chunks of memory which the compiler will treat as a Component class object. Printing those addresses via cout will just give you a big list of meaningless numbers.
What I suspect you want to do is probably not store a vector of Component pointers at all and just store a vector of Components. It's frowned upon in C++ these days to store raw pointers unless you know exactly what you are doing. If you really do want pointers you should use a vector of std::unique_ptr and std::make_unique.
Once you start trying to print Components rather than addresses of them you will most likely see that there is no << operator for Component. You will need to write one. Something like
std::ostream& operator<<(std::ostream &stream, const Component&component)
{
stream << component.string_member;
return stream;
}
Related
As the title describes, I am trying to pass the pointer to the data of a std::vector into a function expecting a double pointer. Take as an example the code below. I have an int pointer d which is passed to myfunc1 as &d (still not sure if call it the pointer's reference or what), where the function changes its reference to the beginning of an int array filled with 1,2,3,4. However, if I have a std::vector of ints and try to pass &(vec.data()) to myfunc1 the compiler throws the error lvalue required as unary ‘&’ operand. I have already tried something like (int *)&(vec.data()) as per this answer, but it does not work.
Just for reference, I know I can do something like myfunc2 where I directly pass the vector as reference and the job is done. But I want to know if it's possible to use myfunc1 with the std::vector's pointer.
Any help will be very much appreciated.
#include <iostream>
#include <vector>
using std::cout;
using std::endl;
using std::vector;
void myfunc1(int** ptr)
{
int* values = new int[4];
// Fill all the with data
for(auto& i:{0,1,2,3})
{
values[i] = i+1;
}
*ptr = values;
}
void myfunc2(vector<int> &vec)
{
int* values = new int[4];
// Fill all the with data
for(auto& i:{0,1,2,3})
{
values[i] = i+1;
}
vec.assign(values,values+4);
delete values;
}
int main()
{
// Create int pointer
int* d;
// This works. Reference of d pointing to the array
myfunc1(&d);
// Print values
for(auto& i:{0,1,2,3})
{
cout << d[i] << " ";
}
cout << endl;
// Creates the vector
vector<int> vec;
// This works. Data pointer of std::vector pointing to the array
myfunc2(vec);
// Print values
for (const auto &element : vec) cout << element << " ";
cout << endl;
// This does not work
vector<int> vec2;
vec2.resize(4);
myfunc1(&(vec2.data()));
// Print values
for (const auto &element : vec2) cout << element << " ";
cout << endl;
return 0;
}
EDIT: What my actual code does is to read some binary files from disk, and load parts of the buffer into the vector. I was having troubles getting the modified vector out of a read function, and this is what I came up with that allowed me to solve it.
When you write:
myfunc1(&(vec2.data()));
You are getting the address of a rvalue. The pointed int* is so a temporary that is destroyed right after the call.
This is why you get this error.
But, as #molbdnilo said, in your myfunc1() function, you are reassigning the pointer (without caring to destroy previously allocated memory by the way).
But the std::vector already manages its data memory on its own. You cannot and you must not put your hands on it.
What my actual code does is to read some binary files from disk, and load parts of the buffer into the vector.
A solution could be to construct your std::vector by passing the iterator to the beginning and the iterator to the end of the desired part to extract in the constructor's parameters.
For example:
int * buffer = readAll("path/to/my/file"); // Let's assume the readAll() function exists for this example
// If you want to extract from element 5 to element 9 of the buffer
std::vector<int> vec(buffer+5, buffer+9);
If the std::vector already exists, you can use the assign() member function as you already did in myfunc2():
vec.assign(buffer+5, buffer+9);
Of course in both cases, you have to ensure that you are not trying to access an out of bounds element when accessing the buffer.
The problem is that you cannot take the address of data(), since it is only a temporary copy of the pointer, so writing to a pointer to it makes not that much sense. And that is good that way. You DO NOT want to pass data() to this function since it would overwrite the pointer with a new array and that would break the vector. You can remove one * from the function and only assign to it and not allocate the memory there. This will work, but make sure to allocate the memory in the caller (with resize, just reserve will result un undefined behavior, since data() is only a pointer to the beginning of the valid range [data(), data() + size()). The range [data(), data() + capacity ()) is not necessary valid.
I have a C++ struct that I need to convert to a list so that I can load into GPU
struct point_cloud_tensor
{
std::vector<float> timestamp;
std::vector<std::vector<double>> position;
// more fields
};
point_cloud_tensor sweep_to_array(const point_sweep &point_sweep)
{
const auto num_points = point_sweep.points.size();
point_cloud_tensor tensor;
point_cloud_tensor.timestamp.reserve(num_points);
point_cloud_tensor.point.reserve(num_points);
for (int i = 0; i < point_sweep.points.size(); i++)
{
const auto point = point_sweep.points.at(i);
tensor.timestamp.push_back(point.timestamp);
std::vector<double> point_triple(3);
point_triple.push_back(point.x);
point_triple.push_back(point.y);
point_triple.push_back(point.z);
tensor.position.push_back(point_triple);
// more fields
}
return tensor;
}
There are about 100K points in the sweep vector and this runs in about 30ms.
Is there a way to substantially reduce this?
In this case, your std::vector is being used for a small sized array, for this you can replace it by std:array
As mentioned, testing how fast a code can be run, is a matter of hardware so I can't be 100% sure if it is faster with this change.
Do not call size() every time if it does not change
Since you already store point_sweep.points.size() into the variable num_points, you can use it in your for loop. When you iterate like that:
for (int i = 0; i < point_sweep.points.size(); i++)
Every iteration you will dereference point_sweep and dereference points to call its method size(). It should be faster to use the local variable instead:
for (int i = 0; i < num_points; i++)
Use a reference when appropriate
When you fetch your point:
const auto point = point_sweep.points.at(i);
You are calling the copy constructor for no reason. You should use a reference to it, using &:
const auto& point = point_sweep.points.at(i);
References can be risky because every modification you perform will be applied to the original object, but since you are using a const reference, you should be safe.
Minimize the calls when pushing elements to the back of a vector
When you fill up your tensor.position vector, you may:
Create the point with an intializer_list
Add the item without a temporary variable, in order to be move-able
So, this code:
std::vector<double> point_triple(3);
point_triple.push_back(point.x);
point_triple.push_back(point.y);
point_triple.push_back(point.z);
tensor.position.push_back(point_triple);
Becomes:
tensor.position.push_back({point.x, point.y, point.z});
Plus it becomes easier to read, in my opinion.
Use another 3D point structure (if possible)
Also, as others have pointed out, if you can change the data structures then you may use an std::array or std::tuple or you may simply write a struct such as struct Point { double x, y, z; }. The array can be accessed almost exactly like a vector, which should make the transition a bit easier. The tuple must be accessed by std::get which needs to rewrite a bit of code. For example if you want to display the contents of the last element:
struct point_cloud_tensor
{
std::vector<float> timestamp;
std::vector<std::tuple<double,double,double>> position;
// more fields
} tensor;
auto last_pos = tensor.position.back();
std::cout << "x=" << std::get<0>(last_pos) << ' ';
std::cout << "y=" << std::get<1>(last_pos) << ' ';
std::cout << "z=" << std::get<2>(last_pos) << std::endl;
However, with tuples you can add items with emplace_back instead of push_back, which saves you a move constructor, e.g.:
tensor.position.emplace_back(point.x, point.y, point.z);
Notice the difference in syntax. With push_back you have one parameter {point.x, point.y, point.z} but with emplace_back you have 3 parameters point.x, point.y, point.z. Basically with emplace_back you are just removing the curly braces.
Did you thought about making step backward and creating a list when constructing points?
I am coming from a C#/Java background into C++, using visual studio community 2017 & plenty of tutorials. I came to the point where am unsure of what is a correct way to write a function to process a vector of data. Should I force a function to use a pointer / reference? Should I let compiler sort it out? What is best practice?
This is my main, I ask for an input on vector size, then pass a pointer to the integer value to function that creates and populates vector with values through a simple for loop.
I then pass the array to another function that performs a shuffle.
vector<int> intVector(int* count)
{
vector<int> vi;
for (int i = 1; i <= *count; i++)
vi.push_back(i);
return vi;
}
vector<int> &randVector(vector<int> *v)
{
shuffle(v->begin(), v->end(), default_random_engine());
return *v;
}
int _tmain(int argc, _TCHAR* argv[])
{
int count;
cout << "Enter vector array size: ";
cin >> count; cout << endl;
cout << "Vector of integers: " << endl;
vector<int> vi = intVector(&count);
for_each(vi.begin(), vi.end(), [](int i) {cout << i << " ";});
cout << endl;
vi = randVector(&vi);
cout << "Randomized vector of integers: " << endl;
for_each(vi.begin(), vi.end(), [](int i) {cout << i << " ";});
cout << endl;
return 0;
}
So my question is, what is the best practice in my case to avoid unnecessary copying. Should I even care about it? Should I rely on compiler to solve it for me?
I am planing to use C++ for game development on desktop and consoles. Understanding memory and performance management is important for me.
You are in charge of enforcing (or avoiding) the copy of objects around.
Regarding your example:
You can avoid using pointers and use a reference instead.
Like in the following:
vector<int>& randVector(vector<int>& v)
{
shuffle(v->begin(), v->end(), default_random_engine());
return v;
}
Note that since you are using a reference, the shuffle operation is already modifying the parameter of randVector so there is no real need to return a reference to it.
As a rule of thumb when you need to pass an object around and you want to avoid a potentially expensive copy you can use references:
void function(<const> Object& v)
{
// do_something_with_v
}
The rules on passing in C++ for typical code are pretty straightforward (though obviously still more complex than languages without references/pointers).
In general, prefer references to pointers, unless passing in null is actually something you might do
Prefer to write functions that don't mutate their inputs, and return an output by value
Inputs should be passed by const reference, unless it is a primitive type like an integer, which should be passed by value
If you need to mutate data in place, pass it by non-const reference
See https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rf-conventional for more details.
The upshot of this is that here are the "correct" signatures for your two functions:
vector<int> intVector(int count);
void randVector(vector<int> &v);
This doesn't take into account iterators which is probably really the correct "generic" way to write the second function but that is a bit more advanced. But, see std::shuffle which lets you randomize any arbitrary container by leveraging iterators: http://en.cppreference.com/w/cpp/algorithm/random_shuffle.
Since you mentioned unnecessary copying, I will mention that when you return things like vector by value, they should never be copied (I'm assuming you're using C++11 or newer). They will instead be "moved", which doesn't have significant overhead. Thus, in newer C++ code, "out parameters" (passing in arguments by reference to mutate them) is significantly discouraged compared to older versions. Good to know in case you encounter dated advice. However, passing in by reference for something like shuffling or sorting is considered an "in/out" parameter: you want to mutate it in place and the existing data is important, not simply being overwritten.
I've got a class that shall behave like matrix.
So the usecase is something like:
Matrix matrix(10,10);
matrix[0][0]=4;
//set the values for the rest of the matrix
cout<<matrix[1][2]<<endl;
code:
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <sstream>
using namespace std;
class Matrix {
public:
Matrix(int x, int y);
class Proxy {
public:
Proxy(int* _array) : _array(_array) {
}
int &operator[](int index) const {
return _array[index];
}
private:
int* _array;
};
Proxy operator[](int index) const {
return Proxy(_arrayofarrays[index]);
}
Proxy operator[](int index) {
return Proxy(_arrayofarrays[index]);
}
const Matrix& operator=(const Matrix& othersales);
private:
int** _arrayofarrays;
int x, y;
};
Matrix::Matrix(int x, int y) {
_arrayofarrays = new int*[x];
for (int i = 0; i < x; ++i)
_arrayofarrays[i] = new int[y];
}
const Matrix& Matrix::operator=(const Matrix& othermatrix) {
new (this) Matrix(x, y);
for (int i = 0; i < 3; i++)
for (int j = 0; j < 3; j++)
_arrayofarrays[i][j] = othermatrix._arrayofarrays[i][j];
return *this;
}
int main() {
Matrix a(2, 3);
a[0][0] = 1;
a[0][1] = 2;
a[0][2] = 3;
a[1][0] = 4;
a[1][1] = 5;
a[1][2] = 6;
cout << a[1][2] << endl;
//prints out 6
const Matrix b = a;
cout << b[1][2] << endl;
a[1][2] = 3;
cout << a[1][2] << endl;
// prints out 3
cout << b[1][2] << endl;
// prints out 3 as well
}
By calling const Matrix b = a; I want to create new instance of Matrix, that will have the same values as a has in that moment. Nevertheless b is being affected by changing the values in a. So if I change some value in a, then it changes in b as well. And I don't want it to behave like this.
So I need to create a copy of b that would not be affected by a itself.
Those might be stupid question, but for me, as a java guy and a C++ newbie are all those stuff really confusing, so thanks for any helpful advices...
There are a few issues with your implementation. The simple one is the error you are getting...
In your Matrix class, operator[] is a non-const member function, which means that it can only be executed on non-const objects. Your operator= takes the right hand side object by const &, and thus you cannot call operator[] on it. The issue here is that you are not offering an implementation of operator[] that promises not to modify the object, once you add that to your type it should compile.
More important than that is the fact that you are leaking memory. When you call operator= on an object you are creating a different Matrix in place, without previously releasing the memory that it held. That is a memory leak.
The implementation of operator= is also not thread-safe. If allocation of memory for any of the internal arrays fails and throws an exception you are leaving your object in a state that is neither the original one nor a valid state. This is bad in itself.
Related to the previous, in as much as correcting one probably leads to the other, your implementation of operator= is not safe if there is aliasing, that is, it fails if you self-assign. The first line will leak the memory and create the new buffers, and from there on you will copy the new buffer into itself, loosing the original information.
Finally, the implementation of the type could be improved if you drop the requirement of using operator[] and use instead operator() with the two indices. User code will have to be adapted (and look less like a bidimensional array) but it provides a bit more freedom of representation (you can store the information internally in any way you want). At the same time, there is no need to allocate an array of pointers and then N arrays of int. You can perform a single memory allocation of NxM ints and do pointer arithmetic to address each location (this is independent of the use of operator[]/operator()), which will reduce the memory footprint and make the layout more compact, improving cache performance (not to mention reducing the number of dynamic allocations by a factor of M)
By calling const Matrix b = a; I want to create new instance of Matrix, that will have the same values of a in that moment. Nevertheless b is being affected by changing the values in a.
Well, this is yet another issue I missed in the first read. The expression const Matrix b = a; does not involve operator=, but rather the copy constructor. Another thing to google: Rule of the Three (basically, if you implement one of copy-constructor, assignment or destructor manually, you probably want to implement all three). Without defining your own copy constructor the compiler will implicitly define one for you that does a shallow copy (i.e. copies the pointers stored in Matrix but does not allocate memory for it). After the copy is made both Matrix share the same memory, and if your destructor releases the memory, you will run into Undefined Behavior when the second destructor runs and tries to delete [] the already deleted memory.
I have the following array:
int* myArray = new int[45];
If I wanted to iterate each element without knowing the actual size of the array, I would need to use a for_each?
If so, then how would you write the for_each? I was looking over the following site and reading up on for_each but can't figure out how to put this together.
http://www.cplusplus.com/reference/algorithm/for_each/
Update: A for_each is not a good choice in this case, due to the fact that the size of the array has to be known. vectors are the proper way to accomplish such task. My reason for using arrays, in this case, was for learning purposes. if this was a serious project I would move to something such as Lists/Vectors.
Note when the question was first posted, the array in question was declared as
int myArray[45];
This answer deals with that particular case.
If you have C++11 support, you can use a range based loop:
for (int& i : myArray) {
std::cout << i << "\n";
}
C++11 also provides std::begin and std::end, which you can use with a fixed size array to obtain iterators:
std::for_each(std::begin(myArray), std::end(myArray), <func>);
Another option, which works for C++03 and you are dealing with fixed size arrays, is to define a function template:
// taken a fixed size array by reference and loop over it
template <typename T, unsigned int N>
void array_for_each( T (&a)[N]) {
for (unsigned int i = 0; i < N; ++i) {
// do something with array elements
std::cout << a[i] << " ";
}
}
int main() {
int a[5];
array_for_each(a);
}
If you use MSVC (Microsoft Visual C++), you can use "for each."
for each(int i in arr) {
cout << i << ' ' << endl;
}
NOTE: This only works in the block of code the array is declared in.
If not, you can also use the new range-based for loop in the C++11 standard.
for(int i : arr) {
cout << i << ' ' << endl;
}
If you're intent upon the std::for_each:
for_each(arr,arr + 10,[] (int i) {
cout << i << ' ' << endl;
});
NOTE: This requires knowledge of the size of the array (in this example, 10).
You could use a for_each. In this case, you have allocated space for 45 elements in your array, but since it is NULL, you'd probably get a segfault if you tried to do anything. You either need to hold a value of the array, or use something like sizeof(myArray)/sizeof(myArray[0]) (which has its own problems).
Anyway, for a for_each here, if we actually had 45 elements:
std::for_each(myArray, myArray + 45, <func>);
Anyway, this is part of the reason to use vectors: .begin() and .end() reduces errors with using incorrect indexing.
You have described an array of int, not a class that implements a InputIterator, which is what the for_each is designed for, even though you can use it to iterate an array, but you need to know the size of the array to iterate it.
If you want to use for_each you need to use a vector, list, or implement a class that keeps track of the number of elements it contains. IMO it is much easier to just use a vector
If you want to just iterate your current array, assuming it is 0 terminated:
for(int *value = myArray; *value != 0; ++value)
printf("%d\n", *value);
Or, you can use indexes:
for(int index = 0; myArray[index] != 0; ++index)
printf("%d\n", myArray[index]);
IMO the pointer method is cleaner.
This code is still dangerous though, you should either keep track of the number of records in a seperate variable, or use a vector.