first and foremost sorry for my bad english, I'm no english native =/
I have a vector of pointers directing at my base class A which is filled by classes B and C.
B and C are polymorphic classes from A, which have only one more method, setTest().
Now I want to call a method from B/C through the vector:
vector (A*) vec;
vec.push_back(new classB());
vec.push_back(new classC());
for(int i=0;i<3;++i)
vec[i]->setTest(true);
But the compiler says there is no method setTest() in my baseclass A.
Any ideas how I can fix this?
Since compiler "think" that deals with A, it cannot deduce that method setTest exists.
To resolve this problem you can do following:
Add abstract method to A:
virtual void setTest(bool value) = 0;
Update
There is another way. Let's create helper interface D with only one method:
struct D
{
virtual void setTest(bool value) = 0;
};
Than using multiple inheritance change signature of B and C:
class B : public A, public D
{
virtual void setTest(bool value)
{
//your impl goes here...
}
};
//do the same with impl of C
And at last let's change iteration:
for(int i=0;i<3;++i)
((D*)vec[i])->setTest(true);
Simple casting allows call expected method. BUT!!! if vector can contains instances of A than it will fail, so using dynamic_cast helps:
for(int i=0;i<3;++i)
{
D *check_inst = dynamic_cast<D*>(vec[i]);
if( check_inst)
check_inst->setTest(true);
}
Related
I have a really strange problem. I can't modify the object I am pointing to with a shared_ptr.
Example code:
#include<memory>
#include<iostream>
using namespace std;
class foo
{
public:
int asd;
foo(){}
~foo(){}
};
void d(shared_ptr<foo> c)
{
c->asd = 3;
}
void main()
{
foo a;
a.asd = 5;
d(make_shared<foo>(a));
cout<<a.asd; //asd is still 5
}
As far as I know you can access the object pointed to by the shared_ptr by using the "->" operator, so what am I doing wrong here? How can I change the asd variable inside the class via the shared pointer?
// create a temporary object by copying a
// the shared pointer you pass to d function actually points to this temporary object
d(make_shared<foo>(a));
// allocate and store foo object in shared_ptr instead
auto p_a(make_shared<foo>());
p_a->asd = 3;
d(p_a);
... so what am I doing wrong here?
From cppreference on std::make_shared [emphasis mine]:
template< class T, class... Args >
shared_ptr<T> make_shared( Args&&... args );
Constructs an object of type T and wraps it in a std::shared_ptr
using args as the parameter list for the constructor of T.
In your case, you supply an instance of foo as argument to std::make_shared, which will be used when constructing a new object of type foo; i.e., making use of the default supplied copy CTOR of foo (foo(const foo&)). This new object will be a temporary and only live for the call to d(...).
I wanted to have a linked list of nodes with below structure.
struct node
{
string word;
string color;
node *next;
}
for some reasons I decided to use vector instead of list.my question is that is it possible to implement a vector which it's j direction is bounded and in i direction is unlimited and to add more two strings at the end of my vertex.
in other words is it possible to implement below structure in vector ?
j
i color1 color2 …
word1 word2 …
I am not good with C/C++, so this answer will only be very general. Unless you are extremely concerned about speed or memory optimization (most of the time you shouldn't be), use encapsulation.
Make a class. Make an interface which says what you want to do. Make the simples possible implementation of how to do it. Most of the time, the simplest implementation is good enough, unless it contains some bugs.
Let's start with the interface. You could have made it part of the question. To me it seems that you want a two-dimensional something-like-an-array of strings, where one dimension allows only values 0 and 1, and the other dimension allows any non-genative integers.
Just to make sure there is no misunderstanding: The bounded dimension is always size 2 (not at most 2), right? So we are basicly speaking about 2×N "rectangles" of strings.
What methods will you need? My guesses: A constructor for a new 2×0 size rectangle. A method to append a new pair of values, which increases the size of the rectangle from 2×N to 2×(N+1) and sets the two new values. A method which returns the current length of the rectangle (only the unbounded dimension, because the other one is constant). And a pair of random-access methods for reading or writing a single value by its coordinates. Is that all?
Let's write the interface (sorry, I am not good at C/C++, so this will be some C/Java/pseudocode hybrid).
class StringPairs {
constructor StringPairs(); // creates an empty rectangle
int size(); // returns the length of the unbounded dimension
void append(string s0, string s1); // adds two strings to the new J index
string get(int i, int j); // return the string at given coordinates
void set(int i, int j, string s); // sets the string at given coordinates
}
We should specify what will the functions "set" and "get" do, if the index is out of bounds. For simplicity, let's say that "set" will do nothing, and "get" will return null.
Now we have the question ready. Let's get to the answer.
I think the fastest way to write this class would be to simply use the existing C++ class for one-dimensional vector (I don't know what it is and how it is used, so I just assume that it exists, and will use some pseudocode; I will call it "StringVector") and do something like this:
class StringPairs {
private StringVector _vector0;
private StringVector _vector1;
private int _size;
constructor StringPairs() {
_vector0 = new StringVector();
_vector1 = new StringVector();
_size = 0;
}
int size() {
return _size;
}
void append(string s0, string s1) {
_vector0.appens(s0);
_vector1.appens(s1);
_size++;
}
string get(int i, int j) {
if (0 == i) return _vector0.get(j);
if (1 == i) return _vector1.get(j);
return null;
}
void set(int i, int j, string s) {
if (0 == i) _vector0.set(j, s);
if (1 == i) _vector1.set(j, s);
}
}
Now, translate this pseudocode to C++, and add any new methods you need (it should be obvious how).
Using the existing classes to build your new classes can help you program faster. And if you later change your mind, you can change the implementation while keeping the interface.
This is a similar question to this SO post, which I have been unable to use to solve my problem. I have included some code here, which will hopefully help someone to bring home the message that the other posting was getting at.
I want to write a CLI/C++ method that can take a void pointer as a parameter and return the managed object (whose type I know) that it points to. I have a managed struct:
public ref struct ManagedStruct { double a; double b;};
The method I am trying to write, which takes a void pointer to the managed struct as a parameter and returns the struct.
ManagedStruct^ VoidPointerToObject(void* data)
{
Object^ result = Marshal::PtrToStructure(IntPtr(data), Object::typeid);
return (ManagedStruct^)result;
}
The method is called here:
int main(array<System::String ^> ^args)
{
// The instance of the managed type is created:
ManagedStruct^ myData = gcnew ManagedStruct();
myData->a = 1; myData->b = 2;
// Suppose there was a void pointer that pointed to this managed struct
void* voidPtr = &myData;
//A method to return the original struct from the void pointer
Object^ result = VoidPointerToObject(voidPtr);
return 0;
}
It crashes in the VoidPointerToObject method on calling PtrToStructure , with the error: The specified structure must be blittable or have layout information
I know this is an odd thing to do, but it is a situation I have encountered a few times, especially when unmanaged code makes a callback to managed code and passes a void* as a parameter.
(original explanation below)
If you need to pass a managed handle as a void* through native code, you should use
void* voidPtr = GCHandle::ToIntPtr(GCHandle::Alloc(o)).ToPointer();
// ...
GCHandle h = GCHandle::FromIntPtr(IntPtr(voidPtr));
Object^ result = h.Target;
h.Free();
(or use the C++/CLI helper class gcroot)
Marshal::PtrToStructure works on value types.
In C++/CLI, that means value class or value struct. You are using ref struct, which is a reference type despite use of the keyword struct.
A related problem:
void* voidPtr = &myData;
doesn't point to the object, it points to the handle.
In order to create a native pointer to data on the managed heap, you need to use pinning. For this reason, conversion between void* and Object^ isn't as useful as first glance suggests.
I do some thin wrapper of some scientific library (http://root.cern.ch) from unmanaged to managed world using C++ cli.
Reading of the special file format (which is the main goal) is implemented through:
1) Once a lifetime call of SetBranchAddress(const char name, void* outputVariable) to let it know an address of your variable
2) Than you N time call GetEntry(ulong numberOfRow) wthich fills this void* outputVariable with the appropriate value;
I put this example of usage :
double myValue; //this field will be filled
//We bind myValue to the 'column' called "x" stored in the file"
TTree->SetBranchAddress("x", &myValue);
// read first "entry" (or "row") of the file
TTree->GetEntry(0);
// from that moment myValue is filled with value of column "x" of the first row
cout<<"First entry x = "<<myValue<<endl;
TTree->GetEntry(100); //So myValue is filled with "x" of 101 row
...
So in C++/CLI code the problem is with binding managed elementary types to this void * pointer;
I have tried 3 approaches:
namespace CppLogicLibrary {
public ref class SharpToRoot
{
double mEventX;
double *mEventY;
IntPtr memEventZ;
///Constructor
SharpToRoot()
{
mEventy = new double();
memEventZ= Marshal::AllocHGlobal(sizeof(double));
}
void SetBranchAddresses()
{
pin_ptr<double> pinnedEventX = &mEventX;
mTree->SetBranchAddress("ev_x", pinnedEventX);
mTree->SetBranchAddress("ev_y", mEventY);
mTree->SetBranchAddress("ev_z", memEventZ.ToPointer());
...
//now I read some entry to test... just in place
mTree->GetEntry(100);
mTree->GetEntry(101);
double x = mEventX;
double y = *mEventY
double z = (double)Marshal::PtrToStructure(memEventZ, Double::typeid);
}
...
All of 3 variants are compiled with no errors, goes with no exceptions... BUT fills its (void *) values with some rubbish value like 5,12331E-305. In unmanaged code all works fine.
What could be the error with such void* to C++/CLI elementary types binding?
The problem was that internally data was presented by floats inside of this library. So, when it was mapped and processed as doubles on the C# side, it gave 5,12331E-305.
Each of this 3 variats worked. And, from my point of view, using of
pin_ptr pinnedEventX = &mEventX;
was improper in this case, because it doesn't persist between functions execution;
What I'm not sure, why this "float" situation was handled in native C++. As I wrote before, there wasn't any problem.
like: vector<void *(*func)(void *)>...
You can declare a vector of pointers to functions taking a single void * argument and returning void * like this:
#include <vector>
std::vector<void *(*)(void *)> v;
If you want to store pointers to functions with varying prototypes, it becomes more difficult/dangerous. Then you must cast the functions to the right type when adding them to the vector and cast them back to the original prototype when calling. Just an example how ugly this gets:
#include <vector>
int mult(int a) { return 2*a; }
int main()
{
int b;
std::vector<void *(*)(void *)> v;
v.push_back((void *(*)(void *))mult);
b = ((int (*)(int)) v[0])(2); // The value of b is 4.
return 0;
}
You can use typedef's to partially hide the function casting syntax, but there is still the danger of calling a function as the wrong type, leading to crashes or other undefined behaviour. So don't do this.
// shorter
std::vector<int (*)(int)> v;
v.push_back(mult);
b = v[0](2); // The value of b is 4.
Storing a function in vector might be a difficult task as illustrated above. In that case if u want to dynamically use a function u can also store a function in pointer which is much easier. Main advantage of this is u can store any type of function either it is a normal function or a paramatrized one(having some input as parametrs). Complete process is described in the link given below with examples...just have a look...!!!
how can we store Function in pointer