cl::vector vs std::vector: different iterator behaviour

问题

EDIT: Added debugging output with memory locations as suggested by PlasmaHH.

I don't understand the different behaviour of the cl::vector<> in the C++ bindings for OpenCL. Consider the following code:

Header Top.hpp:

class Top {
public:
    void setBool(bool b);
    bool getBool();
private:
    bool status;
};

Source Top.cpp:

#include "Top.hpp"   

void Top::setBool(bool b) {
    std::cout << (void*)this << " setBool("<< b<< ")\n";
    status = b;
}

bool Top::getBool() {
    std::cout << (void*)this << " getBool() returns " << status << std::endl;
    return status;
}

Use the above:

#define __NO_STD_VECTOR

#include <iostream>
#include "CL/cl.hpp"
#include "Top.hpp"

using namespace cl;
using namespace std;

cl::vector<Top> js;

int main() {
    js.push_back(Top());
    js[0].setBool(true);
    cout << js[0].getBool() << endl;
    for(cl::vector<Top>::iterator i = js.begin(); i != js.end(); ++i) {
        (*i).setBool(false);
    }
    cout << js[0].getBool() << endl;
}

With __NO_STD_VECTOR the std::vector is overridden. The output is

0x6021c0 setBool(1)
0x6021c0 getBool() returns 1
0x7fffae671d60 setBool(0)
0x6021c0 getBool() returns 1

So the location returned by the iterator is definitely wrong.

Using the above with the std::vector (and changing the namespaces to std of course) however gives the expected output:

0x1be0010 setBool(1)
0x1be0010 getBool() returns 1
0x1be0010 setBool(0)
0x1be0010 getBool() returns 0

This iterator is acting differently, but it's supposed to replace the std::vector to avoid compatibility issues. Am I missing something?

回答1:

Not an expert at OpenCL by any stretch of the imagination, but I'm interested so I went over to CUDA/OpenCL Computing. I appears that their * operator returns a copy rather than a reference:

00706         T operator *()
00707         {
00708             return vec_[index_];
00709         }

Whereas the (first, non-const) vector [] operator returns a reference:

00621     T& operator[](int index)
00622     {
00623         return data_[index];
00624     }
00625   
00626     T operator[](int index) const
00627     {
00628         return data_[index];
00629     }

Try iterating through the vector directly (using the old "int i = 0, ...") and see if that gives different results. If so, you might want to put in a bug report (check first) since this is unexpected behavior for the * operator.

回答2:

Judging from the addresses I suspect that this is a 64-bit build and that the cl vector's iterator's operator* is returning by value rather than by reference, not allowing access to the original element. As an experiment you could try using the -> operator instead i->setBool(false); to see if that's implemented sanely.

来源：https://stackoverflow.com/questions/9081956/clvector-vs-stdvector-different-iterator-behaviour

标签

c++

vector

iterator

opencl