移动构造函数（错误：调用隐式删除的拷贝构造函数）

Question

I am experimenting for the first time with the move constructor and the move assignment operator but when I compile using clang++:

c++ -o image -O3 image.cpp -std=c++0x -stdlib=libc++

I get the following error message:

image.cpp:125:11: error: call to implicitly-deleted copy constructor of 'Image'
    Image res = sample;

I really don't understand what that means and how to fix this? I copied the entire code. Any help would be greatly appreciated.

PS: I looked on the web and couldn't find anything about this error message (apart some post from 2011/2012 relative to a bug in clang++ which I believe would have been fixed by then).

#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <fstream>
#include <cassert>

class Image
{
public:
    Image() : w(512), h(512), d(NULL)
    {
        //printf("constructor default\n");
        d = new float[w * h * 3];
        memset(d, 0x0, sizeof(float) * w * h * 3);
    }
    Image(const unsigned int &_w, const unsigned int &_h) : w(_w), h(_h), d(NULL)
    {
        d = new float[w * h * 3];
        memset(d, 0x0, sizeof(float) * w * h * 3);
    }
    // move constructor
    Image(Image &&img) : w(0), h(0), d(NULL)
    {
        w = img.w;
        h = img.h;
        d = img.d;
        img.d = NULL;
        img.w = img.h = 0;
    }
    // move assignment operator
    Image& operator = (Image &&img)
    {
        if (this != &img) {
            if (d != NULL) delete [] d;
            w = img.w, h = img.h;
            d = img.d;
            img.d = NULL;
            img.w = img.h = 0;
        }
        return *this;
    }
    //~Image() { if (d != NULL) delete [] d; }
    unsigned int w, h;
    float *d;
};

int main(int argc, char **argv)
{
    Image sample;// = readPPM("./lean.ppm");
    Image res = sample;
    return 0;
}

Answer 1

According to the C++ Standard

If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted

That your example would work try the following

Image res = std::move( sample );

Answer 2

Well as the message says - the image class doesn't have a copy constructor. So you can either implement one or if your intention was to test move constructor try this:

int main(int argc, char **argv)
{
  Image sample;// = readPPM("./lean.ppm");
  Image res = std::move(sample);
  return 0;
}

std::move will cast the sample to r-value reference, so that it can be used by move-constructor/move-assignment.