I'm trying to educate myself on move constructors and move assignment so I can get my students started on this feature of C++11. I've seen (and explained elsewhere on this site) that compilers will optimize away the move (or ordinary) construction in most cases so you can't see it at work. With gcc 4.7.0, the -fno-elide-constructors option will turn that off and you can see move construction happening. But that same flag is supposed(?) to be clang option, and when I specify it as "Other C++ flags" in Xcode 4.4.1 with clang, c++11, and stdc++, apparently a returned value doesn't get copied or moved out at all!
Here is the complete test code and the output annotated with where the problem happens:
#include <iostream>
using namespace std;
class Array_Exception {
public:
Array_Exception (int v, const char * msg) :
value (v), msg_ptr(msg) {}
int value;
const char * msg_ptr;
};
class Array {
public:
Array() : size(0), ptr(nullptr), serial_number(++counter) {
cout << "Array " << serial_number << " default constructed";
}
Array(int size_) : size(size_), serial_number(++counter) {
if(size <= 0)
throw Array_Exception(size, "size must be greater than 0");
ptr = new int[size];
cout << "Array " << serial_number << " constructed" << endl;
}
Array(const Array& source) : size(source.size), ptr(new int[size]), serial_number(++counter) {
for (int i = 0; i < size; i++)
ptr[i] = source.ptr[i];
cout << "Array " << serial_number << " constructed from " << source.serial_number << endl;
}
// move constructor - take the guts from the source and leave it in a safely destructable state
Array(Array&& source) : size(source.size), ptr(source.ptr), serial_number(++counter) {
source.size = 0;
source.ptr = nullptr;
cout << "Array " << serial_number << " move constructed from " << source.serial_number << endl;
}
// copy the data from rhs into lhs object using copy-swap
Array& operator= (const Array& source) {
Array temp(source);
swap(temp);
cout << "Array " << serial_number << " assigned from " << source.serial_number << endl;
return *this;
}
// move assignment just swaps source with this.
Array& operator= (Array&& source) {
swap(source);
cout << "Array " << serial_number << " move assigned from " << source.serial_number << endl;
return *this;
}
// swap the member variable values of this object with the other (serial_numbers are NOT swapped)
void swap(Array& other) {
int t_size = size; // could use std::swap
size = other.size;
other.size = t_size;
int * t_ptr = ptr;
ptr = other.ptr;
other.ptr = t_ptr;
}
~Array() {
delete[] ptr;
cout << "Array " << serial_number << " destroyed" << endl;
}
int get_size() const {return size;}
// overloaded plus operator returns an Array containing the sum of the two
Array operator+ (const Array& rhs) { // "this" object is lhs
// must have the same size
if(size != rhs.get_size())
throw Array_Exception(size, "LHS and RHS must have the same size for +");
Array result(size);
for(int i = 0; i < size; i++)
result[i] = ptr[i]+rhs[i];
return result;
}
const int& operator[] (int index) const {
if ( index < 0 || index > size - 1) {
throw Array_Exception(index, "Index out of range");
}
return ptr[index];
}
int& operator[] (int index) {
if ( index < 0 || index > size - 1) {
throw Array_Exception(index, "Index out of range");
}
return ptr[index];
}
private:
int size;
int* ptr;
int serial_number;
static int counter;
};
int Array::counter = 0;
void print_all(const char * msg, const Array& a);
Array reverse_contents(Array a); // call and return by value
int main()
{
Array a(5), b(5), c(5);
for (int i = 0; i < 5; i++)
a[i] = i; // 0, 1, 2, 3, 4
try {
cout << "\ncall by value and assign from returned value:" << endl;
print_all("a has", a);
b = reverse_contents(a);
print_all("b has reversed", b);
cout << "\nassign from rvalue expression" << endl;
c = a + b;
print_all("c has sum", c);
cout << "\nconstruct from rvalue expression" << endl;
Array e(a + c);
print_all("e has copy of a + c", e);
cout << "\nconstruct from function return value" << endl;
Array d(reverse_contents(a));
print_all("d has returned reverse of a", d);
}
catch (Array_Exception& x) {
cout << x.msg_ptr << ' ' << x.value << endl;
}
cout << "\nDone!" << endl;
}
void print_all(const char * msg, const Array& a) {
cout << msg << ": ";
for(int i= 0; i < a.get_size(); i++)
cout << a[i] << ' ';
cout << endl;
}
Array reverse_contents(Array a) {
int n = a.get_size();
Array result(n);
for(int i = 0; i < n; i++) {
result[i] = a[n-i-1];
}
return result;
}
Running this with clang and -fno-elide constructors doesn't get very far: the local variable in the called function gets destroyed during the return and we end up doing a move assignment from a bogus object:
Array 1 constructed
Array 2 constructed
Array 3 constructed
call by value and assign from returned value:
a has: 0 1 2 3 4
Array 4 constructed from 1
Array 5 constructed
Array 5 destroyed << local Array has been destructed
Array 2 move assigned from 0 << Array with serial_number == 0 is bogus object
Array 0 destroyed << bogus object destroyed here
Array 4 destroyed
b has reversed: << nothing in the returned result
The program halts in the next test because b doesn't have anything in it.
The same code running under gcc 4.7.0 seems to make sense and shows move construction at work: g++ -std=c++11 *.cpp -fno-elide-constructors
Array 1 constructed
Array 2 constructed
Array 3 constructed
call by value and assign from returned value:
a has: 0 1 2 3 4
Array 4 constructed from 1
Array 5 constructed
Array 6 move constructed from 5
Array 5 destroyed
Array 2 move assigned from 6
Array 6 destroyed
Array 4 destroyed
b has reversed: 4 3 2 1 0
assign from rvalue expression
Array 7 constructed
Array 8 move constructed from 7
Array 7 destroyed
Array 3 move assigned from 8
Array 8 destroyed
c has sum: 4 4 4 4 4
construct from rvalue expression
Array 9 constructed
Array 10 move constructed from 9
Array 9 destroyed
Array 11 move constructed from 10
Array 10 destroyed
e has copy of a + c: 4 5 6 7 8
construct from function return value
Array 12 constructed from 1
Array 13 constructed
Array 14 move constructed from 13
Array 13 destroyed
Array 15 move constructed from 14
Array 14 destroyed
Array 12 destroyed
d has returned reverse of a: 4 3 2 1 0
Array 15 destroyed
Array 11 destroyed
Done!
Array 3 destroyed
Array 2 destroyed
Array 1 destroyed
So turning off elision gives a sensible result in gcc 4.7, but produces broken code in clang. Is this a bug in clang, or is that compiler option actually not supported in clang?
This looks like this clang bug:
来源：https://stackoverflow.com/questions/12241448/is-clang-xcode-4-4-1-buggy-when-fno-elide-constructors-is-set