我正在尝试对自己进行移动构造函数和移动赋值的教育，以便让我的学生开始使用 C++11 的这一特性。我已经看到（并在本网站的其他地方解释过）编译器会在大多数情况下优化移动（或普通）构造，因此您无法在工作中看到它。在 gcc 4.7.0 中，-fno-elide-constructors 选项将关闭它，您可以看到移动构造正在发生。但是相同的标志应该（？）是 clang 选项，当我在 Xcode 4.4.1 中使用 clang、c++11 和 stdc++ 将其指定为“其他 C++ 标志”时，显然不会复制返回值或者根本搬出去！
这是完整的测试代码和带有问题发生位置注释的输出：
#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;
}
使用 clang 和 -fno-elide 构造函数运行它并不会走得太远：被调用函数中的局部变量在返回期间被销毁，我们最终从一个虚假对象进行了移动赋值：
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
程序在下一个测试中停止，因为 b 中没有任何内容。
在 gcc 4.7.0 下运行的相同代码似乎很有意义，并显示了移动构造的工作： 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
所以关闭省略在 gcc 4.7 中给出了一个合理的结果，但在 clang 中产生了损坏的代码。这是clang中的一个错误，还是clang实际上不支持该编译器选项？