C++移动构造函数，移动赋值和编译选项-fno-elide-constructors

示例代码：

#include 

using namespace std;
class Array2D {
private:
    int m,n;  // m rows, n cols
    int *array;
public:
    Array2D(int _m, int _n):m(_m), n(_n) {
        cout << "ctor" << endl;
        array = new int[m*n];
    }

    Array2D(const Array2D &a) {
        cout << "copy ctor" << endl;
        int j,k;
        m = a.m;
        n = a.n;
        array = new int[m*n];
        for(int i = 0; i < m*n; ++i) {
            k = i % a.n;
            j = (i - k) /n;
            array[i] = a(j, k);
        }
    }

    Array2D(Array2D &&a) noexcept {
        cout << "move ctor" << endl;
        m = a.m;
        n = a.n;
        array = a.array;
        a.array = nullptr;
    }


    ~Array2D() {
        delete[] array;
        cout << "dtor" << endl;
    }
    int& operator()(int x, int y) {
        return array[x*n+y];
    }

    const int& operator()(int x, int y) const {
        return array[x*n+y];
    }

    Array2D& operator=(const Array2D &a) {
        cout << "call operator=" << endl;
        if (this->array != a.array) {
            int j,k;
            this->m = a.m;
            this->n = a.n;
            delete[] this->array;
            this->array = new int[m*n];
            for(int i = 0; i < m*n; ++i) {
                k = i % a.n;
                j = (i - k) /n;
                this->array[i] = a(j, k);  //calling: const int& operator()(int x, int y) const
            }
        }
        return *this;
    }

    Array2D& operator=(Array2D &&a) {
        cout << "call move operator=" << endl;
        m = a.m;
        n = a.n;
        array = a.array;
        a.array = nullptr;
        return *this;
    }

};


Array2D getArray2D() {
    cout << "in getArray2D()" << endl;
    Array2D a2d(1000,1000);
    a2d(100,100) = 100;
    a2d(200,200) = 200;
    a2d(900,900) = 900;
    return a2d;
}

（1）main中的测试代码：

Array2D big1 = getArray2D();
cout << big1(100,100) << " " << big1(200,200) << " " << big1(900,900) << endl;

	C++98	C++11 & C++14		C++17 & C++20
With -fno-elide-constructors	in getArray2D() ctor copy ctor dtor copy ctor dtor 100 200 900 dtor	in getArray2D() ctor move ctor dtor move ctor dtor 100 200 900 dtor		in getArray2D() ctor move ctor dtor 100 200 900 dtor
Without -fno-elide-constructors	in getArray2D() ctor 100 200 900 dtor

（2）main中的测试代码：

Array2D big3(1,1);
big3  = getArray2D();
cout << big3(100,100) << " " << big3(200,200) << " " << big3(900,900) << endl;

	C++98	C++11 & C++14 & C++17 & C++20
With -fno-elide-constructors	ctor in getArray2D() ctor copy ctor dtor call operator= dtor 100 200 900 dtor	ctor in getArray2D() ctor move ctor dtor call move operator= dtor 100 200 900 dtor
Without -fno-elide-constructors	tor in getArray2D() ctor call operator= dtor 100 200 900 dtor	ctor in getArray2D() ctor call move operator= dtor 100 200 900 dtor

说明：使用C++98标准编译时，需要注释掉移动构造函数和移动赋值函数

-fno-elide-constructors

The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

 默认情况下，不带-fno-elide-constructors选项，编译器会进行良好的优化。