如何在构造函数中销毁对象并返回NULL

Question

我正在尝试编写一个必须检查正确初始化的cpp模块.它需要使用至少一个非NULL指针进行初始化.如果没有,它需要删除自己并返回NULL.以下程序似乎确实破坏了对象,但它似乎没有返回null.

这里发生了什么？

#include <iostream>

using namespace std;

class cmod {
public:
        cmod(int *p1=NULL, int *p2=NULL)
        {
                if( p1 == NULL && p2 == NULL){
                        delete(this);
                }
                else
                        cout << __func__ << ": Initialized" << endl;
                if(p1 != NULL)
                        cout << "*p1 = " << *p1 << endl;
                if(p2 !=NULL)
                        cout << "*p2 = " << *p2 << endl;
        }
        ~cmod()
        {
                cout << __func__ << ": Destroyed" << endl;
        }

};

int main()
{
        int a=10, b = 20;
        cmod *p = new cmod();
        if(p == NULL)
                cout << __func__ << ": Unable to initialize" << endl;
        cmod *p1 = new cmod(&a, &b);
}

以下是输出:

~cmod: Destroyed
cmod: Initialized
*p1 = 10
*p2 = 2

为什么线Unable to initialize不打印？

更新:看完所有答案后,我想出了以下内容:

#include <iostream>

using namespace std;

class cmod {
private:
        int *l1,*l2;
        cmod()
        {
                throw std::runtime_error("Failed to construct object.   No arguements");
        }
        cmod(int *p1=NULL, int *p2=NULL)
        {
                if( p1 == NULL && p2 == NULL){
                        throw std::runtime_error("Failed to construct object. Both args NULL");
                }
                else
                        cout << __func__ << ": Initialized" << endl;
                if(p1 != NULL)
                        l1 = p1;
                if(p2 !=NULL)
                        l2 = p2;
        }
        ~cmod()
        {
                cout << __func__ << ": Destroyed" << endl;
        }
public:
        static cmod * initialize(int *p1=NULL, int *p2 = NULL)
        {
                if( p1 == NULL && p2 == NULL){
                        return NULL;
                }
                else
                        return new cmod(p1,p2);

        }
        void dump()
        {
                cout << __func__ << ": a = " << *l1 << endl;
                cout << __func__ << ": b = " << *l2 << endl;

        }
int main()
{
        int a=10, b = 20;
        cmod *p = cmod::initialize(NULL, NULL);
        if(p == NULL)
                cout << __func__ << ": Unable to initialize" << endl;
        cmod *p1 = cmod::initialize(&a, &b);
        if(p!=NULL)
                p->dump();
        if(p1!=NULL)
                p1->dump();
}

这是一个正确的方法吗？

Answer 1

构造函数将始终返回其类的对象,除非它抛出异常.所以你要做的就是这样:

cmod(int *p1=NULL, int *p2=NULL)
{
    if( p1 == NULL && p2 == NULL)
        throw std::runtime_error("Failed to construct object.");
}