#include <iostream>
#include <new>
#include <cstdio>
using namespace std;

struct A
{
    void *operator new(size_t size) 
    {
        cout<<"operator new"<<endl;
        malloc(size);
    }
    void operator delete(void *p)
    {
        cout<<"operator delete"<<endl;
        free(p);
    }
    
    void *operator new(size_t size,const nothrow_t &thorw_value) noexcept
    {
        cout<<"operator new noexcept"<<endl;
        malloc(size);
    }
    void operator delete(void *p,const nothrow_t &nowthrow_value) throw()
    {
        cout<<"operator delete noexpect."<<endl;
        free(p);
    }
}; 

int main()
{
    A *a1=new A;
    delete a1;
    
    A *a2=new(nothrow) A;
    delete a2;//为什么调用的是 operator delete 而不是  operator delete noexpect
    return 0;
}

你这个代码里可全是 UB...
After that, whether or not an exception was thrown by any destructor, the delete expression invokes the deallocation function: either operator delete (for the first version of the expression) or operator delete[] (for the second version of the expression), unless the matching new-expression was combined with another new-expression (since C++14)

https://en.

看懂了，但不是很理解。

就是 C++ 14 之前, 我估计你编译器应该不支持吧, 默认都是用 throw 的那一版本

两处 malloc(size) 应该是 return malloc(size)，你的代码编译过吗？

特殊的 operator delete 是在对应的 operator new 成功，但 构造函数 失败时才被调用。你想一想吧，编译怎么可能知道一个指针内存是哪个重载的operator new分配的？

#include <iostream>
#include <new>
using namespace std;

struct A
{
    A( bool xpt )
    {
        if( xpt )
            throw( xpt );
    }

    void *operator new(size_t size)
    {
        cout<<"operator new"<<endl;
        return malloc(size);
    }
    void operator delete(void *p)
    {
        cout<<"operator delete"<<endl;
        free(p);
    }
  

    void *operator new(size_t size,const nothrow_t &thorw_value) noexcept
    {
        cout<<"operator new noexcept"<<endl;
        return malloc(size);
    }
    void operator delete(void *p,const nothrow_t &nowthrow_value) throw()
    {
        cout<<"operator delete noexpect."<<endl;
        free(p);
    }
};

int main()
{
    A* a1 = new A(false);
    delete a1;

    try {
        A* a2 = new A(true);
        delete a2;
    }
    catch( ... )
    {
    }
  

    A* a3 = new(nothrow) A(false);
    delete a3;

    try {
        A* a4 = new(nothrow) A(true);
        delete a4;
    }
    catch( ... )
    {
    }

    return 0;
}

输出
operator new
operator delete
operator new
operator delete
operator new noexcept
operator delete
operator new noexcept
operator delete noexpect.

好的。谢谢。

好的 谢谢。