new（3）是什么意思？

Question

SelectInst *Sel = new(3) SelectInst(C, S1, S2, NameStr, InsertBefore);

What's the meaning of the new operator here?

What's the meaning of the number 3 after the new operator?

Answer 1

This code comes from LLVM's codebase . There's a custom operator new in scope and it is being used to placement-new initialize the objects (cfr. placement syntax )

void *User::operator new(size_t Size, unsigned Us) {
  return allocateFixedOperandUser(Size, Us, 0);
}

Here's a toy example:

class SelectInst
{
public:
  int x;
};

void *operator new(size_t Size, unsigned Us) {
                                ^^^^^^^^^^^ 3 is passed here
                   ^^^^^^^^^^^ allocation size requested

  return ... // Allocates enough space for Size and for Us uses
}

SelectInst *Create() {
  return new(3) SelectInst();
}

int main()
{
  auto ptr = Create();
  return 0;
}

Live Example

Specifically that code is being used to adjust the allocated space to fit additional data.

Answer 2

Suppose SelectInst provides a user-defined placement operator new , which takes an int as user-defined argument; the invocation syntax means use that user-defined placement operator new for memory allocation. eg

class SelectInst {
public:
    static void* operator new (std::size_t count, int args) {
    //                                            ~~~~~~~~
        ...
    }
};

SelectInst *Sel = new(3) SelectInst(C, S1, S2, NameStr, InsertBefore);
//                   ~~~

Answer 3

You probably have custom operator new in SelectInst or at global scope (similarly to placement new ):

struct SelectInst
{
    SelectInst(/*...*/);
    // ...

    static void* operator new(std::size_t sz, int custom);
    static void operator delete(void* ptr, int custom); // new counter part
};

or

void* operator new(std::size_t sz, int custom);
void operator delete(void* ptr, int custom); // new counter part

See last part of operator_new

Answer 4

On the doc , it says that this is the amount of memory to request in bytes. (If it wasn't overloaded) Here's this is a request of 3 *8 = 24 bit to store the objet in memory. Think of this at some sort of remnant of malloc.