C++中vptr（虚指针）的返回类型是什么？

Great question! Let's break this down clearly, since vptr and vtable are compiler-specific implementation details the C++ standard doesn't formally define, but are universally used across major compilers to enable runtime polymorphism.

First, quick context: A vptr is a hidden pointer embedded in every object that has at least one virtual function (or inherits from a class with virtual functions). It points to the object's corresponding vtable (virtual function table)—an array storing pointers to the object's virtual functions.

The Actual Type of vptr #

In nearly all mainstream compilers (GCC, Clang, MSVC, etc.), the vptr is a pointer to the start of the vtable array. Since the vtable is an array of function pointers, the vptr's type is typically:

• void** (a pointer to a pointer to void): This generic type lets the compiler handle different virtual function signatures uniformly.
• Some compilers might use a more specific type like void (*[])() (a pointer to an array of void function pointers), but void** is the most common implementation for its flexibility.

Example to Illustrate #

Suppose we have a simple class with virtual functions:

class MyClass {
public:
    virtual void greet() { /* ... */ }
    virtual int calculate(int val) { /* ... */ }
};

The compiler generates a vtable for MyClass that acts like an array holding pointers to greet() and calculate(). Each MyClass object will have a hidden vptr pointing to the first element of this array—so the vptr's type is void** (or the equivalent specific function pointer array pointer).

Key Notes #

• You can't directly access or declare a vptr in standard C++—it's a compiler-internal detail. Some compilers offer non-standard extensions (like MSVC's __vfptr) to access it, but this isn't portable across compilers.
• The exact type might vary slightly between compilers, but its core purpose never changes: to point to the vtable so the compiler can resolve virtual function calls at runtime.

内容的提问来源于stack exchange，提问作者Praveer Kumar