c++类拆解
我有以下代码：
class Base {
public:
int x,y;
Base() { x=10; y=20; }
virtual void myfunction() { }
};
int main() {
Base *b = new Base();
return 0;
}
反汇编给了我类似的内容：
push 0Ch ; size of Base
call j_<some giberrish> ; IDA gives the comment "operator new(uint)"
add esp, 4 ; function epilogue
mov [ebp+var_E0], eax
几行后，您将调用构造函数。
mov ecx, [ebp+var_E0]
call j_Base__Base
mov [ebp+var_F4], eax
- 起初我以为 var_E0 会包含指向实例的指针，但现在我很确定 var_F4 会包含构造函数的返回值。
- 那么，
var_E0到底包含什么？为什么在调用构造函数之前将其移至 ecx 中？
I have the following code:
class Base {
public:
int x,y;
Base() { x=10; y=20; }
virtual void myfunction() { }
};
int main() {
Base *b = new Base();
return 0;
}
The disassembly gives me something like:
push 0Ch ; size of Base
call j_<some giberrish> ; IDA gives the comment "operator new(uint)"
add esp, 4 ; function epilogue
mov [ebp+var_E0], eax
A few lines later you have the constructor being called.
mov ecx, [ebp+var_E0]
call j_Base__Base
mov [ebp+var_F4], eax
- At first I had thought that
var_E0would contain the pointer to the instance, but now I'm pretty sure thatvar_F4does as it contains the return value of the constructor. - In that case, what does
var_E0contain at all? Why is it moved intoecxbefore the constructor is called?
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它是编译器临时生成的一些内部变量。
当您编写
new Base时，编译器会生成对全局的调用operator new函数，然后在返回值上调用构造函数地址。显然，您的编译器保存了从返回的地址
operator new在内存中，而不是将其保存在寄存器中。It's some internal variable for a compiler generated temporary.
When you write
new Base, the compiler generates a call to the globaloperator newfunction, then calls the constructor on the returnedaddress. Apparently, your compiler saves the address returned from
operator newin memory, rather than keeping it in a register.Visual C++ 使用内部约定，其中构造函数返回指向对象实例的指针（根据 C++ 标准，构造函数没有返回值）。因此，在您的情况下，var_E0 和 var_F4 都保存实例指针。
请查看我的文章，了解有关 Visual C++ 如何实现 C++ 的更多详细信息。
Visual C++ uses an internal convention where constructors return the pointer to the object instance (by C++ standard, constructors don't have a return value). So in your case both var_E0 and var_F4 hold the instance pointer.
Check my article for more details on how Visual C++ implements C++.
这几乎肯定是您正在查看的调试版本，并且调试版本对其所做的事情非常保守。创建对象是一个两个阶段的过程：分配内存，然后构造对象。您的编译器将分配的内存指针放入临时变量中。如果您构建优化版本，则不会存储此临时变量，因为这会引入不必要的开销（写入/读取 RAM）。
This is almost certainly a debug build you're looking at and debug builds are very conservative with what they do. Creating an object is a two stage process: allocate memory and then construct the object. Your compiler is putting the allocated memory pointer into a temporary variable. If you build an optimised version, this temporary variable won't be stored since that introduces an unnecessary overhead (writing/reading RAM).