汇编角度看值传递类对象

首先奉上源代码

class A{
private:
    int i;
public:
    A() {
        std::cout << "default constructor is called" << std::endl;
        i = 0;
    }

    A(int _i): i(_i) {}
    
    A(const A& obj) {
        std::cout << "copy constructor is called" << std::endl;
        i = obj.i;
    }

    A& operator = (const A& obj) {
        std::cout << "assignment constructor is called" << std::endl;
        i = obj.i;
        return *this;
    }

    A (const std::initializer_list<int>& l) {
        std::cout << "initializer_list constructor is called" << std::endl;
        i = *(l.begin());
    }

    int get()const  {
        return i;
    }

    void set(int j) {
        i = j;
    }
};

void foo(A a){
    a.set(0xFF);
 }

int main() {
    A a(3);
    foo(a);
    return 0;

}

对于main函数中foo(a)这个函数调用，生成的汇编如下：

45	    foo(a);
   0x00005562b9bc5219 <+44>:	lea    rdx,[rbp-0x10]
   0x00005562b9bc521d <+48>:	lea    rax,[rbp-0xc]
   0x00005562b9bc5221 <+52>:	mov    rsi,rdx
   0x00005562b9bc5224 <+55>:	mov    rdi,rax
   0x00005562b9bc5227 <+58>:	call   0x5562b9bc52d6 <A::A(A const&)>
   0x00005562b9bc522c <+63>:	lea    rax,[rbp-0xc]
   0x00005562b9bc5230 <+67>:	mov    rdi,rax
   0x00005562b9bc5233 <+70>:	call   0x5562b9bc51c9 <foo(A)>

从以上汇编可以明显看到，如果foo()的形参是一个值类型的话，首先调用copy构造函数生成一个实参对象的副本，然后把这个副本的地址传递给函数调用，记住，是副本的地址！