Question

How much does using the dot operator to access some data cost speed-wise? Eg:

struct A{
    public:
       A(): a(0){};
       int a;
};

int main(){
       A obj;
       int b = 0;

       cout << obj.a;  // How much slower is this
       cout << b;      // Than this...?

       return 0;
}

I know I should benchmark, but are there any general understandings here?

Answer 1

Let’s take a look at dissasembled code:

       cout << obj.a;  // How much slower is this
0041161D  mov         esi,esp 
0041161F  mov         eax,dword ptr [obj] 
00411622  push        eax  
00411623  mov         ecx,dword ptr [__imp_std::cout (41A34Ch)] 
00411629  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (41A35Ch)] 
0041162F  cmp         esi,esp 
00411631  call        @ILT+430(__RTC_CheckEsp) (4111B3h) 
       cout << b;      // Than this...?
00411636  mov         esi,esp 
00411638  mov         eax,dword ptr [b] 
0041163B  push        eax  
0041163C  mov         ecx,dword ptr [__imp_std::cout (41A34Ch)] 
00411642  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (41A35Ch)] 
00411648  cmp         esi,esp 
0041164A  call        @ILT+430(__RTC_CheckEsp) (4111B3h) 

This was compiled without optimizations. And even so, they take the same time.
With optimizations, the code is shorter, but still remains largely the same:

       cout << obj.a;  // How much slower is this
00401000  mov         ecx,dword ptr [__imp_std::cout (40203Ch)] 
00401006  push        0    
00401008  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (402038h)] 
       cout << b;      // Than this...?
0040100E  mov         ecx,dword ptr [__imp_std::cout (40203Ch)] 
00401014  push        0    
00401016  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (402038h)] 

So for your snippet, the performance is identical. Small differences might occur on different compilers, but the overhead will always be negligible or 0.