(c + +) i++ and ++i, which are higher efficiency

In reading "Programmer interview written book", found such a problem, the book only gives the ++i more efficient, but did not give specific explanations and explanations.

Find the following answers online:

1, from the advanced level to explain

++i is i=i+1, the value of the expression is I itself

I++ is also i=i+1, but the value of the expression is the copy before 1, which is less efficient because you want to save the copy first.

For C + + built-in types, most compilers do optimizations, so there is no difference in efficiency. However, on the custom type, it is not necessarily optimized, ++i efficiency will be higher.

2, from the bottom of the assembly to see the built-in type

int a,i=0; A=++i; The assembly code is as follows: int a,i=0;01221a4e mov dword ptr [I],0a=++i;01221a55 mov eax,dword ptr [i]01221a58 add eax,101221a5b mov dwor D ptr [i],eax01221a5e mov ecx,dword ptr [i]01221a61 mov dword ptr [A],ecxint a,i=0; a=i++; The assembly code is as follows: int a,i=0;009e1a4e mov dword ptr [I],0a=i++;009e1a55 mov eax,dword ptr [i]009e1a58 mov dword ptr [a],eax009e1 A5B mov ecx,dword ptr [i]009e1a5e add ecx,1009e1a61 mov dword ptr [I],ECX

As you can see from the assembly code above, the number of executions is the same for the built-in types, with no difference in efficiency.

3, from overloaded operators to see the custom type

Operator operator::operator++ () {++value;//Internal member variable return *this;} Operator operator::operator++ (int) {Operator Temp;temp.value=value;value++;return temp;}

As can be seen from the above code, the Post + + has an operation to save the temporary object, so the efficiency is naturally lower.

Summarize:

For C + + built-in types, the efficiency of the two is not very different;

For custom classes, ++i is more efficient.

Reference article:

Http://www.cplusplus.me/1303.html

(c + +) i++ and ++i, which are higher efficiency