Recently, I am preparing to attack Likou, and of course it has just begun. I plan to make a thought summary every 50 questions. But in the process of doing it recently, I found a problem, that is, the for loop often given by the official solution is ++i, including other people's solution, the same use of ++i, I am very confused here, is it ++ Is i faster than i++? Otherwise, I should see both writing methods in the solution, but almost all I see are ++i, so obviously, there is a problem!
Without further ado, let's start with the dry goods.
The difference between ++i and i++
To put it simply, we all know that ++iit is self-incrementing first, and then computing. Instead,i++ it is calculated first and then incremented.
Here is an interesting phenomenon:
#include <iostream>
using namespace std;
int main()
{
int a = 0;
int b = 0;
int *c = &(a++);
int *d = &(++b);
return 0;
}
Error reported after compilation
main.cpp:7:19: error: lvalue required as unary ‘&’ operand
int *c = &(a++);
& acts on an lvalue, a++not an lvalue, but ++bthe result of is an lvalue.
Easy to understand lvalue and rvalue
- lvalue: named object, assignable
- Rvalue, temporary object, not assignable
It means that a++it is an rvalue, or "temporary object", why is this?
We can look at the principle of operator overloading
class Test
{
public:
Test& operator++();//前置自增
const Test operator++(int);//后置自增
private:
int curPos; //当前位置
};
/*前置自增实现范式*/
Test& Test::operator++()
{
++curPos; //自增
return *this; //取值
}
/*后置自增实现范式，为了与前置区分开，多了一个int参数，但从来没用过*/
const Test Test::operator++(int)
{
Test tmp = *this; //取值
++curPos; //自增
return tmp;
}
We can see that although the value of curPos is auto-incremented after post-increment, it returns a temporary object, and the next call will be the value of curPos at the current moment.
So in C++, ++i is faster than i++. If possible, try to use ++i for auto-increment in programming.
The old rules are useful Erlian, thank you everyone.