The pit of sizeof operator in C language

Preface

When I was reading in my bedroom today , Asked by my roommate about

sizeof

The problem with the operator , Unexpectedly, he fell into the pit , It is hereby recorded that , So as not to “ Step into the same river again ”

Text

Let's start with a piece of code

#include <stdio.h>

int main()
{
 int i = 1;
 
 printf(&quot;%d\n&quot;,sizeof(i++));
 printf(&quot;%d\n&quot;,i);
 
 return 0;
}


What is the output of this code ？

First

sizeof

Is an operator used to calculate the memory size of a data type , stay VS in

int

Occupy 4 byte , So use

sizeof

After evaluation, it should be 4, And then we did

i++

, Then output i The value of should be 2.

So my answer is ：

4
2


then … I was wrong and !(ÒωÓױ)

The correct answer should be ：

4
1


Why is that ？ Here's why ：

Many beginners may think

sizeof

Is the function , Because its use is very similar to that of functions , It's not ,

sizeof

It's not a function , It's an operator

. If you check any one C When the operator priority table in Language Textbooks , We can all see the operator

sizeof

Is the highest priority

. Its usage is very simple , The result of solitude is the size of memory occupied by operands , When the operand can be

int

、

char

Data type , Or variables . But about

sizeof

Use with care , because

It completes the calculation at compile time , It is already a constant value when the function is running .

So

sizeof(i++)

Medium

i++

You can't calculate .╮(╯▽╰)╭