Summary of some problems about left value and right value [easy to understand]

Hello everyone , I meet you again , I'm your friend, Quan Jun .

stay C In language , We often encounter some small knowledge points that we usually feel will not make mistakes in how to use them , But when it is a little more difficult , Or change it a little , We will no longer use it so handy . The left value and the right value are exactly an example of such a perfect ten . After learning pointer knowledge , A high understanding of left and right values will no longer seem so boring .

This explanation looks a little silly , But I have to say ：

Lvalues are those that can appear to the left of the assignment symbol , Right values are those that can appear to the right of the assignment symbol .

for example ：a=b+25;

here ,a It's left , It identifies a point where the result value can be stored , in other words , He has a fixed space that he can control ,b+25 It's right value , Because he specified a value .

So can they exchange ？

for example ：b+25=a;

The answer, of course, is no ;

Be careful , When the computer calculates b+25 Its result must be stored somewhere in the machine . But programmers have no way to predict where the results will be stored , There is no guarantee that the value of this expression will still exist in the original place next time . The result is b+25 Can't be an lvalue . Because he doesn't have a fixed mark .

however a It can be used as an R-value , Because each position includes a value .

So in summary ：

The left value ： We mainly use his space ;

Right value ： We mainly use his value .

Literal constants cannot be lvalues .

In order to consolidate the theory learned above , In order to let us know more about what happened , Now let's discuss the following examples in depth ：

When there's code ：

char ch=‘a’;

char *cp=&ch;

Let's see what the following code means ？ Can I do lvalue ？ Can I make a right value ？

&ch; //&ch Is the address constant , Only lvalue , Can't do right value ;

cp; //cp It's a variable , Existing storage space , It's worth it again , So both left and right values can be done ;

&cp; //&cp Is the address constant , Can only do right value , You can't be left-handed .

*cp+1; // Its operation order is , Will now cp Quoting , then cp The value of the add 1, So it's a constant （ In the subject , He expressed to a+1, That's the character b）, Can only do right value , Cannot lvalue ;

*（cp+1）; // The operation here means ch Then a space ( Read cp The content of the next space ), You can do lvalue , You can also do the right value ;

++cp; // This is for ch My address plus 1, That is to add 1, You can do the right value , You can't do lvalue ;

cp++; // Reason and ++cp For the same reason ;

*++cp; // Add it first , In dereference , According to the ch The next space （ Read the contents of the next space ）, You can do lvalue , You can also do the right value ;

*cp++; // For the same reason ;

（*cp）++; // It means that you will ch My address plus 1, Is a constant , You can't do lvalue , But you can do the right value ;

++*++cp; // Said to ch The prefix of the next space address ++, Is a constant , You can't do lvalue , But you can do the right value ;

++*cp++; // Said to ch The space address of is prefixed ++, In the back ++, Is a constant , You can't be left-handed , But you can do the right value .

Okay , Come here first

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