postfix (prefix) increment, L-value and R-value (in C and C++)

Question

I just learned the following facts:

• The result of a prefix increment (++var_name) is an R-value in C (at least, I am sure that it is no

Answer 1

C and C++ are different languages. C++ has operator overloading and C does not. The ++ operators, whether prefix or postfix, are operators which can be overloaded in C++. C++ also has references and C does not.

In C, ++i and i++ both yield a value which is not an lvalue. This is desirable, as otherwise you could run afoul with undefined behaviour by trying to modify the same scalar within the same sequence-point boundaries.

Food for thought: In C, the comma operator also produces a value which is not an lvalue, so to "drop" lvalueness, you can do:

(0, lvalue)

Answer 2

It is true that

• pre increment/decrement operator (++var or --var) yields an lvalue (i.e a modifiable object)

• post increment/decrement operator (var++ or var--) yields an rvalue (i.e a temporary object).

Consider following code with pre increment/decrement operator

{

int i = 0;

int* pi = &(++i);

}

It´s OK because in fact its pseudo code is

i = i+1; // pre increment i

int* pi = &i; // then evaluate its address and assign it to pi 

Now consider the same code but with post increment/decrement operator and the consequences if this invalid code is accepted by the compiler

{

int i = 0;

int* pi = &(i++); // Not OK !! because it is a temporary variable

}

Its pseudo code would be

int i = 0;

int tmp = i; // compiler creates a temporary variable to save value of i

int* pi = &tmp; // then would take the address of a temporary variable 

i = i + 1;