I want to understand in detail distinct between expression and statement in C++. Pls pick concrete example to explain that

Question

Now I am learning C++ programming. I didn't understand the distinction between expression, definition, declaration, and definition.

As Wikipedia says,

In "Statement(computer science)"

In most languages, statements contrast with expressions in that statements do not return results and are executed solely for their side effects, while expressions always return a result and often do not have side effects at all.

In "Expression(Computer Science)" page

In many programming languages a function, and hence an expression containing a function, may have side effects. An expression with side effects does not normally have the property of referential transparency. In many languages (e.g. C++), expressions may be ended with a semicolon (;) to turn the expression into an expression statement. This asks the implementation to evaluate the expression for its side-effects only and to disregard the result of the expression (e.g. "x+1;") unless it is a part of an expression statement that induces side-effects (e.g. "y=x+1;" or "func1(func2());"). Caveats

Concretely, What do "side effects" and "result" mean here?

Help me, C++ Geeks!

instead of reading the wikipedia language agnostic article you should rather read a [book about c++](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — 463035818_is_not_an_ai, Jun 22 '20 at 08:24
Have a read of https://en.cppreference.com/w/cpp/language/statements — Richard Critten, Jun 22 '20 at 08:50
Wikipedia isn't half as good as its editors think it is, and these two pages are awful. Get a decent book. — molbdnilo, Jun 22 '20 at 08:53

Agnius Vasiliauskas · Accepted Answer · 2020-06-22T12:37:24.457

Concretely, What do "side effect" and "result" mean here?

Expression has no side-effects, when removing it from source does not change program semantics.

int main(void) {
   int x = 1, y = 2, z = 0;
   // x+y expression calculates sum and ignores resulting answer 
   // NO SIDE EFFECTS, can be removed
   x+y; 
   // x+y expression calculates sum, but then 15 is assigned to z as a result 
   // SIDE EFFECT is that removing given expression breaks program syntax - can't be removed 
   z = (x+y, 15);
}

EDIT

BTW, keep in mind that not all expression statements has side-effects too. For example x=x; is technically equivalent to ; - an empty statement which is compiled into NOP at assembly level or skipped by GCC optimizer at all. So these kinds of expression statements has no side-effects and can be removed safely from a program. But it doesn't mean that you can remove each empty statement without changing program logic. For example as in this snippet :
for (i=0; i < 10; i++);
Here NOP is executed with each CPU cycle, so if you will remove it - program semantics will change radically.

No, `++x;` is an expression **statement**,- expression itself would be just `++x` part. In general expression is generally something between sequence points,- some tokens. For example in `if (a+b > c+d)` if part has expression `a+b > c+d` with 2 sub-expressions `a+b` and `c+d`. — Agnius Vasiliauskas, Jun 22 '20 at 12:05

score -1 · Answer 2 · answered Jun 22 '20 at 08:33

The declaration informs the compiler that the given name is already known. Memory for the object, however, is not allocated. We cannot refer to an object, we cannot assign it value, because it does not yet exist.

extern varType varName;

The definition reserves memory space for a given variable. For which variable, I hear you ask. That's right - the variable must be declared somewhere, so that its name and type are known. Well, there is a trap in it, because each definition is at the same time a declaration (but not vice versa).

int number;
varType varName;

The expression consists of operators, constants, and variables which are arranged according to the rules of the language. It can also contain function calls which return values.

x = a+(b*c);
bool ifTrue = a>b;

You should read more about expressions there.

I want to understand in detail distinct between expression and statement in C++. Pls pick concrete example to explain that

2 Answers2