Question

My program crashes because it reaches a stack.top() it shouldn’t reach, as the stack is empty.
I have an if which checks just that:

    if(!st.empty());
        //do stuff

(I have initialized

stack<int> st;

).

But although I can see in the debug that the stack is empty, it still goes in the if!
I even wrote this code:

    if(st.size()>0);
        cout<<st.size();

And it prints 0!
What is going on and how can I fix it? Thanks!

Answer 1

The semicolons after the if statements are the problem

BAD:

if(st.size()>0); // <-- this should not be here!!!!!!!!
    cout<<st.size();

Properly rewritten:

if(st.size()>0) {
    cout<<st.size();
}

Also, as @WhozCraig pointed out, the other statement has a semicolon too!

BAD:

if(!st.empty()); // <--BAD!
    //do stuff

Good:

if(!st.empty()) {
    //do stuff
}

ALWAYS!! use brackets with branches (if, switch), and loops (for, while, do-while)!!! It pays off big time! (Not to mention, a cute kitten dies each and every time such a block is written without brackets!) ALWAYS!!

For example this can kill a day in debugging:

BAD:

int i=0;
...
while(i++<1000);
    doStuff(i);

Good:

int i=0;
...
while(i++<1000) {
    doStuff(i);
}

Beware (as @WhozCraig pointed out again) this does not automagically solve the problem of semicolon terminated branch and loop statements, as this is perfectly valid syntax:

if (condition);{ ...code... } 

Or

if (condition);
{ 
     ...code... 
} 

But in my opinion and experience (this is totally subjective!) – as I myself have fallen into this trap a couple of times – I experienced that when I have the curly bracket after the aforementioned statements, I didn’t ever make the mistake of typing a semicolon again. Adhering to this convention was a silver bullet – for me, and others could benefit from this too. Also, if there was a semicolon there, it would immediately catch my eye, just by looking, as it is an “uncommon pattern of characters”.