Using stack defined in C++ stl

Question

#include <stack>
using namespace std;

int main() {
    stack<int> s;
    int i;
    for (i = 0; i <= 10; i++) {
        s.push(i);
    }
    for (i = 0; i <= 10; i++) {
        printf("%d", s.pop());
    }
}

Whats wrong with the code above?

Error:

In function int main(): aggregate value used where an integer was expected

score 21 · Accepted Answer · edited Sep 02 '13 at 20:28

21

stack::pop is a void function which just discards the top element on the stack, in order to get the value you want to use stack::top.

The reason this is so is for exception safety reasons (what happens if the object returned throws an exception in its copy constructor?).

edited Sep 02 '13 at 20:28

answered Apr 11 '10 at 11:06

Motti

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3

Ooh, never realized that was the reason. I always just thought it was a cleanliness of design/separation of concerns kind of thing. +1 for enlightening me. :) – jalf Apr 11 '10 at 12:44
2

Also, `pop` would have to create a copy in order to return it. If the user doesn't want that, it's probably going to end up costing performance. – Potatoswatter Apr 11 '10 at 14:39
3

According to Stroustrup (TC++PL), the actual reason was performance: if `pop` returned the value, it would have to create an unnecessary copy (since the value is consecutively destroyed in the stack). Both sound plausible though (and both are made obsolete by move semantics). – Konrad Rudolph Apr 11 '10 at 20:41
1

@Konrad, since move contructor's are [allowed to throw](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2983.html) I don't think you can have an exception safe `pop` that returns the value. Not to mention the question of thread safety. – Motti Apr 12 '10 at 06:16
But what about (N)RVO? Wouldn't `pop` just construct the top value directly, before "popping" it, so in case the exception happens - nothing will be lost. So no move constructors will be invoked anyway. – Dan M. Apr 13 '18 at 01:10

score 2 · Answer 2 · answered Apr 11 '10 at 14:54

2

Minor nitpick, your for loop is actually encoding 11 items and not 10 like you make think from a brief look at the loop count. Consider using < 11 if you mean 11 elements to add.

answered Apr 11 '10 at 14:54

Michael Dorgan

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score 1 · Answer 3 · edited Apr 11 '10 at 12:39

You're treating pop() which is an operation to print to standard output. pop() just removes the topmost element from the stack. The most confusing thing however is your debug output.

I compiled your code fragment with the standard GNU C++ compiler which gave me:

main.cpp|12|error: invalid use of void expression

int main() {
    stack<int> s;
    int i;
    for (i = 0; i <= 10; i++) {
        s.push(i);
    }
    for (i = 0; i <= 10; i++) {
          printf("%i", s.top());
          s.pop();
    }
}

Using stack defined in C++ stl

3 Answers3

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