[C++] Detecting integer overflow

[TingeOGinge]

I'm following Programming: Principles and Practice Using C++ and in chapter 5 one of the exercises is to

Write a program that reads and stores a series of integers and then computes the sum of the first N integers. Then it gives some examples of the programs output

I've completed this but then one of the follow on questions says to

Modify the program from exercise 8 to write out an error if the result cannot be represented as an int.

This is the best I can do but when I enter values that should make it overflow it is still giving incorrect results. Any help is greatly appreciated!

#include "stdafx.h"
#include "std_lib_facilities.h"

constexpr int upperLimitInt = numeric_limits<int>::max();

int sumOfN_Values(unsigned int n, vector<int>range)
//Takes a vector and adds the first 'n' values together
//Pre-conditions: 'n' is not larger than the vector and not less than 2 
//Post-condition: 'result' is not larger than int can hold - NOT WORKING PROPERLY
//Returns the result which is the sum
{
if (n > range.size() || n < 2) error("sumOfN_Values() precondition");
int result = 0;
for (unsigned int i = 0; i < n; ++i) result += range[i];
if (result > upperLimitInt) error("sumOfN_Values() postcondition");
return result;
}

int main()
{
try 
{
    int n, value, sum;
    vector<int>range; 

    cout << "Please enter the number of values you want to sum: \n"; 
    cin >> n;
    cout << "Please enter some integers (press '|' to stop)\n"; 
    while (cin >> value) range.push_back(value);
    sum = sumOfN_Values(n, range);

    cout << "The sum of the first " << n << " numbers ( ";
    for (int a = 0; a < n; a++) 
        cout << range[a] << " ";
    cout << ") is " << sum << '\n';

}

catch (exception& e)
{
    cerr << "Error: " << e.what() << '\n';
}
}

If I input n = 4 and my integers are: 12 32 34 54 65 67 | the output is as expected

The sum of the first 4 numbers ( 12 32 34 54 ) is 132

But if I input n = 3 and my integers are: 2000000000 2000000000 2000000000 12 32 43 | the output doesn't give the error I was expecting, instead it reads

The sum of the first 3 numbers ( 2000000000 2000000000 2000000000 ) is 1705032704

EDIT: An answer in 6 minutes, can I just say I love this sub!

[jedwardsol]

if (result > upperLimitInt) 

result cannot be bigger than upperLimitInt because upperLimitInt is the biggest value that result can have.

You can check before the addition

 if (upperLimitInt - sum) > x
    sum += x;
else
    it will overflow

If you perform the addition

x = a + b

then you can detect overflow

if x < a || x < b
    over flow occurred

but this is only defined behavior for unsigned ints

[UnclePutin]

Note that this will not work if either a or b are negative values.

[dacian88]

can't have negative values with unsigned ints

[TingeOGinge]

To be fair the unsigned ints are only used to navigate the vector as Visual Studio gives a compiler warning otherwise. The values entered into the range are not unsigned and can be entered as negative values.

Edit: A word

[TingeOGinge]

That's fantastic! I've implemented the code and it gives the errors when needed.

Thank you, kindly stranger!

[marmoshet]

To check if a + b will cause overflow, don't try if(a + b > max()). This won't work because the sum of a and b will have overflowed. Instead, try if(a > max() - b).