raz raz - 23 days ago 12
C++ Question

Integer value assignment in c++

I'm pretty new to C++ and I have the following simple program:

int main()
{
int a = 5,b = 10;
int sum = a + b;
b = 6;
cout << sum; // outputs 15
return 0;
}


I receive always the output 15, although I've changed the value of b to 6.
Thanks in advance for your answers!

Answer

There are already answers, but I feel that something is missing... When you make an assignment like

sum = a + b;

then the values of a and b are used to calculate the sum. This is the reason why a later change of one of the values does not change the sum. However, since C++11 there actually is a way to make your code behave the way you expect:

#include <iostream>     
int main() {
    int a = 5,b = 10;
    auto sum = [&](){return a + b;};
    b = 6;
    std::cout << sum(); 
    return 0;
}

This will print :

11

This line

auto sum = [&](){return a + b;};

declares a lambda. I cannot give a selfcontained explanation of lambdas here, but only some handwavy hints. After this line, when you write sum() then a and b are used to calculate the sum. Because a and b are captured by reference (thats the meaning of the &), sum() uses the current values of a and b and not the ones they had when you declared the lambda. So the code above is more or less equivalent to

int sum(int a, int b){ return a+b;}

int main() {
    int a = 5,b = 10;
    b = 6;
    std::cout << sum(a,b);
    return 0;
}