c++,c++11 , Why is the boolean value within a structure within a vector not being updated?


Why is the boolean value within a structure within a vector not being updated?

Question:

Tag: c++,c++11

This might sound like a very basic question, but I have trying to fix a simple bug for over an hour now and I can't seem to understand what's happening.

I have the following structure declaration in my header file:

struct StudentBody
{
    string name;

    Vec2 position;

    bool disabled;

    StudentBody(string name, Vec2 position) : name(name), position(position) {}
};

This structure is being filled into a vector of type:

std::vector<StudentBody> students_real;

Like this:

students_real =
    {
        StudentBody("student1",Vec2(DISPLAY_WIDTH - 50, LOWER_MARGIN + 100)),
        StudentBody("student2",Vec2(DISPLAY_WIDTH - 100, LOWER_MARGIN + 100)),
        StudentBody("student3",Vec2(DISPLAY_WIDTH - 150, LOWER_MARGIN + 100)),
        StudentBody("student4",Vec2(DISPLAY_WIDTH - 200, LOWER_MARGIN + 100))
    };

By default all of the students have their "disabled" set to false.

Then I have an "update" method that gets triggered with the screen refresh rate, and within that method I have the following code:

for (auto it = students_real.begin(); it != students_real.end(); it++)
        {
            auto student_to_check = *it;

            CCLOG("student %s disabled -> %i",student_to_check.name.c_str(),student_to_check.disabled);

            if (student_to_check.name == "student1" || student_to_check.disabled) {
                continue;
            }

            bool disableStudent = true;

            //... A custom condition here checks if "disabledStudent" should become false or stay as true...

            if (disableStudent)
            {
                CCLOG("Disabling %s",student_to_check.name.c_str());

                student_to_check.disabled = true;

                CCLOG("student %s disabled -> %i",student_to_check.name.c_str(),student_to_check.disabled);
            }
        }

The problem here is that the "disabled" flag is not staying as true. When I check the condition at first it's false. Then I check my second condition too and if it's satisfied I set it to true. However the next time this for loop is started the condition is back to false.

This makes me believe that my "auto student_to_check = *it;" gives me a copy of the structure to handle it but not the structure itself? Or what is going on? Why can't I modify the value of the structure within the vector?


Answer:

This:

auto student_to_check = *it;

declares a local variable that is a copy of the structure in the vector. The iterator points to the structure in the vector, so you can use:

auto student_to_check = it;

and:

student_to_check->disabled = true;

or more simply the following to access anything in the vector's structure. Then you don't need the local variable:

it->disabled = true;

Even better would be to use C++11's range-based for loop, as @sp2danny commented:

for(auto& student_to_check : students_real)

student_to_check will reference the structure in the vector instead of a local copy, and the rest of your code remains as is.


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