For loop not incrementing only when qInfo looks for array value-CodePudding

#include <QCoreApplication>

int ages[4] = {23,7,75,1000};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    qInfo() << ages;
    for (int i = 0; i < 5; i  ){

        qInfo() << i;

    }

    a.exec();
    return 0;
}

returns:

0x7ff7d01f3010 0 1 2 3 4

To the terminal

But

#include <QCoreApplication>

int ages[4] = {23,7,75,1000};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    qInfo() << ages;
    for (int i = 0; i < 5; i  ){

        qInfo() << i;
        qInfo() << ages[i];
    }

    a.exec();
    return 0;
}

returns:

4713 0 4714 0 4715 0 4716 0 4717 0 4718

etc. to the terminal.

As a beginner this behavior is not intuitive and I do not understand what the difference is.

The first code was my attempt to see if my for loop was written incorrectly, but it acts as expected.

CodePudding user response：

You have undefined behaviour in your code because you are indexing the array out of its size. Your array has four elements, but you seem to try to access 5 elements in its. So, the last iteration is undefined behaviour.

You could write this instead:

#include <QCoreApplication>

int ages[4] = {23,7,75,1000};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    qInfo() << ages;
    for (int age : ages)
        qInfo() << age;

    a.exec();
    return 0;
}

Depending on your compiler, you could use the sanitizer options to catch these, for example:

-fsanitize=undefined

I personally use these in my projects:

if (CMAKE_CXX_COMPILER_ID MATCHES "(Clang|GNU)")                                
  add_compile_options(-Wall -Wpedantic -Wextra -Werror)                         
  add_compile_options(-fsanitize=address -fsanitize=undefined -fno-omit-frame-pointer)
  add_link_options(-fsanitize=address -fsanitize=undefined -fno-omit-frame-pointer)
elseif (CMAKE_CXX_COMPILER_ID MATCHES "MSVC")                                   
  set_property(DIRECTORY APPEND PROPERTY COMPILE_OPTIONS "/w")                  
endif()                                                                         
                                                                                
add_compile_definitions(QT_DISABLE_DEPRECATED_BEFORE=0xFFFFFF)

I would also like to point out that you should use size_t instead of int for the loop counter as a good practice.

It is also worth pointing out that you should avoid using raw arrays in a C , and especially Qt program. There are better choices, like:

std::array
std::vector
QList

etc.

Furthermore, you could even merge these two lines:

a.exec();
return 0;

Into the following (this is how it is typically written):

return a.exec();