在gcc中显示char数组不起作用

Question

I wrote a piece of code and tested with gcc compiler

#include <iostream>

int main()
{
    char arr[ 1000 ];
    for( int index( 0 ); index < 1000; ++index )
    {
        std::cout << arr[ index ] << std::endl;
    }
    return 0;
}

I was hoping it to print the garbage values but to my surprise, it did not print anything. When I simply changed the datatype of arr from char to int, it displayed the garbage values as expected. Could somebody please explain this to me?

Answer 1

The overloads for << for character types do not treat them as integral types, but as characters. If the garbage value corresponds to a printable character (eg 97, which corresponds to 'a' ), you will see it. If it doesn't (eg 0), you won't. And if the garbage values correspond to some escape sequence which causes your terminal to use a black foreground on a black background, you won't see anything else, period.

If you want to see the actual numerical values of a char (or any character type), just convert the variable to int before outputting it:

std::cout << static_cast<int>( arr[index] ) << std::endl;

Answer 2

What you're trying to do has an undefined behavior . Some compilers will clear out the memory for you, others will leave it as it was before the creation of your buffer.

Overall, this is a useless test.

Answer 3

Some platforms may choose, for example for security purposes, to fill the uninitialized char array with zeroes, even though it's not static and wasn't explicitly initialized. Therefore, that is why no garbage is showing up - your char array was just automatically initialized.

Answer 4

On your platform garbage characters don't print. On another platform it might be different.

As an experiment try this

std::cout << '|' << arr[ index ] << '|' << std::endl;

See if anything appears between the || characters.

Answer 5

You're getting undefined behaviour because you're attempting to use values from an uninitialised array. You can't expect anything in particular to happen. Maybe every character happens to be a non-printing character. Maybe it just decided that it didn't want to print anything because it doesn't like your little games. Anything goes.