C++ cout prints entire contents of 2D char array

Question

I am writing a tic-tac-toe game to learn C++.

I would like to store the game board in a 2D char array:

    char playerBoard[5][14] {
        {'2',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n'},
        {'1',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n'},
        {'0',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n'},
        {' ',' ',' ',' ','^',' ',' ',' ','^',' ',' ',' ','^','\n'},
        {' ',' ',' ',' ','0',' ',' ',' ','1',' ',' ',' ','2','\n'}
    };

I would like to print the array with cout:

    for (int i = 0; i < 5; i++) {
            cout << playerBoard[i];
        }

When I try, the entire contents of playerboard, starting at the first array, are printed with each iteration:


    2 > _ | _ | _
    1 > _ | _ | _
    0 > _ | _ | _
        ^   ^   ^
        0   1   2
    1 > _ | _ | _
    0 > _ | _ | _
        ^   ^   ^
        0   1   2
    0 > _ | _ | _
        ^   ^   ^
        0   1   2
        ^   ^   ^
        0   1   2
        0   1   2

The entire contents of playerBoard is printed with:

    cout << playerBoard[0];

What is going on here?

Adrian Mole · Accepted Answer · 2021-01-08T23:14:36.150

None of your five array sub-elements is a nul-terminated string (except possibly the last one, but that relies on undefined behaviour).

The std::cout << operation with a char* operand (or a char[] operand, which decays into a char*) assumes a nul-terminated string; in your case, as the terminators are missing, the buffer is printed until one is found, somewhere. This would appear to be after the entire array block.

To fix this, increase the size of the sub-arrays to 15 characters and add the required nul (zero) terminators:

int main()
{
    char playerBoard[5][15]{
        {'2',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n','\0'},
        {'1',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n','\0'},
        {'0',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n','\0'},
        {' ',' ',' ',' ','^',' ',' ',' ','^',' ',' ',' ','^','\n','\0'},
        {' ',' ',' ',' ','0',' ',' ',' ','1',' ',' ',' ','2','\n','\0'}
    };
    for (int i = 0; i < 5; i++) {
        std::cout << playerBoard[i];
    }
    return 0;
}

The answer can be improved by explaining that `playerBoard[i]` has type `char[14]` which decays into `char*`. — Eugene, Jan 08 '21 at 23:11

score 1 · Answer 2 · answered Jan 08 '21 at 23:12

You have undefined behaviour, since std::cout << playerBoard[i]; expects playerBoard[i] to be (a pointer to the first element of) a null terminated array, and there is no null character (char{0}).

To fix this, use std::cout.write(s, count) or std::string_view which work for non-null terminated character arrays:

for (int i = 0; i < 5; i++) {
    std::cout.write(playerBoard[i], std::size(playerBoard[i]));
    // Or
    std::cout << std::string_view(playerBoard[i], std::size(playerBoard[i]));
}

Or alternatively, add a single null terminator at the end:

char playerBoard[][14] {
    {'2',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n'},
    {'1',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n'},
    {'0',' ','>',' ','_',' ','|',' ','_',' ','|',' ','_','\n'},
    {' ',' ',' ',' ','^',' ',' ',' ','^',' ',' ',' ','^','\n'},
    {' ',' ',' ',' ','0',' ',' ',' ','1',' ',' ',' ','2','\n'},
    {0}
};

// Print the whole array
std::cout << *playerBoard;

// (You can still print individual lines the same way like as above)

C++ cout prints entire contents of 2D char array

2 Answers2